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22 Jul 2001

Volume 115, Issue 4, pp. 1619-1979

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Triple excitation effects in coupled-cluster calculations of indirect spin–spin coupling constants

Alexander A. Auer and Jürgen Gauss

J. Chem. Phys. 115, 1619 (2001); http://dx.doi.org/10.1063/1.1386698 (4 pages) | Cited 28 times

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The effect of triple excitations in coupled-cluster calculations of indirect spin-spin coupling constants is investigated in coupled-cluster singles and doubles (CCSD) calculations augmented by a perturbative treatment of triples [CCSD(T)], in calculations based on the CC3 model as well as in coupled-cluster singles, doubles, and triples (CCSDT) calculations. Though triple excitation effects are in most cases not particularly pronounced, it is demonstrated that among the approximate schemes for handling triples only the CC3 model with no orbital relaxation included (unrelaxed CC3) provides an adequate description. The otherwise successful CCSD(T) aproach appears to either significantly overestimate triple excitation effects or to yield corrections with the wrong sign in comparison to CCSDT. © 2001 American Institute of Physics.
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33.25.+k Nuclear resonance and relaxation
31.15.bw Coupled-cluster theory

Influence of the environment on the excited state deactivation in functionalized quinque-thienyls

G. Lanzani, G. Cerullo, S. De Silvestri, G. Barbarella, and G. Sotgiu

J. Chem. Phys. 115, 1623 (2001); http://dx.doi.org/10.1063/1.1386936 (3 pages) | Cited 6 times

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The transient photoinduced transmission of quinquethiophene-S,S-dioxide in solution is reported. Stimulated emission is observed in the viscous solvent, but quenched in polar and nonpolar low viscosity solvents. The lifetime of the photoexcited state changes of more than one order of magnitude depending on the solvent properties, pointing out conformational effects. Low-energy coherent transient is observed and assigned to real time conformational dynamics. © 2001 American Institute of Physics.
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33.80.Be Level crossing and optical pumping
33.50.Hv Radiationless transitions, quenching
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.-b Photon interactions with molecules
33.15.Bh General molecular conformation and symmetry; stereochemistry
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back to top Theoretical Methods and Algorithms

Impact of electron–electron cusp on configuration interaction energies

David Prendergast, M. Nolan, Claudia Filippi, Stephen Fahy, and J. C. Greer

J. Chem. Phys. 115, 1626 (2001); http://dx.doi.org/10.1063/1.1383585 (9 pages) | Cited 13 times

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The effect of the electron–electron cusp on the convergence of configuration interaction (CI) wave functions is examined. By analogy with the pseudopotential approach for electron–ion interactions, an effective electron–electron interaction is developed which closely reproduces the scattering of the Coulomb interaction but is smooth and finite at zero electron–electron separation. The exact many-electron wave function for this smooth effective interaction has no cusp at zero electron–electron separation. We perform CI and quantum Monte Carlo calculations for He and Be atoms, both with the Coulomb electron–electron interaction and with the smooth effective electron–electron interaction. We find that convergence of the CI expansion of the wave function for the smooth electron–electron interaction is not significantly improved compared with that for the divergent Coulomb interaction for energy differences on the order of 1 mHartree. This shows that, contrary to popular belief, description of the electron–electron cusp is not a limiting factor, to within chemical accuracy, for CI calculations. © 2001 American Institute of Physics.
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31.15.V- Electron correlation calculations for atoms, ions and molecules

Can optimized effective potentials be determined uniquely?

