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  • 11
    Digitale Medien
    Digitale Medien
    Radial moments of the electron density: Gas phase results and the effects of solvation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1113-1121 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The radial moments of the electron density, 〈rn〉, have been calculated for N2 and H2O at the quadratic configuration interaction (with singles and doubles) levels of theory by use of a variety of basis sets. An optimal basis set was chosen from the first set of calculations and then used with several conventional ab initio and density functional methods to compute the moments for a representative set of molecules: N2, CO, CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, CH3OH, and CH3SH. The effects of solvation on the radial moments were also studied using the Onsager model and an extension of the Tomasi polarized continuum model. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480666
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  • 12
    Digitale Medien
    Digitale Medien
    A self-consistent field study of the wetting transition in binary polymer blends (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1257-1263 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A self-consistent field approach is used to investigate the partial to complete wetting transition for an A:B polymer blend at coexistence where polymers A and B have equal numbers of segments, N. The surface free energy, Fs, is modeled using the quadratic form suggested by Schmidt and Binder [J. Phys. II (France) 46, 1631 (1985)], namely, Fs=−μφ1−0.5gφ12, where μ and g are the surface equivalents of the bulk chemical potential and interaction energy, respectively, and φ1 is the surface volume fraction of the surface preferred component (A). For selected values of g and the bulk volume fraction of A, φ∞, the volume fraction profile and A surface excess, z*, are calculated as a function of increasing μ. The first and second order wetting transitions are indicated by a discontinuity and divergence, respectively, of z* and φ1. In our simulations, at high values of φ∞ only first order transitions are detected for both N=100 and N=1000. However, both first and second order wetting transitions are observed for low values of φ∞ depending on the value of g. The latter results contrast with those of Carmesin and Noolandi [Macromolecules 22, 1689 (1989)], who found that only first order wetting transitions are possible polymer mixtures. However, our results are in agreement with recent Monte Carlo simulations carried out by Wang and Binder [J. Chem. Phys. 94, 8537 (1991)] and Pereira and Wang [J. Chem. Phys. 104, 5294 (1996)]. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473222
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  • 13
    Digitale Medien
    Digitale Medien
    A combined quantum mechanics and molecular dynamics study of small Jahn–Teller distorted hydrocarbons: Another difficult test for density-functional theory (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 12059-12069 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Temperature, vibrational, and matrix effects on the geometry and hyperfine coupling constants of the methane and ethane radical cations are investigated with a combined quantum mechanics and molecular dynamics technique. Density-functional theory (the B3LYP functional) is implemented as the quantum mechanical method. Results obtained for the methane cation are discouraging. The hyperfine coupling constants (HFCCs) obtained from the simulations are in poor agreement with experimental results. These deficiencies are ascribed to the inadequacy of density-functional theory to describe the potential energy surface in this radical. Results obtained for the ethane radical cation with the identical method are more promising. The HFCCs obtained from the simulations are in better agreement with experimental results obtained at 4 K than those obtained from static, gas-phase calculations, indicating vibrational effects are important for this radical even at low temperatures. Temperature effects on the HFCCs in the ethane radical cation observed experimentally are also well reproduced by the simulations.© 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.479141
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  • 14
    Digitale Medien
    Digitale Medien
    Density functional theory investigation of hyperfine coupling constants in peroxyl radicals (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 7738-7748 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The geometries and 17O hyperfine coupling constants in several peroxyl radicals have been determined through the use of density functional theory. Becke's three-parameter hybrid exchange functional (B3) together with the correlation functional of Lee, Yang, and Parr (LYP) in combination with a variety of basis sets was used to study basis set effects. Subsequently, the effects of different gradient-correlated functionals were also examined. Results comparable to experimental values are obtained for all of the alkyl peroxyl radicals at the B3LYP level with IGLO-III or s-shell decontracted IGLO-III, 6-311G(d,p), 6-311+G(2df,p), and the augmented correlation-consistent polarized-valence triple-zeta basis set of D. E. Woon and T. H. Dunning [J. Chem. Phys. 98, 1358 (1993)], R. E. Kendall, T. H. Dunning, and R. J. Harrison [J. Chem. Phys. 96, 6796 (1992)], and T. H. Dunning [J. Chem. Phys. 90, 1007 (1989)]. Calculations imply that the spin density ratio between the inner and outer oxygens is 0.3:0.7, supporting earlier theoretical work [S. L. Boyd, R. J. Boyd, and L. R. C. Barclay, J. Am. Chem. Soc. 112, 5724 (1990)]. Erratic and strongly fluctuating results are exhibited for the fluoroperoxyl radical. Geometries close to the experimental values can be obtained at the B3LYP level, but at the expense of considerable spin contamination. A high degree of spin contamination can also be observed in calculations of the hyperfine coupling constants for this molecule. Possible explanations for the apparent failure to obtain converged results for FOO, apart from the considerable spin contamination, include vibrational, multireference, and matrix effects. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.473774
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  • 15
    Digitale Medien
    Digitale Medien
    Structure sensitivity and cluster size convergence for formate adsorption on copper surfaces: A DFT cluster model study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9562-9568 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core–valence correlation effect on the adsorbate–surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)〉Cu(100)〉Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481573
