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Notes: Previously published self-diffusion data of HF are analyzed with newly measured densities of the liquid, from 258 K to 373 K, and at pressures up to 200 MPa. The results confirm the importance of hydrogen bonding upon the translational mobility in liquid hydrogen fluoride. © 1998 American Institute of Physics.

Notes: Using the Voter and Chen version of the embedded atom model, we performed molecular dynamics simulations to compute the thermodynamic properties of liquid Ni up to 3000 K, i.e., well above the melting temperature. Our results show good general agreement with available experimental data. Comparison between simulated and experimental heat capacities requires subtraction from the latter of the electronic contribution, which for liquid transition metals is usually an order of magnitude greater than for simple metals. © 1998 American Institute of Physics.

Notes: In addition to the traditional methods of identification of order–disorder transformations in nanoscale clusters containing highly symmetrical molecules, we have developed a simple projection method to recognize such transformations. The three-dimensional orientation distributions of the molecules are projected onto a two-dimensional spherical surface without any loss of information. We have applied the method in the analysis of the phase changes of clusters containing up to 180 tellurium hexafluoride molecules. The molecular configurations are generated by constant energy molecular dynamics simulations of the thermal history of free clusters. We observe two successive transitions as the clusters cool from their melting point. The projection method clearly reveals the mechanism of the structural transformations in the studied clusters: the orientation order of the molecular axes at low temperatures is accompanied by lattice distortion. © 1998 American Institute of Physics.

Notes: Using the Gibbs description of an interphase, the necessary conditions for equilibrium of a closed, two-phase fluid system in the presence of gravity are the Laplace and Young equations and a condition on the chemical potentials. The last condition has been neglected in all previous examinations of contact angles in a gravitational field. After introducing explicit expressions for the chemical potentials, we find that the condition on the chemical potentials can be used to determine the pressure profile within the system. In a "two-interface" system in which a liquid phase is both above and below a vapor phase and the vapor phase forms a solid–vapor interphase in one region, the pressure profile in the liquid phases is the same as it would have been if the vapor phase were not there; thus in a gravitational field, the pressure is smaller in the liquid phase above the vapor phase than it is in the liquid phase below the vapor phase. This results in the contact angle at the upper three-phase line necessarily being smaller than that at the lower three-phase line. This difference in contact angles is conventionally referred to as contact angle hysteresis; however, we show that it is simply an equilibrium property of a capillary system in a gravitational field. The contact angle difference predicted to exist in the presence of gravity does not violate the Young equation, but the Young equation does impose a restriction on the equilibrium adsorption isotherms at the solid–vapor and solid–liquid interfaces. © 1998 American Institute of Physics.

Notes: The uncertainty over the symmetry of the ground state of NiH2 is resolved, showing that it is a bent 1A1. The computations have been performed using a complete active space self-consistent field wave function (CASSCF), a second order perturbation method (CASPT2), and quasirelativistic corrections to the energy and geometry.© 1998 American Institute of Physics. [S0021-9606(98)30333-5]

Notes: Ab initio and semiempirical calculations of large cluster models have been performed in order to study water adsorption and dissociation on pure, defective (vacancies) and doped (Li, Na, K, Ca, Fe) MgO (001) surfaces. The geometries of the adsorbed and dissociated molecules have been optimized preparatory to analysis of binding energies, stretching frequencies, charge transfers, preferential sites of interaction, and bond distances. We have used Mulliken, natural bond order, and electrostatic-derived atomic and overlap populations to analyze charge distributions in the clusters. We have also investigated transition structures, activation energies, energy gaps, HOMO, density of states, SCF orbital energies as well as the acid–base properties of our cluster model. Numerical results are compared, where possible, with experiment, interpreted in the framework of various analytical models, and correlated with site coordination numbers, corner and edge site preferential locations, and direction of charge transfer. A thorough charge analysis indicates substantial charge redistribution in the magnesium oxide crystal as a result of water adsorption and dissociation in pure, defective, and doped MgO crystals. The introduction of heavier impurities and vacancies could produce substantial changes in the physical and chemical properties of the catalyst and increase the binding and dissociation energies. Some of the largest changes originate from the introduction of vacancies. Two and three-dimensional potential energy surfaces are used to investigate activation energies of hydroxylation on the MgO surface. Stretching frequencies are correlated with magnesium and oxygen coordination numbers. © 1998 American Institute of Physics.

Notes: The transition state region of the Li-HF system is theoretically studied via infrared excitation of the ground state of the complex in the reactant valley. The absorption spectrum shows intense peaks for which LiF is produced with high efficiency ((approximate)90%), while the reaction has a very low cross section during the collision at the same energies. The reason is that the resonances reached through optical excitation are in the vicinity of the transition state. © 1998 American Institute of Physics.

Notes: We study smooth transformations V(r)=g(−1/r)+f(1/r2) of Kratzer's potential −a/r+b/r2 in N≥2 spatial dimensions. Eigenvalue approximation formulas are obtained which provide lower or upper energy bounds for all the discrete energy eigenvalues Enl and all N≥2, corresponding, respectively, to the two cases that the transformation functions g and f are either both convex (g″≥0) and f″≥0) or both concave (g″≤0 and f″≤0). Detailed results are presented for V(r)=−a/r+b/rβ and V(r)=−(v/r)[1−ar/(1+r)]+b/r2. © 1998 American Institute of Physics.

Notes: In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess Helmholtz free energy in terms of the potential energy distribution, instead of the configurational partition function. We obtained in this way the temperature dependence of thermodynamic functions in the canonical ensemble assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the temperature dependence of thermodynamic properties in an exact way in the isothermal-isobaric and grand canonical ensemble, using the distribution of the appropriate heat function. For both ensembles restrictions on and implications of these distributions are discussed, and the thermodynamics assuming a Gaussian or (diverging) Gamma distribution is derived. These cases have been tested for water at constant pressure, and the results for the latter case are satisfactory. Also the distribution of the heat function of some theoretical model systems is considered.© 1998 American Institute of Physics.

Notes: Rotationally resolved photoelectron spectra are reported for rovibronically state-selected autoionizing levels of water. These photoelectron spectra are helpful for the spectroscopic assignment of the autoionizing levels and provide considerable dynamical information on the mechanisms for the transfer of energy and angular momentum between the ion core and the Rydberg electron. As a result of angular momentum restrictions, photoelectron spectra for J=0 autoionizing levels provide a direct partial wave analysis for the ejected photoelectrons. © 1998 American Institute of Physics.

Notes: Ab initio calculated potential energy surfaces for low-lying doublet electronic states of C2H2+ are employed to investigate the structure of spectra involving these species. Particular attention is paid to the X 2Πu, A 2Ag, and B 2Σu+ states arising by loss of an electron from one of the three uppermost molecular orbitals populated in the ground electronic state of the neutral molecule. © 1998 American Institute of Physics.

Notes: The pair-potentials calculations of McCaffrey and Kerins [J. Chem. Phys. 106, 7885 (1997)] used with success in simulating the emission spectroscopy of the Zn–RG matrix systems are extended to examine the different temporal decay characteristics exhibited at low temperature, T〈13 K, by the singlet emission bands in the Zn–Ar matrix system. The 238 nm band, assigned in the earlier theoretical work to the body mode Q2, exhibits a 0.1 ns risetime, the 219 nm band assigned to the waist mode Q3, is prompt. By extracting the gradients and the second derivatives of the Q3 and Q2 mode potentials of a Zn⋅Ar18 cluster, decay rates of 3 and 2 ps, respectively, are calculated at the Franck–Condon regions of these potentials accessed in absorption, leading to effective competition between the Q2 and Q3 modes for relaxation of excited-state population and thereby to the coexistence of the 238 nm emission with the 219 nm band. A quasi-bound region is located at 0.32 Å in the body mode, Q2, which slows down the relaxation on this mode and is identified as responsible for the recorded risetime on the 238 nm emission. The temperature dependence exhibited in the Zn–Ar system at higher temperatures (T〉14 K) in which the intensity of the 219 nm band can reversibly be put into the 238 nm band, was examined by generating the (PES) potential-energy surface for coupled Q2×Q3 vibronic modes. The theoretically predicted activation energy barrier is 380 cm−1, which is only in qualitative agreement with the value of 130.6 cm−1 extracted in the kinetics study. Possible reasons for the overestimation in the theoretical value are discussed. © 1998 American Institute of Physics.

Notes: Lattice dynamics and bond characteristics in SnCl4⋅5H2O have been investigated using 35Cl nuclear quadrupole resonance (NQR). Two chemically inequivalent chlorine sites in SnCl4⋅5H2O were distinguished from the temperature dependences of the NQR frequency and from the spin–lattice (T1Q) and spin–spin relaxation times (T2Q). The anomalous temperature dependence of the NQR frequency of one of the two sites was attributed to a thermal weakening of the crystal field from uncoordinated water molecules. In addition, high temperature activated molecular motion was observed. © 1998 American Institute of Physics.

Notes: Employing the scaling arguments, we show that the temperature dependence of the chemical diffusion coefficient near the points corresponding to continuous phase transitions has power-law or inverse logarithmic singularities, Dc∝|ΔT|α/(1−α) for α〉0, ∝|ΔT|−α for α〈0, or ∝1/|ln|ΔT|| for α=0, where α is the specific heat exponent. Monte Carlo simulations, executed with the parameters corresponding to the O/W(110) system, indicate that these singularities, occurring in a very narrow temperature interval, can be reproduced only if the lattice size is large (L〉500). Outside the critical region, the temperature dependence of Dc is regular and the deviations from the ideal Arrhenius behavior are relatively weak. In particular, at appreciable coverages, the variations of the activation energy for chemical diffusion are about 10 kJ/mol (this value amounts to (approximate)10% of the activation energy). © 1998 American Institute of Physics.

