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  • 1
    Electronic Resource
    Electronic Resource
    Ab initio theoretical studies on photodissociation of HNCO upon S1(1A″)←S0(1A′) excitation: The role of internal conversion and intersystem crossing (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5004-5016 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field (CASSCF) method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory (CASPT2). The three product channels that participate in the photodissociation process are [1] HN(X 3Σ−)+CO at 86.0 (calculated 79.6) kcal/mol, [2] H+NCO(X 2Π) at 109.7 (108.7) kcal/mol, and [3] HN(a 1Δ)+CO at 122.2 (120.8) kcal/mol. The four electronic states, S0, S1, T1, and T2, that interconnect these channels were studied in detail. S1 exhibits dissociation barriers to both, channel [2] and [3], whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T1 and T2, show barriers of similar heights for HN bond fission, while S0 has no barriers to either channel. Various key isomerization transition states as well as numerous minima on the seam of surface crossings (MSX's) were also found. At photoexcitation energies near channel [3] threshold, products to channel [3] are likely to be formed via S1→[3] (if enough energy in excitation) and S1→S0→[3]. Channel [2] can be formed via S1→S0→[2]; (HN-mode quanta)+S1→T1→[2]; S1→T2→[2]; S1→T2→T1→[2], and channel [1] via S1→S0→T1→[1], S1→T1→[1] and S1→T2→T1→[1]. At higher photoexcitation energies the S1→[3] pathway is expected to be dominant while S1→[2], with the higher activation energy, is expected to drop rapidly. Also addressed are such important issues as the impact of a vibrationally excited HN mode on a channel [2] yield, and the band origin of the S1←S0 excitation spectrum. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479758
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical study on the mechanism of CH4+C2H2+ reaction: Mode-enhancement effect (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 56-62 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High level ab initio calculations have been performed to investigate the mechanism of the ion–molecule reaction of CH4+C2H2+. Except for some subtle differences, the profile for the H-abstraction channel obtained here at the G2M//B3PW91/6-311G(d,p) level is very similar to that found in a previous study at the G2//MP2/6-31G(d) level. For the complex formation channel, however, a different transition state has been located; the geometry and energetics of which are more consistent with experimental findings. Calculations of a few direct trajectories have been carried out to investigate the possible reason for the significant mode enhancement observed experimentally for the H-abstraction channel. Although none of them is reactive, a trajectory with an asymmetric C2H bend excitation exhibits a clear signature for being more reactive than those without vibrational excitation or with a symmetric bend excitation. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476539
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  • 3
    Electronic Resource
    Electronic Resource
    An ab initio investigation of spin-allowed and spin-forbidden pathways of the gas phase reactions of O(3P)+C2H5I (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1544-1551 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The singlet and triplet potential energy surfaces involved in the gas phase reactive collisions of O(3P) and C2H5I have been studied with ab initio electronic structure computations. The collisions produce both spin-forbidden HOI+C2H4 and spin-allowed OI+C2H5 products. The calculations indicate that HOI is formed via a triplet complex and through a triplet/singlet intersystem crossing, followed by passage through a singlet intermediate and transition state for the intramolecular abstraction of β-hydrogen. All the relevant structures for this pathway are lower in energy than the reactants, and this pathway is accessible even at low impact energies. The calculations also indicate that OI may be formed by two channels. One is the same to the above singlet pathway up to the singlet intermediate, which now dissociates endothermically without barrier to give the products. The second channel is the direct dissociation of the triplet intermediate, and is open only when an enough excess energy to surmount a triplet transition state is provided. The product energy distribution is also discussed based on the structures of transition states. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475525
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular properties from combined QM/MM methods. I. Analytical second derivative and vibrational calculations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1133-1149 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytical second derivatives for combined QM/MM calculations have been formulated and implemented in the CHARMM program interfaced with the ab initio quantum mechanical GAMESS and CADPAC programs. This makes possible evaluation of vibrational frequencies and infrared intensities in large systems that cannot be treated effectively by QM or MM alone; examples are polarizable molecules in solution and substrates or transition states in enzymes. Test calculations on a number of systems, including formamide in water, butanol, a model transition state structure for triosephosphate isomerase and the active site model of myoglobin, show that the MM description of the environment can capture much of its polarization effects on the QM region. Thus the implementation of analytical second derivatives within the QM/MM framework has considerable potential for the study of large systems. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480658