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  • 16
    Digitale Medien
    Digitale Medien
    A spin-density polarization index (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2824-2830 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: By analogy with the Carbo overlaplike similarity measure, a spin-density polarization index (SPI) is defined. This index gives the Euclidean distance between the α and β electron densities for an open-shell molecule. An extensive study of the O2 molecule reveals that the SPI is highly dependent on the theoretical method, but is less sensitive to the basis set provided flexible extended basis sets are used. The radial distribution of the O2 spherically averaged electron density also reveals that near the center of the bond there is an inversion of the α and β electron densities and that there is a minimum in the β electron density near the nuclei within the internuclear region. Finally, the SPI is calculated for a variety of small molecules and the results are compared with those obtained for O2. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475673
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  • 17
    Digitale Medien
    Digitale Medien
    The convergence of basis set contractions: A case study of the molecular hyperfine structure of NH〈sup LOCATION="PRE"〉142 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6270-6274 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti's general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is found that the correlation due to core electrons is recovered well with the simple choice of the most populated ANOs. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475133
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  • 18
    Digitale Medien
    Digitale Medien
    A hybrid quantum mechanical force field molecular dynamics simulation of liquid methanol: Vibrational frequency shifts as a probe of the quantum mechanical/molecular mechanical coupling (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 7261-7269 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies of the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree–Fock/3-21G(d,p) level. Good overall agreement is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanical (QM/MM) coupling schemes are investigated using gas-to-liquid vibrational frequency shifts as a probe. The results suggest, somewhat surprisingly, that the method with the weakest QM/MM coupling gives the best overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme overestimates the shifts towards the red direction due to overestimation of the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in describing the O–H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.471439
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  • 19
    Digitale Medien
    Digitale Medien
    Hydrogen on polycrystalline diamond films: Studies of isothermal desorption and atomic deuterium abstraction (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 992-1002 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Studies of hydrogen isothermal desorption and abstraction from polycrystalline diamond surfaces are presented. The surface H and D coverages (θH and θD) are measured in real time by mass analyzing the recoiled ions generated in a time-of-flight scattering and recoil spectroscopy (TOF-SARS) experiment. For surface temperatures (Ts) from 825 and 920 °C, isothermal H2 desorption is 1st order in θH with a measured activation energy, ET, of 69±6 kcal/mol and a pre-exponential factor, ν, of 1010.5±0.9 s−1. For H2 desorption from diamond, the estimated ΔET based on bond energy calculations is ≈88 kcal/mol, substantially higher than the experimentally measured ET. This difference suggests π-bonding of the surface after H2 desorption is involved. Using a simple bond order argument, the π-bonding contribution is estimated to be ≈21 kcal/mol. The abstraction and replacement of absorbed H by atomic deuterium (Dat) is explained by three first-order reactions. Under a constant Dat flux, the rate of abstraction of adsorbed H by Dat is 1st order in θH, with an "apparent activation energy'' (Ea) of 0.8±0.2 kcal/mol and ν=(1.3±0.2)10−3 s−1. The low Ea and 1st order kinetics imply that H is abstracted from the surface by a generalized Eley–Rideal or direct mechanism. Using the relative rate of Dat abstraction of H to Dat adsorption on clean diamond, we estimate an upper limit for the abstraction activation barrier of 16 kcal/mol. Under identical reaction conditions, the rate for Hat abstraction of D is ≈1/3 the rate for Dat abstraction of H. We interpret this isotope effect using momentum transfer arguments. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.469167
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  • 20
    Digitale Medien
    Digitale Medien
    Mean-field theory of the interface between a homopolymer and a binary-polymer mixture (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 10134-10144 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In this paper we investigate the properties of the interface between a homopolymer (A) and a binary polymer mixture (B:C). We have extended the self-consistent field (SCF) model of Helfand [Macromolecules 25, 1676 (1992)] by including the effects of the numbers of segments of polymers on the polymer volume fractions, the interfacial excess of B, zB*, the interfacial tension, γABC, the interfacial width, wABC, and the A–B interfacial overlap, wAB. Other parameters include the polymer interaction parameters, χAB, χAC, and χBC, and the B volume fraction in the B:C mixture, φB∞. As expected, the B component segregates to the A/B:C interface when A–B interactions are more favorable than the A–C ones (χAB〈χAC). This interfacial adsorption lowers the interfacial tension and width compared to the pure A/C case, demonstrating the compatibilizing effect of B. As χBC varies from favorable to unfavorable, B segregation is found to increase. Similarly, at constant interaction parameters, B segregation increases as the number of the B segments increases. Furthermore, the addition of a small amount of high molecular weight B to an immiscible A/C blend is found to reduce greatly γABC and thus stabilize the system. Calculations for the case of an athermal mixture B:C with χAB=χAC reveals that the shorter chains are "entropically driven'' to the A/B:C interface. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.472936
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