So Hirata, Stanislav Ivanov, Ireneusz Grabowski, Rodney J. Bartlett, Kieron Burke, and James D. Talman

J. Chem. Phys. 115, 1635 (2001); http://dx.doi.org/10.1063/1.1381013 (15 pages) | Cited 93 times

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Local (multiplicative) effective exchange potentials obtained from the linear-combination- of-atomic-orbital (LCAO) optimized effective potential (OEP) method are frequently unrealistic in that they tend to exhibit wrong asymptotic behavior (although formally they should have the correct asymptotic behavior) and also assume unphysical rapid oscillations around the nuclei. We give an algebraic proof that, with an infinity of orbitals, the kernel of the OEP integral equation has one and only one singularity associated with a constant and hence the OEP method determines a local exchange potential uniquely, provided that we impose some appropriate boundary condition upon the exchange potential. When the number of orbitals is finite, however, the OEP integral equation is ill-posed in that it has an infinite number of solutions. We circumvent this problem by projecting the equation and the exchange potential upon the function space accessible by the kernel and thereby making the exchange potential unique. The observed numerical problems are, therefore, primarily due to the slow convergence of the projected exchange potential with respect to the size of the expansion basis set for orbitals. Nonetheless, by making a judicious choice of the basis sets, we obtain accurate exchange potentials for atoms and molecules from an LCAO OEP procedure, which are significant improvements over local or gradient-corrected exchange functionals or the Slater potential. The Krieger–Li–Iafrate scheme offers better approximations to the OEP method. © 2001 American Institute of Physics.
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31.15.E- Density-functional theory

A new separable potential operator for representing a chemical bond and other applications

I. V. Abarenkov and I. I. Tupitsyn

J. Chem. Phys. 115, 1650 (2001); http://dx.doi.org/10.1063/1.1380712 (11 pages) | Cited 14 times

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A new type of potential operator has several kinds of applications in electronic structure calculations. Three uses are envisaged here. First when some special region of a covalently bonded solid or very large molecule is modeled by a modest sized cluster, each dangling bond at the cluster surface can be saturated in a way that exactly reproduces the bond in the complete system. Second a similar approach can be used at the matching surface in an embedding scheme for calculations on the same type of systems. The third application is to atomic pseudopotentials where the new potential operator avoids the possibility of “ghost” states that sometimes plague the widely used Kleinman–Bylander form of the pseudopotential. The theory of the new separable potential and its application to the dangling bond problem are the main subjects of the present paper. Starting from a given potential or pseudopotential, the new separable operator modifies some of the required eigenfunctions and eigenvalues in a controlled way while conserving all other eigenvalues. The method has been tested on the molecules X–SiH3 where X=H, F, Cl, Br, or I. © 2001 American Institute of Physics.
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61.50.Lt Crystal binding; cohesive energy
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
71.55.-i Impurity and defect levels

Ab initio simulation of charged slabs at constant chemical potential

A. Y. Lozovoi, A. Alavi, J. Kohanoff, and R. M. Lynden-Bell

J. Chem. Phys. 115, 1661 (2001); http://dx.doi.org/10.1063/1.1379327 (9 pages) | Cited 34 times

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We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential μ, rather than at constant number of electrons Ne. To this end, we define the chemical potential relative to a plane (or “reference electrode”) at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The “constant μ” mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry. © 2001 American Institute of Physics.
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82.45.Fk Electrodes

Moving adaptive grid methods for numerical solution of the time-dependent molecular Schrödinger equation in laser fields

HuiZhong Lu and André D. Bandrauk

J. Chem. Phys. 115, 1670 (2001); http://dx.doi.org/10.1063/1.1383033 (8 pages) | Cited 11 times

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We present a moving adaptive grid method for solving the time-dependent Schrödinger equation, TDSE, for molecules in intense laser fields, applicable in the nonperturbative nonlinear regime where dissociation ionization occurs. The method is based on a Lagrangian, moving coordinate system. In this representation, the reference system is moving with the laser pulse so that the classical movement of free particles in the field, i.e., in the asymptotic region where electron–molecule potentials are negligible but the laser field is still present, is exactly described. As a consequence, the asymptotic quantum wave functions are exact in presence of a laser pulse. We have tested several discrete propagator methods for the TDSE in different gauges in a Born–Oppenheimer simulation of H2+ in a short, intense laser pulse. Our comparison of convergence between the same discretization methods for different gauges have demonstrated the superiority of the present Lagrangian adaptive grid method to treat the response of molecules to intense time-dependent electromagnetic fields. © 2001 American Institute of Physics.
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42.50.-p Quantum optics
32.80.Fb Photoionization of atoms and ions
33.80.Eh Autoionization, photoionization, and photodetachment