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  • 5
    Electronic Resource
    Electronic Resource
    The spin-forbidden reaction CH(2Π)+N2→HCN+N(4S) revisited. II. Nonadiabatic transition state theory and application (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 9469-9482 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transition state theory is extended straightforwardly to treat nonadiabatic processes and applied to study the rate constant of the spin-forbidden reaction CH(2Π)+N2→HCN+N(4S). A one-dimensional model was set up to calculate the intersystem crossing probability with the distorted wave approximation and using an ab initio value of the spin–orbit coupling. The effect of orthogonal degrees of freedom was then considered by energy convolution with the vibrational frequencies, obtained from ab initio calculations, orthogonal to the crossing seam at the minimum of the seam of crossing (MSX), also obtained from ab initio calculations. An expression for the cumulative reaction probability, N(E), of the reaction was obtained by a straightforward extension of the unified statistical theory, where the MSX was treated as a transition state. The calculated thermal rate constant, k(T), seems to be too low by two orders of magnitude compared to experimental measurements and an empirical transition state study where empirical vibrational frequencies at the MSX are lower by a factor of 2 than those derived here. The disagreement strongly suggests that the current treatment of the multidimensional dynamics needs to be improved. In particular, it may be a poor assumption that spin-forbidden transition takes place with uniform probability on the seam in the case we are considering. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478949
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  • 6
    Electronic Resource
    Electronic Resource
    An ab initio study of the dissociation of HNCO in the S1 electronic state (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1452-1458 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Regions of the S1 potential energy surface of HNCO relevant to N–H and C–N bond photodissociation have been investigated with ab initio calculations. Geometries of minima and transition states on S1 as well as those of the product photofragments and the HNCO ground state have been optimized with the CASSCF method, and their energies calculated with MRSDCI and CASPT2 methods. Deep planar trans and cis minima exist on the S1 surface, and are connected by transition states for isomerization. The S0→S1 electronic transition is brighter for trans configurations than for cis, and the initial excitation and dynamics are most likely to proceed through trans configurations. The N–H fission on S1 has a substantial barrier; it occurs more easily through the planar cis transition state, which is about 20 kcal/mol above the dissociation threshold, than through the trans transition state. The C–N fission on S1 can take place both via the planar trans and the planar cis transition state with a low barrier over the dissociation threshold; the reverse barrier is estimated to be a few kcal/mol. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475517
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular orbital study of H2 and CH4 activation on small metal clusters. I. Pt, Pd, Pt2, and Pd2 (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8418-8428 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic structure of Pd/Pt dimer and the detailed reaction mechanism of H2 and CH4 activation on these clusters have been studied with density functional (B3LYP) and complete active space second-order perturbation (CASPT2) theories. It was found that B3LYP calculations gave reliable results on the electronic structures of the Pd/Pt dimers, in comparison with our CASPT2 calculations and data from previous theoretical investigations. Full geometry optimization has been carried out in the current study in contrast to previous work where only limited potential energy scans have been carried out, which led to dramatically different reaction mechanisms. In the case of Pt2+H2/CH4, H–H/C–H activation preferentially takes place at first on one metal atom via structures far from planar, then one of the H atoms migrates to the other Pt atom with negligible barrier. On both the singlet and the triplet state, H–H activation is barrierless, while C–H activation has a distinct barrier on the singlet state for reaction starting from the ground triplet state Pt2. In contrast, Pd2 is found to activate the H–H bond without barrier on the singlet state, while the triplet states are very high in energy. In the CH4 activation, two paths, referred as symmetric and asymmetric paths, respectively, have been found. The characters of the metal dimers and the differences between Pd2 and Pt2 systems have been analyzed based on MO diagrams. Results from the current study are consistent with the recent experimental observations of Cox et al. on the reactivities of unsupported Pdn and Ptn. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476269