Notes: The coarsening of a random interface in a fluid of surface tension γ and viscosity μ is analyzed using a curvature distribution function A(Km,Kg,t) which gives the distribution of the mean curvature Km and Gaussian curvature Kg on the interface. There is a variation in the area distribution function due to the rate of change of Km, Kg and the compression of the interface due to tangential motion. The rates of change of mean and Gaussian curvature at a point are related to the rate of change of the normal velocity in the tangential directions along the interface. The fluid velocity is governed by the Stokes equation for a viscous flow, and the velocity field at a point is determined as an integral of the product of the Oseen tensor and the normal force at other points on the interface. Using a general form for this integral, it is shown that there is a characteristic variable K*=Kg/(Km2−4Kg)1/2 which is independent of time even as Km and Kg decrease proportional to t−1 and t−2, respectively. In the late stages, analytical forms for the distribution function are determined in the limit Km(very-much-less-than)K* using a similarity variable η=(γKmt/μ). Two reasonable approximations are used for the characteristic length for the correlation of the curvature and normal along the interface, and the results for these two approximations are quadratic polynomials in |η| which are nonzero for a finite interval about η=0. It is expected that the actual distribution function is in between these two limiting cases. © 1998 American Institute of Physics.

Notes: A density functional theory is presented that combines an exact expression for the ideal gas free energy functional with a weighted density approximation for the excess free energy functional. The weighting function required in the theory is obtained from the Curtin-Ashcroft recipe, with a bulk fluid direct correlation function from the polymer reference interaction site model integral equation theory. The theory is in quantitative agreement with computer simulations for the density profiles of freely jointed tangent sphere hard chains at a hard wall, about as accurate as the Curtin-Ashcroft theory is for hard spheres at a hard wall. For a more realistic fused-sphere chain model with fixed bond angles and bond lengths, the theory is in excellent agreement with simulations at low and intermediate densities but overestimates the magnitude of layering at high densities for short chains. The theory becomes more accurate as the chain length is increased. © 1998 American Institute of Physics.

Notes: In J. Chem. Phys. [107, 10823 (1997)], Rozyczko and Bartlett report an open-ended formula for equation-of-motion coupled cluster (EOM-CC) hyperpolarizabilities. We demonstrate that this formula is incompatible with the generic definition of EOM-CC properties and converges to a wrong full configuration limit. We derive the correct expressions for the EOM-CC quadratic and cubic response functions. © 1998 American Institute of Physics.

Notes: The difficulty of widely used density functionals in describing the dissociation behavior of some homonuclear and heteronuclear diatomic radicals is analyzed. It is shown that the self-interaction error of these functionals accounts for the problem—it is much larger for a system with a noninteger number of electrons than a system with an integer number of electrons. We find the condition for the erroneous dissociation behavior described by approximate density functionals: when the ionization energy of one dissociation partner differs from the electron affinity of the other partner by a small amount, the self-interaction error will lead to wrong dissociation limit. Systems with a noninteger number of electrons and hence the large amount of self-interaction error in approximate density functionals arise also in the transition states of some chemical reactions and in some charge-transfer complexes. In the course of analysis, we derive a scaling relation necessary for an exchange-correlation functional to be self-interaction free. © 1998 American Institute of Physics.

Notes: Among algorithms that are used to solve the equations of motion, the symplectic integrator (SI) has the advantage of conserving the phase space volume and ensuring a stable simulation. However, incorporating the explicit formula of the SI in a molecular simulation is feasible only for the systems whose Hamiltonian is described by K(p)+V(q), where the kinetic energy K and the potential energy V depend only on momenta p and coordinates q, respectively. Due to this limitation, explicit SI integrators cannot directly be applied to the Nosé-Hoover equations of motion for the constant temperature molecular dynamics (MD) simulation. In this article, by applying the formula of the decomposition of the exponential Liouville operator to the Nosé-Hoover equations, we have obtained a series of integrators for the constant temperature simulation which have the correct form of the Jacobian of the Nosé-Hoover equations. The systems examined here are liquid water and a protein in water. From the results of the constant temperature simulations, where several variations of the integrators were employed, we show that a combination of the Suzuki's second order formula and the fourth order symplectic integrator of Calvo and Sanz-Serna generates a trajectory of much higher accuracy than the nonsymplectic Gear predictor-corrector method for a given amount of CPU time. © 1998 American Institute of Physics.

Notes: We present a molecular–statistical theory of the phase transition from the two-dimensional isotropic liquid to the phase which has the elements of the orientational and translational order combined in a single order parameter. This phase possesses the glide symmetry, similar to the herringbone order in crystals, and is related to the mesophases observed in Langmuir monolayers. The microscopic definition of the herringbone order parameter is presented and the transition temperatures from the isotropic to the nematic, smectic, and the herringbone phases are expressed in terms of the direct correlation function of the two-dimensional isotropic fluid. The relative stability of these phases is discussed. The transition temperature into the herringbone phase is estimated using the simple model of hard discs interacting via the quadrupole–quadrupole potential that promotes the herringbone order. © 1998 American Institute of Physics.

Notes: We present a new algorithm for Reverse Monte Carlo (RMC) simulations of liquids. During the simulations, we calculate energy, excess chemical potentials, bond-angle distributions and three-body correlations. This allows us to test the quality and physical meaning of RMC-generated results and its limitations. It also indicates the possibility to explore orientational correlations from simple scattering experiments. The new technique has been applied to bulk hard-sphere and Lennard-Jones systems and compared to standard Metropolis Monte Carlo results. © 1998 American Institute of Physics.

Notes: The multiconfiguration time-dependent Hartree (MCTDH) method is employed to calculate initial-state selected reaction probabilities for the two isotopic reactions H+H2(D2) with initial states ν=0,j=0–3 and total angular momentum J=0. To compute the reaction probabilities, an initial wave packet is prepared and propagated in time employing the recently developed constant mean-field integrator, thus reducing the computational effort by an order of magnitude. An adiabatic correction scheme is introduced which allows the initial wave packet to be moved from the asymptotic region of the educt channel close to the interaction region. The calculations are performed on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) potential surface which is expanded in products of one-dimensional functions of the Jacobian coordinates. Initial-state selected reaction probabilities are computed for total energies up to 2.5 eV utilizing a combined flux operator/complex absorbing potential approach. © 1998 American Institute of Physics.

Notes: Numerical calculations and Monte Carlo simulations have been performed to explore the dynamic scattering behavior of solutions of copolymer chains composed by two nonsegregated blocks in an athermal solvent. The experimental investigation of this property for real copolymers has shown the presence of a variety of modes, with a complex variation of their locations and intensities, which we try to partially understand with the present simulation work. For this purpose, we use chain samples with a small amount of polydispersity in the block lengths. We have introduced opposite scattering contrast factors for the A and B units. In this way, zero-averaged contrast conditions have been set for the overall scattering, avoiding the presence of the collective mode that otherwise would manifest for any nondilute system of polymer chains. Static and dynamic scattering functions have been obtained for the different samples. For the dynamic scattering functions, three modes are observed in agreement with existing experiments. The first two modes, predominant for low values of the scattering variable, q, are assigned to correspond to the compositional heterogeneity and the first internal relaxation of the chains, as predicted by the theory. The variation of the mode intensities and positions with polymer concentration is analyzed. It semiquantitatively agrees with the existing experimental data. © 1998 American Institute of Physics.

Notes: Dynamic Monte Carlo simulations of single star-branched polymer models were made. Star macromolecules were confined to a simple cubic lattice with the nearest-neighbor attractive interactions. Every star consisted of f=3 arms of equal length. Length of a star varied between 49 and 799 statistical segments. Static and dynamic properties of model stars were calculated in good solvent conditions, aitch-theta-state and in the collapsed state. Change of the chain dimensions, diffusion coefficients, and their scaling exponents with the temperature was shown and discussed. The locations of the aitch-theta temperature and the collapse transition temperature TC were estimated for all chain lengths under consideration. The differences in motion of inner and outer parts of a star-branched polymer in different solvent conditions were described. The existence of a high-density core in the center of star macromolecules was confirmed and the influence of the temperature on its magnitude was studied. An analysis of motion of different parts of star polymers and of the number of inter- and intra-armal contracts was performed. © 1998 American Institute of Physics.

Notes: We report surfactant-induced instabilities along the wetting meniscus of an oil–water–solid system which are discussed through density fluctuations of the interfacial molecular film, coupled with variations of the meniscus shape according to Laplace's relation γJ∼(Δρg)z, where J=(1/R1+1/R2). The results show that two critical parameters exist which govern these instabilities and can be tuned to either magnify or suppress them. These are (i) the initial flux close to the meniscus and, (ii) the size and structure of the amphiphiles. The first parameter determines through the interfacial adsorption amount (∂Γ/∂t) both the magnitude and rate of the meniscus reconformation ∼∂(γJ)/∂t. The higher ∂(γJ)/∂t, the higher the inertia of the moving meniscus fluid (ρv2/2) and the more the interfacial film is compressed beyond its equilibrium position and enters the unstable regime. The molecular size and structure are found to determine the stability and response of the film to the oscillations of the meniscus. A description of these instabilities, their concentration-profile- and molecular-size-dependent behaviors is then proposed. © 1998 American Institute of Physics.

Notes: Solvation structure and conformational preferences of the dicarboxylic suberic acid, HOOC(CH2)6COOH, both neutral and protolyzed in water and neutral in methanol have been studied using molecular-dynamics (MO) computer simulations. According to results from MD simulations in water solution, the backbone hydrocarbon chain shows a very clear tendency to curl up into a helical structure, forming either a tgggt or a tg′g′g′t conformation. The carboxylic head groups are strongly hydrated in the water solution, while the hydrophobic hydrocarbon skeleton is surrounded by water molecules in a packed structure. In the helical conformation, the surface area of the nonpolar part of the fatty acid is minimized against water phase. Transitions between the right and the left-handed helices are observed in neutral and mono-anionic forms of suberic acid in water solution. Suberic acid dissolved in methanol does not show any conformational preferences. Along the hydrocarbon chain, g, g′, and t conformers are equally populated. The head group torsional angles, however, strongly prefer trans conformation due to dipolar interactions between the carboxyl groups and the solvent hydroxyl groups. In addition to MD simulations, corresponding water and methanol solutions are prepared and 13C NMR (nuclear magnetic resonance) chemical shifts are measured in both solutions. Using a time-averaged geometry for suberic acid from MD simulations in water, chemical shielding constants are calculated quantum chemically. Agreement between the theoretical and the experimental chemical shifts is good, and gives indirect support to the simulated conformations of suberic acid in the investigated solutions. The simulation results are also consistent with recent Raman investigations of suberic acid in both water and methanol solutions. © 1998 American Institute of Physics.

Notes: Self-consistent theory of the early spinodal decomposition stage is developed. Calculations show the existence of two maxima in the wave number dependence of the structure factor S(q,t). The main maximum grows with time and has a position between qsaddle (14) and qm (1) at a finite wave number q+. The second one is localized near q(similar, equals)0, its height being practically independent of time. Characteristics of the main maximum are sensitive to the initial conditions. Experiments confirm these predictions. © 1998 American Institute of Physics.