Non-Hamiltonian molecular dynamics: Generalizing Hamiltonian phase space principles to non-Hamiltonian systems

Mark E. Tuckerman, Yi Liu, Giovanni Ciccotti, and Glenn J. Martyna

J. Chem. Phys. 115, 1678 (2001); http://dx.doi.org/10.1063/1.1378321 (25 pages) | Cited 110 times

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The use of non-Hamiltonian dynamical systems to perform molecular dynamics simulation studies is becoming standard. However, the lack of a sound statistical mechanical foundation for non-Hamiltonian systems has caused numerous misconceptions about the phase space distribution functions generated by these systems to appear in the literature. Recently, a rigorous classical statistical mechanical theory of non-Hamiltonian systems has been derived, [M. E. Tuckerman, et al., Europhys. Lett. 45, 149 (1999)]. In this paper, the new theoretical formulation is employed to develop the non-Hamiltonian generalization of the usual Hamiltonian based statistical mechanical phase space principles. In particular, it is shown how the invariant phase space measure and the complete sets of conservation laws of the dynamical system can be combined with the generalized Liouville equation for non-Hamiltonian systems to produce a well defined expression for the phase space distribution function. The generalization provides a systematic, controlled procedure for designing non-Hamiltonian molecular dynamics algorithms which can be used to generate nonmicrocanonical ensembles, stationary nonequilibrium flows, and/or the dynamics of constrained systems. In light of this new general analysis, molecular dynamics algorithms for the canonical and isothermal–isobaric ensembles are examined, potential difficulties are illuminated, and the limitations of previous theoretical treatments are elucidated. © 2001 American Institute of Physics.
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71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
02.30.Rz Integral equations
02.70.Ns Molecular dynamics and particle methods
02.60.Nm Integral and integrodifferential equations

Ab initio Hartree–Fock study of electron transfer in organic molecules

Ranjit Pati and Shashi P. Karna

J. Chem. Phys. 115, 1703 (2001); http://dx.doi.org/10.1063/1.1381409 (13 pages) | Cited 12 times

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Electron transfer (ET) in σ-bonded organic cage structures (bicyclo[1.1.1]pentane, cubane, and bicyclo[2.2.2]octane) has been studied with the help of ab initio Hartree–Fock calculations in the framework of a two-state model. The calculated values of the ET coupling matrix element VAB exhibit strong dependence on the basis set employed. A minimal basis set underestimates the value of VAB with respect to an extended (double-zeta and polarization) basis set. The ET shows correlation with the electronic and geometrical structure of the molecules studied. It is found that the more strained the chemical bonds in the cage structure are, the stronger is the coupling between the two states participating in ET. Furthermore, the ET matrix element VAB is calculated to have its maximum value when the two end groups attached to the cage structures are coplanar, and its minimum value when two end π groups are perpendicular to each other. However, for coplanar end-groups, minimal changes are noted in the value of VAB with respect to the rotation of the σ-bonded cage. The dependence of ET on the relative orientation of the planes of the end groups offers a mechanism for designing molecular switches. © 2001 American Institute of Physics.
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31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
34.70.+e Charge transfer