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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio molecular orbital studies on the structure, energies, and photodissociation of the electronic excited states of C2H (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 626-636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High level ab initio calculations have been carried out to study seven electronic states of C2H. The calculated equilibrium structure, energetics and vibrational frequencies for the 3 2A′ state at the CASPT2/PVTZ level are in good agreement with those obtained experimentally by Hsu et al. The transition dipole moments from the ground state C2H to electronic excited states depend sensitively on the H–C–C bending angle and often peak at nonlinear configurations. Based on this and the dissociation behavior of the excited states, we predict A 1Πu and c 3Σu+ C2 fragments to be rich in population, the former of which is experimentally detected recently by Jackson et al. The ground X 1Σg+ and the a 3Πu state C2 are expected to be formed via the nonadiabatic process 3 2A′→2 2A′ or 4 2A′→3 2A′→2 2A′, which is in accord with the experimentally observed lifetime pattern by Hsu et al. No reverse barrier for CC–H dissociation was found on the X and A electronic states of C2H in the linear configuration. The 2 2A′ state, however, develops a distinct "barrier" (not a true saddle point) along dissociation coordinate when the H–C–C is significantly bent, due to the interaction with upper electronic state. Since the 2 2A′ state energetically prefers a linear dissociation, we suspect that the upper bound of the CC–H bond energy measured by Hsu et al. is not severally affected by this "barrier." © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475424
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  • 9
    Electronic Resource
    Electronic Resource
    Ab initio studies on the electronic excited states and photodissociation of O3 anion (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 7684-7694 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Extensive ab initio calculations have been performed for the low-lying electronic states of O3− to elucidate the mechanism of photodissociation processes. The identity of the mysterious state implied by the recent experiment of Continetti et al. has been discussed based on the current calculations. Calculations reveal that 2B2 is a truly bound state favoring a strongly bent geometry with (angle)O–O–O∼90.0°, and crosses with the X 2B1 at a similar angle. Therefore, if O3− is produced in a highly bent geometry, 2B2 might be preferentially populated. The large transition dipole moment and the vertical excitation energy for 2B2→2A1 also suggest that 2B2 may be electronically excited efficiently to 2A1 at the wavelength of 523 nm. The computed energetics of 2B2 and 2B1 and the corresponding dissociation limits may explain the larger maximum kinetic energy release (KER) observed in the second experiment of Continetti and the smaller O2–O− bond energy derived from the experiment of Hiller, if we assume that 2B2 is the parent state in both cases. Furthermore, meta-IRC (intrinsic reaction coordinate) calculations suggest rather different final state distribution of the photofragments from 2B1→2A2 and 2B2→2A1 processes, in qualitative agreement with the experimental observations. Although the vibrationally excited ground state O3− might also produce rotational hot, vibrational cold photofragments through the angular dependence of the seam between the two diabatic excited 2A″ states, the exact effect of parent vibrational excitation requires future dynamics calculations. At the current stage, our calculations strongly support that the 2B2 electronic state has been accessed in the second experiment of Continetti et al. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476322
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  • 10
    Electronic Resource
    Electronic Resource
    Ab initio study on the mechanism of C2H2++NH3 reaction: Efficient charge transfer and proton transfer processes competing with stable complex formation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4021-4030 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High level ab initio calculations have been performed to investigate the mechanism of the ion–molecule reaction NH3+C2H2+. Three channels, covalent complex formation (CC), proton transfer (PT), and charge transfer (CT) have been studied. Among the two pathways found for the PT channel, one leads the reactants NH3+C2H2+ to NH4++C2H(2Π) through a moderately bound complex without any barrier, and the other leads NH3++C2H2 to the H-atom transferred products NH4++C2H(2Σ+) with a modest barrier. These findings support the fast "stripping" mechanism proposed by Anderson et al. As to the CC channel, several isomers of C2H5N+ and the isomerization transition states have been located. No significant barrier relative to the reactants has been found on either the ground or the 2A″ excited state. To rationalize the experimental fact that no CC channel products have been observed, it is argued that the reactants NH3+C2H2+ correlate adiabatically to excited states of covalent C2H5N+ species, whose formation requires significant alternation of the C2H2+ geometry and electronic structure. Therefore, the system is most likely to follow the PT or the CT channel instead of visiting the CC channel. For the CT channel, limited potential energy surface scans of the three electronic states (1,2 2A′+2A″) indicate that CT at different approach angles or between electronic states of different symmetries (A′→A′,A″→A′) may produce final products of different characteristics, and might account for the two pathways proposed by Anderson et al. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476230
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