Notes: The photochemistry of carbonyl sulfide (OCS) adsorbed to small silver clusters is shown to exhibit a striking odd–even dependence on the number of Ag atoms in the cluster. OCS is found to desorb nondissociatively from even numbered silver clusters. In contrast, on odd silver clusters, a new product channel corresponding to AgnS is observed. Parallels are found with the photochemistry of adsorbates on extended surfaces. Cross-section measurements for OCS desorption from Ag10 and wavelength-dependent measurements of OCS dissociation on Ag9 both indicate that electronic excitation of the cluster initiates chemistry, analogous to substrate mediated surface photochemistry. The size dependence is reasonably explained in terms of a charge-transfer mechanism involving an ion-pair state of the AgnOCS complex. © 1998 American Institute of Physics.

Notes: Utilizing high-resolution transmission electron microscopy, in tandem with molecular dynamics simulations, we conduct a fundamental study of the mechanical force–structure relationship of carbon nanotubes. We study the response of tubes to asymmetrical radial compressive forces, which are directly related to observed structure distortions in HRTEM (high-resolution transmission electron microscopy). We show that the magnitude of such forces can be readily estimated and is related to the shape of the distortion, characterized by percent compression and the radius of the tube, as well as the number of layers for very narrow tubes. The elasticity and resilience of the walls depend on the tube radius and the number of layers. We further suggest that even very large forces produce reversible, elastic distortions, indicating that radial mechanical forces should not be a feasible method to cut a nanotube without causing severe structural damage. © 1998 American Institute of Physics.

Notes: We have investigated the relaxational dynamics for a protein model at various temperatures. Theoretical analysis of this model in conjunction with numerical simulations suggests several relaxation regimes, including a single exponential, a power law, and a logarithmic time dependence. Even though a stretched exponential form gives a good fit to the simulation results in the crossover regime between a single exponential and a power-law decay, we have not been able to directly deduce this form from the theoretical analysis. © 1998 American Institute of Physics.

Notes: The replica Ornstein–Zernike (ROZ) equations, supplemented by the hypernetted chain and mean spherical closures, were solved for an ionic fluid adsorbed in a disordered charged matrix. To obtain the numerical solution of the ROZ equations we performed renormalization of the initial equations. Both the matrix and adsorbed fluid were modeled as charged hard spheres in a dielectric continuum, i.e., in the so-called restricted primitive model. As a result, the pair distribution functions between fluid ions and for fluid-matrix correlations were obtained. Structural properties were studied as a function of the matrix density, the concentration of adsorbed electrolyte and for different prequenching conditions. The isothermal compressibility, excess internal energy, and the chemical potential were calculated and discussed with respect to of the model parameters. Comparison with the Monte Carlo computer simulations of Bratko and Chakraborty [J. Chem. Phys. 104, 7700 (1996)] indicates that the theory yields qualitatively correct results for the model system. © 1998 American Institute of Physics.

Notes: A simplified form of the Wertheim theory of chemical association has been used to address how ellipsoidal particles with three highly localized interaction sites (head, tail, side) polymerize to form aggregates. Aggregation is allowed for heads with tails subject to an energy parameter, εa, and for heads or tails with the side spot subject to a different energy parameter, εc. No side-by-side interactions are allowed. As εc/εa→0, the aggregate becomes rodlike, whereas as εc/εa→∞, the object becomes a freely rotating chain or a herringbone pattern. When the present theory is applied to self assembly of type I collagen, it is found that each εc and εa correspond to the anchoring caused by two H bonds. © 1998 American Institute of Physics.

Notes: Properly combined with the Lanczos iteration algorithm, the density matrix renormalization group enables us to directly calculate the dipole-allowed singlet states and the transition moment elements between these states and the ground state for longer chains. Our new scheme does not resort to reflection symmetry and electron-hole symmetry, nor to the construction of different superblocks for the ground state and these singlets separately. As an example, we calculate the ground state and first-excited state in the extended Hubbard model. Our calculations show that for a large U, the transition matrix elements decrease with the chain length; for V around U/2, the transition matrix elements can increase with the chain length, which is consistent with recent experimental results. © 1998 American Institute of Physics.

Notes: We present the results of a spectroscopic study carried out on a solid sample of 5,5″-bis(dicyanomethylene)-5,5″-dihydro-Δ2,2′:5′,2″-terthiophene. This compound bears a heteroquinonoid structure in its ground state and could be considered as a model of the structural changes induced by the different types of charge carriers in doped polythiophene. We have identified two very strong Raman bands that are indicative of the attainment of a pure heteroquinonoid structure in α-oligothienyl compounds. The large frequency downshifts of these Raman bands with respect to the pure heteroaromatic structure are a measure of the extent of the electronic modification caused by the quinoidization of the ring molecular system. © 1998 American Institute of Physics.

Notes: The "resolution of the identity" integral approximation applied to second-order many-body perturbation theory, or RI-MP2, method offers improved computational performance compared to traditional (exact) second-order perturbation theory calculations, but introduces a new auxiliary or "fitting basis set" into the method. We develop fitting basis sets for use with the correlation consistent cc-pVDZ and cc-pVTZ atomic orbital basis sets for the atoms H-Ne. These fitting sets are designed to reproduce exact second-order results for a set of 32 test cases, including a variety of reaction energies, weak interactions, and electrostatic properties, to better than 1% error averaged across all tests and less than 2% error in any individual case. Although the RI-MP2 method is primarily targeted to large-scale calculations, it offers substantial performance improvements even for the small molecules used in these test cases. © 1998 American Institute of Physics.

Notes: We report a theoretical framework for the study of the optimal control of multisurface molecular systems via a set of nondegenerate excitation fields. The resulting control equations in the strong response regime are presented in terms of both the Liouville-space density matrix dynamics and the Hilbert-space wave function evolution. We further derive a pair of eigenequations for the optimal pump-pump fields in the pure-state control of three-surface molecular systems in the weak response regime. The globally optimal pair of pump-pump fields in this case are identified. Application to the control of a rovibronic level on the final excited surface reveals a symmetry relation within the optimal pair of pump-pump fields in the weak response regime. For numerical demonstrations, we consider the control of the I2 molecular system involving the initial ground X, the intermediate B, and the final E surface. The target is chosen as an outgoing vibrational wave packet in the bound region of the final E electronic state. The optimal control fields in both the strong and weak response regimes are calculated and further parameterized to fit simple experimentally realizable laser pulses. © 1998 American Institute of Physics.

Notes: The reactions of NO+ and NO2+ with water are considered to make an important contribution to the formation of proton hydrates in the upper atmosphere. There have been several recent studies of the relevant reactions, with the result that discrepancies have arisen in each case over the critical number of water molecules required to promote the appearance of a proton hydrate. Presented here are new results based on the collision-induced reactions of NO+⋅(H2O)n and NO2+⋅(H2O)n cluster ions, which we believe clarify the situation. © 1998 American Institute of Physics.

Notes: A block-localized wave function method is introduced to evaluate the electronic delocalization effect in molecules. The wave function for the hypothetical and strictly localized structure is constructed based on the assumption that all electrons and primitive basis functions can be divided into several subgroups; each localized molecular orbital is expanded in terms of primitive orbitals belonging to only one subgroup. The molecular orbitals belonging to the same subgroup are constrained to be mutually orthogonal, while those belonging to different subgroups are free to overlap. The final block-localized wave function at the Hartree–Fock level is expressed by a Slater determinant. In this manner, the energy difference between the Hartree–Fock wave function and the block-localized wave function can be generally defined as the electronic delocalization energy. The method is applied to two cases. The first concerns the resonance stabilization in the allyl ions. We find that the vertical resonance energies for the planar cation and anion are −45.7 (or −44.7) and −46.7 (or −48.2) kcal/mol at the HF/6-31G* (or 6-31+G*) level, respectively. Their rotational barriers are decomposed in terms of conjugation, hyperconjugation, steric effect, and pyramidalization. The n→σ* negative hyperconjugation in the staggered allyl anion is very strong and stabilizes the system by as much as −13 kcal/mol. The second concerns the hyperconjugation effect in propene. Our calculations suggest that the theoretical hyperconjugation energy in propene is about −5 kcal/mol, which is close to the experimental estimate (−2.7 kcal/mol) derived from the hydrogenation heats of propene and ethylene. Comparisons between the results based on the present block-localized wave function method and those based on the natural bond orbital method are presented and discussed. The examples demonstrate that the block-localized wave function method can be employed as a useful model to analyze chemical bondings and intuitive concepts. © 1998 American Institute of Physics.

Notes: We report on the dynamical structure of water in NaCl aqueous solutions as functions of temperature and concentration by a low-frequency Raman spectroscopy. The spectral profiles in the frequency range from −50 to 250 cm−1 have been analyzed with a superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The distributions of the characteristic frequencies of the intermolecular vibrational modes observed around 50 and 180 cm−1 in NaCl aqueous solutions are always wider than those in pure water. In NaCl aqueous solutions with various concentrations, the spectral line width g1=1/(2πcτ) of a central component, which corresponds to the reciprocal relaxation time, linearly changes with temperature from about 255 K to 300 K, while the relaxation time above 300 K holds an Arrhenius-type behavior. The slope of the spectral linewidth against temperature below 300 K decreases with increasing concentration. The distribution parameter of the relaxation time in the Cole–Cole type relaxation formula decreases with decreasing temperature and it becomes smaller with increasing concentration. © 1998 American Institute of Physics.

Notes: A novel broadband femtosecond version of the stimulated emission pumping (SEP) technique is demonstrated. A nonstationary ground state of a molecular sample in the condensed phase is prepared by two optical pulses. The first picosecond PUMP pulse resonantly excites the sample. The second femtosecond DUMP pulse, which is tuned to the molecular fluorescence band, is applied after relaxation in the excited state and creates a "particle" in the ground state and a "hole" in the excited state. The relaxation of this system is probed by a femtosecond supercontinuum. An advantage of the proposed scheme is that the hole contribution is constant for certain conditions, and hence, the transient absorption spectrum of the particle may be isolated. As an application of the technique, the ground-state evolution of coumarin 102 in acetonitrile is studied. Intramolecular vibrational redistribution (IVR), with a characteristic time τIVR∼10 fs, is observed in the frequency domain. Subsequently, the absorption band shifts to the blue and shows isosbestic points in the course of relaxation. © 1998 American Institute of Physics.