Approximate accelerated stochastic simulation of chemically reacting systems

Daniel T. Gillespie

J. Chem. Phys. 115, 1716 (2001); http://dx.doi.org/10.1063/1.1378322 (18 pages) | Cited 257 times

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The stochastic simulation algorithm (SSA) is an essentially exact procedure for numerically simulating the time evolution of a well-stirred chemically reacting system. Despite recent major improvements in the efficiency of the SSA, its drawback remains the great amount of computer time that is often required to simulate a desired amount of system time. Presented here is the “τ-leap” method, an approximate procedure that in some circumstances can produce significant gains in simulation speed with acceptable losses in accuracy. Some primitive strategies for control parameter selection and error mitigation for the τ-leap method are described, and simulation results for two simple model systems are exhibited. With further refinement, the τ-leap method should provide a viable way of segueing from the exact SSA to the approximate chemical Langevin equation, and thence to the conventional deterministic reaction rate equation, as the system size becomes larger.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.20.Wt Computational modeling; simulation
02.50.Ey Stochastic processes
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

193 nm photolysis of vinyl bromide: Nascent product distribution of the C2H3Br→C2H2 (vinylidene)+HBr channel

Dean-Kuo Liu, Laura T. Letendre, and Hai-Lung Dai

J. Chem. Phys. 115, 1734 (2001); http://dx.doi.org/10.1063/1.1382812 (8 pages) | Cited 15 times

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The internal energy content of the photofragments HBr and C2H2 from the minor channel of the photolysis of C2H3Br with 193 nm radiation has been measured using time-resolved infrared Fourier transform IR emission spectroscopy with 0.5 μs resolution. Vibrational level population and the rotational population of the HBr fragment are determined from 1.0 μs following the photolysis until complete HBr relaxation. The nascent distribution of HBr is extrapolated, from a collision quenching model with a Boltzmann distribution, to be at 8690 and 7000 K for the vibration and rotation respectively. The product vibrational energy distribution supports a reaction mechanism based on the 3-centered HBr elimination process yielding vinylidene and HBr. The nascent internal energy of vinylidene is deduced to be 24 kcal/mol. Vinylidene isomerizes to acetylene and the acetylene emission bands, ν3, ν4+ν5 and ν5, are detected. © 2001 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
82.37.Vb Single molecule photochemistry
82.20.Hf Product distribution
33.20.Ea Infrared spectra

The gaseous reaction of vinyl radical with oxygen

Hui Wang, Baoshan Wang, Yong He, and Fanao Kong

J. Chem. Phys. 115, 1742 (2001); http://dx.doi.org/10.1063/1.1382814 (5 pages) | Cited 8 times

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The gaseous reaction of vinyl radical with oxygen has been experimentally investigated. C2H3 radical was produced by laser photolysis of C2H3Br at 248 nm. The vibrationally excited products of the reaction were detected by time-resolved Fourier transform infrared emission spectroscopy. H2CO(ν1), HCO(ν1,ν3), and CO2(ν3) are ascertained as the main emitters. The most favorable product channel is HCO and H2CO. The reaction channel leading to CO2+CH3 has been found for the first time. The minor reactions leading to C2H2+HO2, C2H3O+O, and C2H2O2+H may also occur. A secondary reaction product of CO is observed, which is generated from the primary reaction product HCO. Combining theoretical analysis with the present experimental results, the reaction pathways are clarified. The results are of importance for understanding the combustion processes of hydrocarbon. © 2001 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.20.Ea Infrared spectra
82.20.-w Chemical kinetics and dynamics

Density functional study of the first-row transition-metal complexes M–CH2, M–CHF, and M–CF2

Ilza Dalmázio and Hélio Anderson Duarte

J. Chem. Phys. 115, 1747 (2001); http://dx.doi.org/10.1063/1.1383289 (10 pages) | Cited 6 times

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Carbenes and fluorocarbenes are important intermediates in the hydrodechlorination of CFCs catalyzed by metal surfaces. However, the reaction mechanism at a molecular level is not completely understood. In this work, density functional calculations have been performed for the first-row transition-metal complexes M–CH2, M–CHF, and M–CF2 aiming to conbribute to the understanding of the metal/carbene interaction mechanism. Relative energies, geometries, and frequencies of the M–CXY complexes in different electronic states are reported. The binding mechanism is described through an analysis of the molecular orbitals. The binding energy of the M–CF2 is about 30% smaller than the respective M–CH2 binding energy. The electronic configuration of all complexes studied is presented in a diagram that allows one to predict qualitatively properties such as geometries, multiplicities, charge transfers, and relative bond lengths. © 2001 American Institute of Physics.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Vibrations of nitrous oxide: Matrix isolation Fourier transform infrared spectroscopy of twelve N2O isotopomers