Notes: The static structure of a bulk electrolyte solution or colloid system is investigated in the framework of a dressed-ion theory (DIT). The number–number, charge–number, and charge–charge static structure factors are calculated and are seen to depend only on the linear response function of the DIT α(circumflex)(k), the α function therefore determining the charge structure of the fluid in what is an expression of the fluctuation–dissipation theorem. The expression of the static structure factors for one-component charged spheres (OCCS) is evaluated in the random-phase approximation and in a modified version of the mean-spherical approximation (MSA), using the hard-sphere fluid as a reference system, and an explicit expression for the linear response function and dielectric function is obtained. The effective screening length (κ−1) and the transition from monotonic exponential to oscillatory behavior obtained from the modified MSA expression of the α function are seen to improve the ones derived from the second moment condition at intermediate concentrations. The internal charge density distribution of a dressed ion and the renormalized ion charges (q*) are also investigated. The oscillatory behavior of the charge distributions suggests an "onionlike model," with the central ion surrounded by spherical charge shells. The effective charges calculated from the modified MSA are seen to diverge in the neighborhood of the transition from the monotonic exponential regime to the oscillatory regime. In the limit of vanishing concentration, Debye–Hückel (DH) results are recovered.© 1998 American Institute of Physics.

Notes: The high-level ab initio potential energy surface (PES) for NeCl2 in the ground electronic state predicts the energy minimum in the linear geometry (L-well) to be slightly deeper than that in the T-shaped geometry (T-well). The experimental D0 and R0 values are reproduced within uncertainties of measurements by both adding the calculated perturbation of the Ne–Cl interactions due to intramolecular forces in Cl2 to empirical NeCl potentials, and by linearly extrapolating or simply scaling the ab initio PES. These procedures lead to equal or even reversed relative depths of the two wells, in accord with both predictions of an atom-atom model using equivalently accurate ab initio NeCl potentials and variation of the ab initio PES with increasing accuracy of calculations. The D0 value for the L-well is predicted to be less than that for the T-well by 2.4 to 5.2 cm−1 for different scaling schemes. The calculated lowest energy rovibrational states associated with each of two conformers show negligible mutual influence, while the effect of the L-well on the rovibrational wave functions for the next vibrational states associated with the T-well is found to be rather important. Microwave spectra are predicted for each PES obtained, and include portions originating from the L-well. The calculated scattering cross section reproduces well the experimental data and is found to be significantly contributed by the L-well. © 1998 American Institute of Physics.

Notes: Relative state-to-state cross sections are obtained for OH colliding with CO, N2, and CO2. Hexapole state selection is used to prepare a beam of OH molecules in the upper Λ-doublet component of the lowest rotational state. The collision induced rotational energy transfer is monitored state selectively by means of LIF (laser induced fluorescence) spectroscopy. A study is made of the symmetry effects in the obtained cross sections. The results are compared with previously reported cross sections for Ar, He, and H2. A general propensity is found for parity conserving transitions to the Π3/2, J=〈fraction SHAPE="CASE"〉52 and Π1/2, J=〈fraction SHAPE="CASE"〉12 states. © 1998 American Institute of Physics.

Notes: The primary photochemistry of gas phase dichlorine monoxide (Cl2O) and of hypochlorous acid (HOCl) following excitation at 235 nm has been investigated using photofragment ion imaging to obtain the recoil velocity and angular distributions of the ground (2P3/2) and spin-orbit excited (2P1/2) atomic chlorine products. In the case of Cl2O, both Cl spin-orbit products exhibit angular distributions characterized by an anisotropy parameter, β=1.2±0.2, consistent with previous interpretations of the ultraviolet (UV) absorption spectrum of Cl2O which associate the broad intense absorption feature peaking at λ∼255 nm with excitation to a (bent) dissociative state of 1B2(C2v) symmetry. The recoil velocity distributions of the two Cl spin-orbit products are markedly different. The ground state atoms (which constitute 〉90% of the total Cl atom yield) are partnered by ClO fragments carrying significantly higher average levels of internal excitation. The slowest Cl atoms are most readily understood in terms of three body fragmentation of Cl2O to its constituent atoms. These findings are rationalized in terms of a model potential energy surface for the 1 1B2 state, which correlates diabatically with ClO(X) radicals together with a spin-orbit excited Cl atom, with efficient radiationless transfer to one (or more) lower energy surfaces at extended Cl-O bond lengths accounting for the dominance of ground state Cl atom fragments. The image of the ground state Cl atoms resulting from photolysis of HOCl at 235 nm is consistent with parent excitation via a transition for which the dipole moment is closely aligned with the Cl-O bond, followed by prompt dissociation (β=1.7±0.2) with the bulk of the excess energy partitioned into product recoil. Such conclusions are consistent with the results of laser induced fluorescence measurements of the OH(X) products resulting from 266 nm photodissociation of HOCl which reveal OH(X) products in both spin-orbit states, exclusively in their zero-point vibrational level, and carrying only modest levels of rotational excitation (well described by a Boltzmann distribution with Trot∼750±50 K). © 1998 American Institute of Physics.

Notes: The photodissociation of diphenylmethane by excitation to the S1 state at 266 nm in n-heptane solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of the diphenylmethyl radical is identified by its fluorescence, which is induced by excitation at 308 nm, and by its absorption. The growth rate of (3.7±0.4)×107 s−1 for the radical is equal to the decay rate of (3.8±0.4)×107 s−1 for the precursor fluorescence. The quantum yield of the radical is of the order of ∼10−3. Neither dissociation to the radical nor intersystem crossing to the T1 state is thermally activated, whereas activated internal conversion to the S0 state is observed. The formation of the radical depends linearly on the photolysis pulse fluence. The data are consistent with a mechanism that the molecule undergoes intersystem crossing from thermally equilibrated levels of the S1 state to vibrationally excited levels of the T1 state at which it dissociates in competition with vibrational relaxation. The mechanism is explained in terms of electronic coupling between the precursor and product states. The S1 state does not correlate adiabatically to the ground state of the C–H bond fission products, so intersystem crossing to the T1 state precedes dissociation. In the T1 state, avoided crossing between the ππ* (benzene) configuration and the σσ* (C–H) repulsive configuration results in the adiabatic potential energy surface which evolves to the ground state of the C–H bond fission products allowing rapid dissociation. © 1998 American Institute of Physics.

Notes: The all-trans→13-cis⋅9-cis photoisomerization reaction of retinal in aerated nonpolar solvents has been studied by femtosecond time-resolved ultraviolet-visible (UV-VIS) absorption spectroscopy. The excited-state absorption spectra in the wavelength region 400–800 nm indicate that there is no all-trans→13-cis⋅9-cis isomerization reaction pathway that is complete in the electronic excited singlet manifold of S1, S2, and S3. The ground-state bleaching recovery of all-trans retinal monitored in the near UV (ultraviolet) wavelength region 310–390 nm shows that a perpendicular excited singlet state (p*) takes part in the all-trans→13-cis⋅9-cis isomerization reaction. The lifetime of p* is about 7 ps, and the precursor of p* is most probably the S2 state. The isomerization quantum yield derived from the femtosecond UV absorption data agrees well with those determined by the HPLC analysis of the photoproduct. The temperature dependence of the isomerization quantum yield indicates the existence of a potential-energy barrier as high as (1.2±0.6)×103 cm−1 on the reaction pathway from the S2 state to the p* state. © 1998 American Institute of Physics.

Notes: A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion–water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion–water interaction potential: The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)6]n+, and the water molecules interact with the hydrate by means of an ab initio [M(H2O)6]n+–H2O interaction potential. A new ab initio interaction potential has been developed to describe the Mn+–(H2O)first-shell interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion–water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns of simulation for a sample formed by [Cr(H2O)6]3+ and 512 H2O are presented. Rigidity effects of the cluster are examined by comparing the present results with those previously obtained with a model of rigid hexahydrate [J. Phys. Chem. B 102, 3272 (1998)]. A new definition of hydrated ion based on the rotational properties of its hydrate is supported. © 1998 American Institute of Physics.

Notes: Neutron diffraction experiments with isotopic substitution on water confined in porous Vycor glass at two hydration states are presented and analyzed in terms of the experimentally accessible site-site distribution functions. The bias on these functions as well as their limitations, due to the presence of regions where water molecules are not allowed (excluded volume effects), and to the contribution of water–Vycor interference to the measured cross sections, are discussed. In particular the relative weight of these cross correlation terms is estimated for the first time. It is shown that the traditional analysis of diffraction data of these kinds which ignore cross correlations may be erroneous. A full account of the excluded volume effects is reported in paper II [A. Soper, F. Bruni, and M. A. Ricci, J. Chem. Phys. 109, 1486 (1998), following paper]. © 1998 American Institute of Physics.

Notes: This work addresses the consideration of the energy landscape roughness in protein sequence design. The proteins are modeled by 2D lattice chains, initially designed to maximize the energy gap between the folded and unfolded states. Additional optimization and control of the folding properties is achieved by specific sequence mutations that alter the energetic and geometric roughness of the landscape. It is found that mutations that reduce the energetic roughness at the expense of increasing the native-state energy generally lead to a fast folding and stable protein at lower temperatures. Such mutations are also found to modify the geometric roughness (related to nucleation effects) creating variations in the folding time that depends specifically on each sequence and can lead in many cases to a reduction of the total landscape roughness. An additional reduction of the geometric roughness is achieved by adding local bond-angle propensities to selected sequence sites. © 1998 American Institute of Physics.

Notes: Ab initio folding of the avian pancreatic polypeptide using a diffusion-process-controlled Monte Carlo method is presented. This method differs from other Monte Carlo methods in that two successive conformations must be kinetically connected in a small period of time. The 36-residue polypeptide is represented using a hybrid level of structure description: the backbone is treated at an all-atom level, while the side chains are modeled as spheres. The conformations are evaluated on the basis of pairwise contact energies between the side chains, a main chain hydrogen bonding potential, and local bonded potentials. Starting from various extended conformations, the chain reaches the basin of lowest energy in ∼1000–3500 Monte Carlo steps and the predicted conformations deviate by ∼3.0 Å rms from the x-ray structure. The eight trajectories suggest a three-step mechanism: (1) early formation of the α helix in the region 14–33, (2) cooperative formation of long-range interactions, and (3) stabilization of the polyprolinelike conformation in the region 1–8 in the final steps of folding. © 1998 American Institute of Physics.

Notes: We propose an efficient numerical algorithm for solving integral equations of the theory of liquids in the reference interaction site model (RISM) approximation for infinitely dilute solution of macromolecules with a large number of atoms. The algorithm is based on applying the nonstationary iterative methods for solving systems of linear algebraic equations. We calculate the solvent–solute atom–atom correlation functions for a fragment of the B-DNA duplex d(GGGGG)⋅d(CCCCC) in infinitely dilute aqueous solution. The obtained results are compared with available experimental data and results from computer simulations. © 1998 American Institute of Physics.

Notes: The 2A2 and 2B1 states formed in the ionization of the outermost π orbitals in furan, pyrrole and thiophene are shown to interact vibronically via nontotally symmetric b2 vibrational modes. The interaction is strongest in pyrrole and thiophene, where the conical intersection between the two adiabatic surfaces occurs near the minimum of the upper (2B1) state. The resulting nonadiabatic effects manifest themselves in the 2B1 bands by a lack of resolved structure in case of pyrrole and thiophene, and by a line broadening in case of furan. The spectra are investigated using a linear vibronic coupling model. All totally symmetric a1 (tuning) modes and nontotally symmetric b2 (coupling) modes describing the ring motion are taken into account. The parameters of the model are obtained with the aid of ab initio calculations. The ground state optimized geometries and vibrational frequencies are computed at the level of the second-order Møller–Plesset perturbation theory, while the dependence of the ionization energies on the nuclear configuration is evaluated using the outer valence Green's function method. Where appropriate, assignments of the observed structure are given. © 1998 American Institute of Physics.

Notes: For general mixtures of polyatomic molecules and their constituent atoms, we first rigorously derive an exact statistical mechanical result relating the background pair correlation function y(1,2,...,m) to a certain excess chemical potential difference involving its components, βΔμe, extending and generalizing our previous results. Second, using only thermodynamic methods, we develop a perturbation theory for the equation of state (EOS) which involves βΔμe; we then express this EOS in an alternative form involving y(1,2,...,m). The latter form coincides with results recently obtained by Zhou and Stell using a different approach and with the EOS of the Wertheim first-order perturbation theory (TPT1); our approach explicitly exposes the underlying thermodynamic approximations involved. Third, we show for the case of tangent fused-hard sphere (FHS) systems, under the approximation that βΔμe is independent of composition, that implementation of the former form of the theory yields results analytically equivalent to those obtained from the Boublik–Nezbeda (BN) EOS; and that the alternative implementation is only slightly less accurate, due to a (numerically small) internal inconsistency in this EOS. This sheds light on the remarkable accuracy obtained for several previous implementations of TPT1 for such systems. We present new computer simulation results for a particular ternary tangent FHS heteronuclear diatomic mixture, which support the approximation that βΔμe for mixtures of such molecules is nearly composition independent. Finally, for several FHS mixture model systems, we test the Lewis–Randall rule and several other approximations for calculation of the mixture chemical potentials. The Lewis–Randall rule is generally superior for the individual chemical potentials, and is competitive for βΔμe. © 1998 American Institute of Physics.

Notes: A novel method, Hamiltonian scaling grand canonical Monte Carlo, has been used to determine the phase behavior of 12 variations of the modified Buckingham exponential-6 potential. Hamiltonian scaling grand canonical Monte Carlo enables the determination of thermodynamic properties of several related potential models from a single set of simulations. The main advantage of the method is that by appropriate selection of the chemical potential and temperature for each Hamiltonian, the densities that are sampled are relevant for multiple Hamiltonians, thus increasing the simulation efficiency. The method was combined with mixed-field finite-size scaling to determine the critical parameters and phase coexistence curves of several modified Buckingham exponential-6 potentials to a high level of accuracy. In particular, the critical parameters of the models have been calculated to within 0.3% for the critical temperature and 1% for the critical density and pressure. A new potential for methane based on the Buckingham exponential-6 model is presented. The vapor pressure curve of methane from the Buckingham exponential-6 potential model is much more accurate than is possible with the Lennard-Jones potential model. © 1998 American Institute of Physics.

Notes: We apply a stochastic method introduced by Dellago et al. [J. Chem. Phys. 108, 1964 (1998)] to sample transition paths in high-dimensional systems. The method connects two endpoint regions (for example a reactant and a product region) by a set of space-time paths. This approach is an importance sampling for rare events that does not require prior knowledge of the location of dynamical bottlenecks. Transition paths are generated with a weight corresponding to a chain of Metropolis Monte Carlo steps. We derive Monte Carlo algorithms and apply the technique to the dynamics of hydrogen-bond breaking in liquid water. We obtain averages in a transition path ensemble for the structure and energy along the trajectory. While characterized by a rate constant, hydrogen-bond breaking in water occurs frequently enough to be studied by standard methods. The process therefore provides a useful test of path sampling methods. The comparison between path sampling and standard Monte Carlo demonstrate the feasibility of transition path sampling for a many-body system with a rough potential energy surface. © 1998 American Institute of Physics.

Notes: A continuum microscopic model for symmetric amphiphilic mixtures, is generalized by considering explicitly water-oil asymmetry, through the interactions between amphiphiles and water and oil. The phase diagram, including lamellar phases, and the properties of water-oil interfaces are studied, using an approximate free energy density-functional, for a wide range of amphiphilic interactions. We also consider the structure and stability of spherical micelles and study the dilute micellar regime. By combining the microscopic density-functional description with the phenomenologic Helfrich elastic free energy, we calculate the elastic properties of the amphiphilic film. Our results for the elastic constant, ks=2k+k¯, are compared with experimental data. © 1998 American Institute of Physics.

Notes: A simple mean-field model of azimuthal ordering in Langmuir monolayers induced by anisotropic dispersion forces is presented. The approach is removed from previous studies of this interaction in systems of grafted rods insofar as the tilt of amphiphiles is decoupled from azimuthal ordering and treated as an external variable. We examine the phase space of fixed tilt against temperature, finding that azimuthal ordering is second order for most tilts, except in the range 57°–66°, where it is first order. This is related to the existence of three different types of azimuthal order; polar, nematic, and polar–nematic. © 1998 American Institute of Physics.

Notes: Exposing an O2-saturated Pt(111) surface at 85 K to a beam of D atom leads to desorption of O2 and D2O. A series of post D-exposure thermal desorption spectra shows that D2O is produced by consecutive D-addition reactions via adsorbed OD intermediate, i.e., O2(ad)→DO(ad)+OD(ad)→DD2O(ad)+D2O(g). When CO is coadsorbed with O2 on Pt(111) at 85 K, the incident D atom also induces prompt desorption of CO2 but not CO. We propose that CO is oxidized by the nascent hot O* and OD* formed in a highly exothermic initiation reaction D(g)+O2(ad)→DO2≠→O(ad)+OD(ad) with an energy release of ∼−4.6 eV before they become accommodated to the surface. Possible mechanisms for O2 desorption are also briefly discussed. © 1998 American Institute of Physics.

Notes: A formulation for the calculation of nuclear magnetic resonance (NMR) shielding tensors, based on density functional theory (DFT), is presented. Scalar-relativistic and spin-orbit coupling effects are taken into account, and a Fermi-contact term is included in the NMR shielding tensor expression. Gauge-including atomic orbitals (GIAO) and a frozen-core approximation are used. This formulation has been implemented, and 1H and 13C NMR shifts of hydrogen and methyl halides have been calculated and show good agreement with experiment. 13C NMR shifts of 5d transition metal carbonyls have been calculated and show improved agreement with experiment over previous scalar-relativistic calculations. For the metal carbonyls it is shown explicitly that the combination of spin-orbit coupling and magnetic field mixes spin triplet states into the ground state, inducing a spin density that then interacts with the nuclei of the metal carbonyl via the Fermi-contact term. Results indicate that the Fermi-contact contribution to the 13C NMR of the metal carbonyl ions increases with increasing oxidation state of the ion. It is reasoned that as the oxidation state increases, π back bonding decreases and σ bonding increases, within the metal–carbon bond, thus facilitating a greater transfer of spin density from the metal to the carbon nucleus, and thus increasing the Fermi-contact contribution to the NMR shielding of the carbon. © 1998 American Institute of Physics.

Notes: The formation mechanisms of C1+, C2+, and C3+ ions by laser ablation of graphite are investigated using time-of-flight (TOF) quadrupole mass spectroscopy. The laser ablation is performed in conjunction with a pulsed Ar expansion to elucidate the third-body effect on the formation of the small carbon cluster ions. Drastic enhancement of the C2+ ion signal is observed by an increase of the local pressure near the target, indicating that C2+ ions are efficiently formed by recombination of carbon atoms and subsequent ionization. The branching ratio of C1+, C2+, and C3+ ions and their mean translational energies are different from those of neutrals. Also, the TOF spectra for Cn+ ions show multiple peak structures, which implies that different mechanisms are involved in the formation of Cn+ ions. © 1998 American Institute of Physics.

Notes: High resolution stimulated gain Raman spectroscopy is used to investigate the collisional parameters of pure rotational S0(j=0–4) lines of H2 in pure H2 and H2–He mixture. Measurements are performed between 300 and 1000 K in a density regime where the lines are essentially collisionally broadened (typically 10 amagat). For the first time, these highly accurate measurements of the frequencies of pure rotational lines allow one to correct previously measured values that did not take into account the collisional frequency shift. For both collisional systems, the shifting coefficients exhibit a linear behavior with the square root of temperature, similar to the behavior already observed in the Q branch. The broadening coefficients of the S0 branch increase nonlinearly with temperature contrary to the Q branch. For the H2–He system, both these new S0(j) data and previously measured Q(j) data are analyzed using a modeling of the broadening coefficients in terms of elastic and inelastic contributions. These different contributions are analyzed as a function of temperature and of the rotational quantum number j. Preliminary quantum calculations are used to assess the validity of the model. Further calculations will be presented in paper II. © 1998 American Institute of Physics.

Notes: The optimum size of the cavity accommodating a solute in the reaction field theory of solvation is considered by empirical calibration of the results of electronic structure calculations against experiment. To isolate the long range electrostatic free energy contributions treated by reaction field theory from the many other short range contributions not explicitly considered, computational results are compared to experimental determinations of conformational free energy differences in polar solutes having two or more stable or metastable isomers. When the cavity shape is defined by a solute electronic isodensity contour, it is found that the best overall agreement with experiment is obtained with a cavity size corresponding to the 0.001 a.u. contour. © 1998 American Institute of Physics.

Notes: New concepts are described for nuclear magnetic resonance (NMR) implementations of spin ensemble quantum computing in one and two dimensions. Similarities and differences between ensemble and pure state quantum computing are discussed by using a Liouville space formalism based on polarization and single transition operators. The introduction of an observer spin, that is coupled to the spins carrying the quantum bits, allows a mapping of the states of a quantum computer on a set of transitions between energy levels. This is spectroscopically favorable compared to a mapping on the energy levels themselves. Two complementary parallelization schemes for quantum computing are presented: one exploits the parallel processing feature inherent in multidimensional NMR, while the other employs mixed superposition states represented by operators in Liouville space. The spin swap operation, introduced in this paper, allows a convenient extension of quantum computing to spin systems where not all spin–spin couplings are resolved. The concepts are illustrated by implementations of logic operations and identities consisting of a sequence of basic logic gate operations. © 1998 American Institute of Physics.

Notes: Analytical representations of the global potential energy surface of XYn molecules are developed and applied to model the potential surface of methane in the electronic ground state. The generic analytical representation allows for a compact, robust, and flexible description of potentials for XYn systems irrespective of the specific nature of the atomic interactions. The functions are global in that structures near several minima of the potential hypersurface as well as saddle points and dissociation limits are well described. Clusters of atoms Yn can be represented as well by this type of function. Care is taken to implement conditions resulting from the symmetric group Sn and to construct positive definite bilinear forms of special functional forms of certain coordinates (such as bond lengths and bond angles), in order to avoid artifacts in exceptional ranges of the potential hypersurface. These special functional forms include intrinsic, symmetry allowed couplings between coordinates such as bending and stretching. We include linear potential terms in bond angle coordinates, which result in effectively quadratic potential terms for highly symmetric structures. True logical multidimensional 01-switching functions Ssw(r) of bond lengths r are used to interpolate between limiting ranges in the hypersurface. The particular form Ssw(r)∼exp(−(rsw/r)nsw) allows us to describe the potential as a multipole expansion representation in the limit of large r(→∞). In the application to methane, first the representations are fitted to data from high level ab initio calculations using multireference configuration interaction techniques. Additional conditions which help to improve the description of experimental data are considered during the fit. Typically, these conditions involve some parameters or parameter groups and refer to the equilibrium geometry and harmonic force field. Other constraints apply to the energies of dissociation channels. We describe the model potentials METPOT 1 to METPOT 4 in the present work. © 1998 American Institute of Physics.

Notes: An investigation of the electron impact ionization and fragmentation of helium clusters that contain Ne atoms and Nek subclusters has been performed. The charge transfer probability from He+ to Ne and the branching ratios for fragmentation of the Nek subclusters were found by analyzing the dependence of the ion signal intensities on the Ne pressure in the "pickup" region. The measured charge transfer probability from He+ to Ne ranges from 0.06±0.01 for clusters of mean original size 〈N〉=3300 to 0.43±0.02 for 〈N〉=1100. Charge transfer to a single Ne atom within the helium clusters never yields bare Ne+ ions. Instead, fragments of the type NeHen+ are produced. The charge transfer from He+ to Ne2 subclusters yields mainly Ne2+ for smaller initial cluster sizes, but NeHen+ or Ne2Hen+ fragments are more probable for larger clusters. This shows that He droplets of a few thousand atoms are able to cage Ne2 subclusters by dissipating the entire energy released by charge transfer and formation and vibrational relaxation of the Ne2+ ion. Interestingly, it was found that in these relatively small helium clusters the Ne3 and Ne4 subclusters never survive the charge transfer from He+. Fragments such as Ne2+ and Ne2Hen+ are more likely to survive than are Ne3+ and Ne4+. In general, the results presented here are qualitatively similar to those for a recent study of the ionization of Ar in helium droplets. In both cases fragmentation to the bare ion is rare, while fragmentation to the dimer ion dominates. However, the helium cluster caging effect is more efficient for Ne subclusters than for Ar subclusters. Also, there is no evidence for shell structures in the NeHen+ ion fragment distributions. © 1998 American Institute of Physics.

Notes: An accurate determination of frequency-dependent molecular hyperpolarizabilities is at the same time of possible technological importance and theoretically challenging. For large molecules, Hartree–Fock theory was until recently the only available ab initio approach. However, correlation effects are usually very important for this property, which makes it desirable to have a computationally efficient approach in which those effects are (approximately) taken into account. We have recently shown that frequency-dependent hyperpolarizabilities can be efficiently obtained using time-dependent density functional theory. Here, we shall present the necessary theoretical framework and the details of our implementation in the Amsterdam Density Functional program. Special attention will be paid to the use of fit functions for the density and to numerical integration, which are typical of density functional codes. Numerical examples for He, CO, and para-nitroaniline are presented, as evidence for the correctness of the equations and the implementation. © 1998 American Institute of Physics.

Notes: Results from ab initio calculations at the CCSD(T) level of theory are presented for krypton monofluoride (KrF), krypton monofluoride cation (KrF+), linear, ground-state krypton difluoride (KrF2), the triplet state of krypton difluoride, and the krypton–fluorine van der Waals complex (Kr–F2). These are the first calculations demonstrating that KrF is a bound molecule, in agreement with experimental observation. When corrected for basis-set superposition error, the calculated potential displays quantitative agreement with the attractive wall of the experimentally measured potential curve. Results are also presented for KrF+ and linear KrF2 which yield accurate values for their dissociation energies. The triplet state of KrF2 is found to have a minimum energy below that of separated atoms, and its structure is bent, with a small F–Kr–F bond angle (71 deg). The van der Waals complex, Kr–F2, appears to consist of an unperturbed F2 molecule attached to a krypton atom in the expected T-shaped structure. © 1998 American Institute of Physics.

Notes: State-selective infrared excitation of o-H2–OH via the pure OH overtone transition has been used to induce a half-collision inelastic scattering event between the OH radical and ortho-H2 under restricted initial orientation conditions. The time evolution and final state distribution of the OH products from vibrational predissociation have been evaluated by ultraviolet probe laser-induced fluorescence measurements. The half-collision scattering takes place with ∼3350 cm−1 of energy available to the OH (v=1)+o-H2 products, an energy that exceeds the classical barrier to reaction. The OH (v=1) products are preferentially populated in high rotational levels with a distribution that is consistent with an energy gap law. A significant fraction of the OH fragments are promoted to the excited spin–orbit state in the predissociation process. A strong lambda-doublet propensity is also found, indicating that the OH unpaired pπ orbital is preferentially aligned perpendicular to the rotational plane of the OH products. Finally, the OH rotational and fine structure distributions are compared with those obtained in previous full collision inelastic scattering studies at energies below the threshold for reaction. © 1998 American Institute of Physics.

Notes: The multiphoton ionization dynamics of NaK molecules is investigated experimentally using one-color pump–probe femtosecond spectroscopy at 795 nm and intermediate laser field strengths (about 10 GW/cm2). Both NaK+ and Na+ ions are detected as a function of pulse separation time, pulse intensities, and strong pulse–weak pulse order. To aid in the analysis, the potential energy curves of the two lowest electronic states of NaK+ and the electronic transition dipole moment between them are calculated by the GAUSSIAN94 UCIS method. Different ionization pathways are identified by Franck-Condon analysis, and vibrational dynamics in the A 1Σ+ and 3 1Π states, as well as in the ground state, is observed. Further, the existence of a highly excited (above the adiabatic ionization limit) neutral state of NaK is proposed. By changing the strong pulse–weak pulse order of the pulses, the ionization pathways for production of both ions can be varied and thus controlled. © 1998 American Institute of Physics.

Notes: The irregular vibronic structure in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene is a result of a classic E⊗e Jahn–Teller effect [A. Furlan et al., J. Chem. Phys. 96, 7306 (1992)]. This is well characterized and can be used as an effective probe of intramolecular perturbations. Here we examine the S1←S0 R2PI spectrum of 9-hydroxytriptycene and the fluorescence from various excited state vibronic levels. In this system the pseudorotation of the Jahn–Teller vibration is strongly coupled to the torsional motion of the bridgehead hydroxy group. This torsional motion results in a tunneling splitting in both the ground and excited states. The population of the upper level in the ground electronic state results in additional vibronic transitions becoming symmetry allowed in the R2PI spectrum that are forbidden in the bare triptycene molecule. The assignment of the R2PI and fluorescence spectra allows the potential energy surfaces of these vibrational modes to be accurately quantified. The full C3v vibronic point group must be used to interpret the spectra. The time scale of the internal rotation of the–OH group and the butterfly flapping of the Jahn–Teller pseudorotation are of similar magnitude. The tunneling between the nine minima on the three dimensional potential energy surface is such that the Jahn–Teller pseudorotation occurs in concert with the–OH internal rotation. The Berry phase that is acquired during this motion is discussed. The simple physical picture emerges of the angle between two of the three benzene moieties opening in three equivalent ways in the S1 electronic state. This geometry follows the position of the hydroxy group, which preferentially orients itself to point between these two rings. © 1998 American Institute of Physics.

Notes: The results of large-scale quantum mechanical calculations of the CH(v=2) 1st overtone spectrum for 30-mode benzene are reported. This overtone was chosen for investigation because of its high degree of fragmentation and resulting complexity compared to spectra for the fundamental and higher overtones. These calculations use the best available ab initio force field supplemented by higher-order terms for the CH stretch–wag interaction. The dynamical calculations were conducted in large active spaces with 12 000 or 16 000 vibrational basis functions. The recursive residue generation method was used to compute residues (intensities) and eigenvalues. From these quantities, the lineshape function, survival probabilities, and vibrograms were computed. Wherever possible, these results were compared to experimental overtone spectra and to other computational results. © 1998 American Institute of Physics.

Notes: α and β relaxation time of glass forming solution LiCl–6H2O have been measured in three states; liquid, supercooled liquid, and glass. The β process presents an original behavior which is analyzed. By using the Adam Gibbs and the Coupling Rearranging Region theory, we try to describe the α process. The predictive model of Perez based on these two theories give excellent results and a coherent interpretation of the observed phenomenon. © 1998 American Institute of Physics.

Notes: We consider the nature of the fluid–solid phase transition in a polydisperse mixture of hard spheres. For a sufficiently polydisperse mixture (σ〉0.085) crystallization occurs with simultaneous fractionation. At the fluid–solid boundary, a broad fluid diameter distribution is split into a number of narrower fractions, each of which then crystallize. The number of crystalline phases increases with the overall level of polydispersity. At high densities, freezing is followed by a sequence of demixing transitions in the polydisperse crystal. © 1998 American Institute of Physics.

Notes: Two types of divacancy at the (001) surface of MgO are theoretically studied and compared with the corresponding defect in the bulk: the pit, where a surface magnesium and the oxygen ion underneath are removed, and the tub, where both removed ions are at the surface. All calculations have been performed by means of the EMBED program which adopts an embedded-cluster approach in the frame of the Hartree-Fock (HF) approximation [C. Pisani F. Corà, R. Nada, and R. Orlando, Comput. Phys. Commun. 82, 139 (1994); C. Pisani and U. Birkenheuer, ibid. 96, 152 (1996)]; the semi-infinite host crystal for the study of the surface defects has been simulated with a four-layer slab. The energy released on formation of the divacancy from the two charged isolated vacancies is very high, almost 300 kcal/mol. The tub divacancy is the most stable, both as a neutral and as a singly charged defect. For the paramagnetic center (one electron trapped in the cavity), spin density data are provided and discussed with reference to results from electron paramagnetic resonance experiments and molecular cluster calculations [E. Giamello M. C. Paganini, D. Murphy, A. M. Ferrari, and G. Pacchioni, J. Phys. Chem. 101, 971 (1997)]. It is suggested that the tub divacancy is a common defect, if not the most common, at the highly dehydrated MgO surface. © 1998 American Institute of Physics.

Notes: A resonance Raman intensity analysis is performed on the intramolecular charge-transfer molecule 1-aza-adamantane-4-ylidenemalononitrile in acetonitrile solution. We explore the extent to which changes in molecular structure upon charge transfer can be obtained from resonance Raman intensity analysis, and extend the analysis method for charge-transfer excitation to take into account the possible influence of nearby locally excited states. Absolute scattering cross sections are measured at five excitation wavelengths spanning both the charge-transfer band at 324 nm and the lowest locally excited band at 231 nm, and the absorption spectra and resonance Raman intensities are modeled self-consistently to obtain the mode-specific reorganization energies accompanying electronic excitation to both states. Interference effects between the two states are considered but are found to be of minimal importance for this particular charge-transfer molecule. The reorganization parameters in terms of dimensionless normal coordinates are converted to actual bond length and bond angle changes by making use of a previously developed ground-state normal mode analysis and by comparing with electronic structure calculations on models for the donor and acceptor ends to reduce the indeterminacy in the signs of the dimensionless displacements. The geometry changes upon excitation to the LE state are dominated by lengthening of the ethylenic C(Double Bond)C bond, while for CT excitation the distortions are distributed over the donor, acceptor, and adamantane bridge, with a smaller C(Double Bond)C bond length change. © 1998 American Institute of Physics.

Notes: The kinetics of CO chemisorption on both the (1×5) and (1×1) surfaces of Ir{100}, including the CO-induced surface restructuring process, have been studied by measuring the sticking probability as a function of the surface temperature and beam flux. Due to competition between desorption from the (1×5) phase and growth of (1×1) islands, the sticking probability on the initial (1×5) surface is strongly flux-dependent at surface temperatures Ts in the range 480≤Ts≤510 K. It is shown that this is due to a strongly nonlinear dependence of the (1×1) growth rate on the local CO coverage on the (1×5) substrate, with an apparent reaction order of around 5. Desorption energies and pre-exponentials of desorption for CO from both the (1×1) and (1×5) surfaces have been determined by means of a modified lifetime measurement technique. Equilibrium coverages as well as isothermal desorption rates of CO were determined for both surface phases. The zero coverage desorption energy of CO from the (1×1) substrate is 196±5 kJ/mol and from the (1×5) surface it is around 150 kJ/mol. This difference in adsorption energies is the driving force for the CO-induced (1×5) to (1×1) phase transition. TEAS data show that the local CO coverage on the growing (1×1) islands during the phase transformation is 0.5 ML. © 1998 American Institute of Physics.

Notes: In the present paper, we extend the dynamic mean-field density functional method which describes microphase separation phenomena in polymer liquids, to account for viscoelastic effects. The effect of simple steady shear on polymer orientation and elongation is taken into account by adapting the polymer configurational distribution function. We propose a simplified model for polymer chains in a simple steady shear flow and show numerically that this model correctly reproduces expected conformational changes. The conformational effect is only of importance for high viscosity liquids and/or high shear rates. © 1998 American Institute of Physics.

Notes: A lamellar phase confined between parallel walls changes its structure when compared with the bulk system. The system is studied here in the Monte Carlo simulations of the Landau–Ginzburg model of a ternary mixture of oil, water, and surfactant. In the case of strongly hydrophilic boundary conditions at the walls, we observe strong topological fluctuations in the form of passages. As we change the distance between the walls we observe the formation of two surfactant layers, then the microemulsion between two layers, and finally four surfactant layers. The transition is marked by the peaks in the average Euler characteristic and in its variance. In the case of strongly hydrophilic boundary conditions at one wall and strongly hydrophobic boundary condition at the other, we observe under dilation a permanent deformation of layers in the middle of the system. In the case of weakly hydrophilic boundary conditions, the system exhibits strong topological fluctuations (passages and droplets) and the lamellar phase which forms is perpendicular to the bounding walls. In this case, edge dislocations form close to the walls. We also simulate an onion vesicle in a cubic pore and edge dislocations in slits, and show that the passages appear near a dislocation core. © 1998 American Institute of Physics.

Notes: The sedimentation equilibrium of adhesive spheres mimicking a system of interacting spherical colloidal particles in suspensions in planar pores is considered. The density profiles of the adhesive fluid in a gravitational field, and its distribution between the pores and the homogeneous phase are studied on the basis of the solution to the hypernetted chain/Ornstein–Zernike equation, obtained by using the analytic results for the direct correlation function of the bulk fluid. In a few cases, the Percus–Yevick closure is also used. In the hard sphere limit, both integral equation approaches are compared with the results of a grand canonical ensemble Monte Carlo simulation. This comparison shows, in particular in narrow pores, that the hypernetted chain approximation provides a better estimate for the structure of the hard sphere fluid in the pore, as well as for its partitioning between the bulk and the confined system. The calculated density profiles consist of an oscillatory part near the lower wall revealing layering, and a monotonically decreasing tail approaching the upper wall, their shapes being very sensitive to the strength of interparticle attraction, the strength of the gravitational field, and the degree of confinement. Increasing interparticle adhesive attraction together with gravity results in the particles occupying the region of lower altitudes in the gap and being partly squeezed out from the pore. © 1998 American Institute of Physics.

Notes: We have considered the effect of a nematic solvent on the configurations of liquid crystalline polymers (LCPs) modeled as semiflexible chains. The model for the chain is a generalization of the Kratky–Porod model satisfying the global inextensibility of the chain for all values of coupling with the nematic field. We have calculated analytically the components of the mean-square end-to-end distance, persistence length and radius of gyration parallel and perpendicular to the nematic director, and the asymptotic behaviors of the structure factor. We also discuss the effect of architecture on the configurations of the polymer. We conclude that the configurations of main-chain and side-on side-chain LCPs are more sensitive to the nematic solvent than those of end-on side-chain LCPs. © 1998 American Institute of Physics.

Notes: Small-angle x-ray scattering is used to study size distributions of noble metal nanoparticles embedded in polyelectrolyte hydrogels with oppositely charged surfactants. A procedure is proposed to subtract matrix scattering and to extract pure scattering due to the nanoparticles allowing to evaluate their size distribution functions by means of a regularization technique. Two kinds of collapsed gel-surfactant complexes were studied: a complex of a cationic gel of poly(diallyldimethylammonium chloride) with an anionic surfactant sodium dodecyl sulfate (PDADMACl/SDS), and that of an anionic gel of poly(methacrylic acid) with a cationic surfactant cetylpyridinium chloride (PMA/CPC). Addition of a gold compound (HAuCl4⋅3H2O) to the PDADMACl/SDS system forms the metal compound clusters and leads to a partial distortion of the gel structure. After subsequent reduction of the gold compound with sodium borohydride (NaBH4) ordering in the gel disappears and gold nanoparticles are formed. Their size distribution includes a fraction of small particles with approximately the same size as the compound clusters before reduction and a fraction of larger particles with the radii up to 40 nm. For the collapsed PDADMACl/SDS gels, aging does not change the size distribution profile; for the noncollapsed PDADMACl gels without surfactant, metal particles are found to grow with time. This suggests that the aggregation of metal colloids is prevented by the ordering in the collapsed gel-surfactant complex. The addition of HAuCl4⋅3H2O and the subsequent reduction of the metal ions in the PMA/CPC system does not distort the gel structure as the degree of incorporation of AuCl4− ions is very low. Particle sizes in the PMA/CPC system are found to be somewhat larger than those in the PDADMACl/SDS system. The PDADMACl/SDS gels loaded with the PtCl4 compound were also studied to analyze the influence of the reducing agent type on the particle size distribution distributions. Fast reduction with NaBH4 yielded mostly small particles with the radii around 2 nm grown from the compound clusters similar to those observed for the gold-loaded gels. In contrast, slow reduction with N2H4⋅H2O was found to produce larger nanoparticles and the size distribution function shows a major fraction of the particles with the radii up to 30 nm. © 1998 American Institute of Physics.

Notes: In this Communication we analyze the relaxation to equilibrium of a biomolecule and its sampling of the equilibrium ensemble, by using simple collective observables. We demonstrate that the customary use of the root mean square positional deviation parameter from the initial structure of a molecular dynamics trajectory, to determine the attainment of a stationary state in a simulation of biomolecules, necessarily leads to an overestimation of the relaxation time; this causes a loss of precious data that otherwise could be used in the calculation of equilibrium properties. A simple and reliable alternative is suggested, by computing the root mean square deviation from several different reference conformations along the trajectory. © 1998 American Institute of Physics.

Notes: Although constant-pressure molecular-dynamics simulations can be performed through the use of constraint methods, achieving a desired pressure can be difficult. A technique for setting the pressure is proposed in this paper. The pressure is fixed via an automatic, differential pressure controller which provides a first-derivative coupling to external pressure perturbations. This generates neither pressure overshoot during transient periods nor unrealistic fluctuations at equilibrium. The implementation of this algorithm is outlined and discussed. Results of simulations of a Lennard-Jones fluid are presented. © 1998 American Institute of Physics.

Notes: In our previous work [P. Itskowitz and M. L. Berkowitz, J. Phys. Chem. A 101, 5687 (1997)], we showed how in the framework of the density functional theory the energy of a molecule can be expressed as a functional of the perturbations on atomic densities. In the present work we discuss the forms of the atomic hardness kernels that enter the energy expression and apply our approach to the problem of finding the response of molecules to an applied electric field. We obtain a system of linear equations for the density perturbations on each atom in a molecule due to the applied electric field. The calculated values of polarization tensor components of several planar molecules are reported. © 1998 American Institute of Physics.

Notes: The nonequilibrium grand ensemble method previously reported is rigorously implemented for a nonequilibrium dilute gas mixture sheared in plane Couette flow geometry and analytic results are presented for the nonequilibrium thermodynamic quantities of the sheared gas. The calortropy is shown to contain all the constitutive information of the system. The notions of temperature and pressure for the nonequilibrium gas are examined on the basis of the calortropy calculated from the nonequilibrium grand partition function. The shear rate dependence of the nonlinear shear and first normal stress coefficients is calculated numerically and also by means of an iterative method. The first iterative solutions are found to give a qualitatively correct behavior for all Peclet numbers. © 1998 American Institute of Physics.

Notes: Pure liquid helium droplets of mean size 〈N〉=100–15 000 atoms ionized by electron impact show surprisingly similar ion fragment distributions. For all cluster sizes He2+ is the most probable cluster ion fragment, accounting for 30%–70% of the total ion yield. The high relative intensity of He2+ for the larger clusters shows that the droplets dissipate the ionization energy through an impulsive process, which ejects He2+ from the cluster, rather than by thermal evaporation. The other helium ion fragments that have been the focus of previous studies are most likely formed by a similar mechanism. © 1998 American Institute of Physics.

Notes: The geometrical structure of Ni39 is probed via molecular adsorption of nitrogen on its surface. Nitrogen uptake patterns are determined at various reaction temperatures, and the results are interpreted in terms of the number and nature of nitrogen binding sites on possible structures. It is found that an adsorbate-induced isomerization occurs at partial nitrogen coverage, but that at saturation a reverse isomerization returns the cluster to its initial structure. The two lowest energy structures calculated by Wetzel and DePristo [T. L. Wetzel and A. E. DePristo, J. Chem. Phys. 105, 572 (1996)] are completely consistent with the observed saturation and isomerization behavior. The structures consist of atom caps packed around a central pentagonal bipyramid, and reflect a marked change from the octahedral structure determined earlier for Ni38.

Notes: The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2−+(h-dash-bar)ω351.1 nm→HO2+e− EA[HO2,X˜ 2A″]=1.089±0.006 eV, (CH3)3COO−+(h-dash-bar)ω351.1 nm→(CH3)3COO+e− EA[(CH3)3COO,X˜ 2A″]=1.196±0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ΔE(X˜ 2A″–A˜ 2A′)[HO2]=0.871±0.007 eV and ΔE(X˜ 2A″–2A′)[(CH3)3COO]=0.967±0.011 eV. The gas phase acidity of (CH3)3COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be ΔacidG298=363.2±2.0 kcal mol−1 and we find ΔacidH298[(CH3)3COO–H]=370.9±2.0 kcal mol−1. Use of ΔacidH298[(CH3)3COO–H] and EA[(CH3)3COO] leads to the bond energies DH298[(CH3)3COO–H]=85±2 kcal mol−1 and D0[(CH3)3COO–H]=83±2 kcal mol−1. The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2]X˜ 2A″→[RO2]*A˜ 2A′→aldehydes/ketones+HO(2Π), [RO2]X˜ 2A″→[RO2]*A˜ 2A′ →alkenes+HO2(X˜ 2A″). © 1998 American Institute of Physics.

Notes: Time dependent density functional theory in its "extended linear" or "surrogate" form is used to investigate the dynamics of selective ion solvation in binary dipolar solvents. It is shown that simple analytical approximations that trap the basic physics of the solvation process can be obtained. In particular, it is found that the relaxation of the solvent number densities about a charged solute is governed by two distinct modes clearly associated with electrostriction and redistribution processes. This is consistent with the physical picture suggested by molecular dynamics (MD) simulations. The solvent polarization relaxation is also dominated by two modes associated with the two rotational diffusion constants of the binary solvent. In addition to the analytical approximations, full numerical solutions of the extended linear theory are obtained and the dependence of the relaxation on solvent density and solute charge is discussed. Detailed comparisons of the theory with MD simulations for a closely related model indicate that the theory is qualitatively correct, but quantitatively poor generally predicting relaxation rates which are too fast. This is due mainly to the neglect of inertial or non-Markovian effects in the theoretical approach. © 1998 American Institute of Physics.

Notes: Protein monolayers were built up by allowing protein molecules in solution to deposit at the solid/liquid interface. The structural evolution of these layers was followed using optical waveguide lightmode spectroscopy (OWLS). The measured effective refractive indices were interpreted using the uniform thin film model. For a relatively rigid, spheroidal protein (transferrin), the thickness of the layer was already established at very low surface coverages, and the refractive index increased during deposition. For a highly elongated and flexible protein (fibronectin), the converse occurred. The former case is consistent with classical random sequential adsorption. The latter result implies constant conformational rearrangement during deposition producing a compact proteinaceous membrane. © 1998 American Institute of Physics.

Notes: Within the Gaussian phantom-chain model of a polymer network, we demonstrate and apply the formal analogy between the problem of computing the force constants acting on a set of rigid filler particles and that of computing the capacity of a system of conductors in a dielectric medium. We find that a single spherical particle undergoes a mean-square displacement 〈(ΔX)2〉 from its equilibrium position which is inversely proportional to its radius R. It is thus subject to an isotropic harmonic potential with force constant [3kBT/〈(ΔX)2〉]∝R. Quantitative evaluation of the proportionality constant for typical unswollen networks shows that 〈(ΔX)2〉/R decreases from 10−1 to 10−5 as R increases from 10 nm to 1 μm. The time scale of these fluctuations is independent of R and falls in the range 10−4–101 s. The fluctuations of two neighboring particles are not additive. A distance-dependent "stiffening" of the network is demonstrated through the calculation of the appropriate response and force constant matrices. © 1998 American Institute of Physics.

Notes: The photochemical dynamics of aqueous chlorine dioxide (OClO) are investigated using time-resolved resonance Raman spectroscopy. Stokes and anti-Stokes spectra are measured as a function of time following photoexcitation of OClO using degenerate pump and probe wavelengths at 390 nm. The temporal evolution of OClO Stokes intensity is found to be consistent with the reformation of ground-state OClO by subpicosecond geminate recombination of the primary ClO and O photofragments. Anti-Stokes intensity is observed for transitions corresponding to the symmetric stretch of OClO demonstrating that upon geminate recombination, excess vibrational energy is deposited along this coordinate. Dissipation of this energy to the surrounding solvent occurs with a time constant of ∼9 ps. Finally, a delay in the appearance of OClO anti-Stokes intensity relative to geminate recombination is observed demonstrating that the excess vibrational energy available to OClO is initially deposited along the resonance Raman inactive asymmetric stretch coordinate with the exchange of energy between this coordinate and the symmetric stretch occurring with a time-constant of ∼5 ps. © 1998 American Institute of Physics.

Notes: We propose a stochastic method to reduce the autocorrelation time of a general Monte Carlo (MC) method and apply it to the variational quantum Monte Carlo (VMC) simulation of full-core atoms. We achieve a reduction in autocorrelation time of at least a factor of four compared with the standard method. Further, we find an approximate analytic fit to our results which gives a comparable reduction in autocorrelation time at essentially no cost. Our analytic form is independent of the geometry of the system being modeled and, therefore, can be easily applied to the VMC simulation of solids; it may also prove useful in any MC simulation where there are widely varying length scales. Results are presented for C, F, and Si. © 1998 American Institute of Physics.

Notes: Spectroscopy of buffer-gas cooled vanadium monoxide (VO) is performed in the presence of a magnetic trapping field and at low field. VO is created via laser ablation. A helium buffer gas, chilled by a dilution refrigerator, cools 1012 VO molecules to 1.8±0.2 K within 10 ms. The measured rotational temperature is 1.5±0.8 K. Spatially resolved Zeeman spectra allow the magnetic broadening terms of several optical transitions to be determined. The density of VO decays with a characteristic time of 60 ms, thus precluding the observation of trapping. © 1998 American Institute of Physics.

Notes: We discuss the vibrational and rotational state distributions of ground-state O2 following the photodissociation of O3 in the Chappuis band. They are obtained from time-dependent wave packet calculations employing ab initio potential energy surfaces for the 1 1A″ and 2 1A″ electronic states and the nonadiabatic elements, which couple these states. The satisfying agreement with experimental results underlines that the essential mechanisms of this two-state process are well described. © 1998 American Institute of Physics.

Notes: We have extended the time-dependent wave packet method to calculate cross sections and rate constants for rotationally excited initial states by using the centrifugal sudden (CS) approximation. A detailed study of the effects of rotational excitation of reagents on the title reaction on the WDSE PES has been carried out. It is found that (a) OH rotational excitation very mildly enhances the total cross section, (b) H2 rotational excitation quite substantially reduce the cross section, and (c) simultaneous OH and H2 rotational excitation has a largely uncorrelated effect. As a result, we found that the thermal rate constant can be obtained fairly accurately by only taking into account the effect of H2 rotation. A model calculation by changing the mass of an O atom reveals that the weak dependence of the cross section on OH rotation is not because the O atom is left relatively stationary by OH rotation. We speculate that it may be a general feature for the diatom-diatom reaction that the nonreactive diatom acts as a spectator not only vibrationally but also rotationally. It was also found that the "J-shifting" approximation works quite well for the reaction. On the other hand, the effect of K on the dynamics is found to be much stronger and more complicated than the J effect, making the "K-shifting" approximation not good for the reaction. © 1998 American Institute of Physics.