Andrzej Łapiński, Jens Spanget-Larsen, Jacek Waluk, and J. George Radziszewski

J. Chem. Phys. 115, 1757 (2001); http://dx.doi.org/10.1063/1.1383031 (8 pages) | Cited 7 times

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Isotopically labeled nitrous oxide has been produced in solid nitrogen matrices using mixtures of nitrogen and water containing 14N, 15N, 16O, 17O, and 18O. All twelve possible N2O isotopomers have been obtained, and their fundamental, overtone and combination frequencies were assigned by the joint use of infrared spectroscopy and quantum chemical calculations (B3LYP/AUG-cc-pVTZ). Specific influence of the nitrogen matrix upon frequency and anharmonicity of the vibrations has been discussed. © 2001 American Institute of Physics.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

Collisional decomposition of the sulfur hexaflouride anion (SF6−)

R. L. Champion, I. V. Dyakov, B. L. Peko, and Yicheng Wang

J. Chem. Phys. 115, 1765 (2001); http://dx.doi.org/10.1063/1.1380692 (4 pages) | Cited 2 times

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Insulating gas mixtures containing SF6 have been promoted to serve as replacements for pure SF6 in order to reduce SF6 atmospheric emission. It has been argued that some synergism may be achieved by choosing proper buffer gases in mixtures with SF6 such that the buffer gases efficiently slow down electrons into an energy range where the electron attachment cross section for SF6 is large. A complete understanding of the dielectric properties of SF6 mixtures obviously requires information about electron detachment from SF6− as collisional electron detachment may be the principal source of discharge initiation in SF6 mixtures. In this paper, we report total cross-section measurements for electron detachment and collision induced dissociation for collisions of SF6− with N2 for collision energies ranging up to a few hundred eV. The experimental results are analyzed using a two-step collision model where the unimolecular decomposition of collisionally excited SF6− ions is described in a statistical framework. © 2001 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Absorption and emission spectroscopy of matrix-isolated benzo[g,h,i]perylene: An experimental and theoretical study for astrochemical applications

Xavier Chillier, Pascal Boulet, Henry Chermette, Farid Salama, and Jacques Weber

J. Chem. Phys. 115, 1769 (2001); http://dx.doi.org/10.1063/1.1376632 (8 pages) | Cited 9 times

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The absorption and emission spectra of benzo[g,h,i]perylene, a six ring polycyclic aromatic hydrocarbon molecule (C22H12), embedded in a rare gas matrix are reported. Time dependent emission shows that this molecule exhibits sharp phosphorescence in the red. Supporting theoretical calculations using the recently developed time-dependent density-functional response theory formalism (TD–DFRT) allow a tentative assignment for the observed transitions. The astrochemical significance of the results is briefly discussed. © 2001 American Institute of Physics.
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33.20.Kf Visible spectra
33.20.Lg Ultraviolet spectra
33.20.Ea Infrared spectra
33.50.Dq Fluorescence and phosphorescence spectra
31.15.E- Density-functional theory

Measurement and theoretical simulation of the HCCO− anion photoelectron spectrum

Boris Schäfer-Bung, Bernd Engels, Travis R. Taylor, Daniel M. Neumark, Peter Botschwina, and Miljenko Perić

J. Chem. Phys. 115, 1777 (2001); http://dx.doi.org/10.1063/1.1378041 (12 pages) | Cited 8 times

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The photoelectron spectrum of HCCO− at the photodetachment wavelength of 355 nm is reported. A theoretical model for the simulation of the photodetachment process is described and the influence of various parameters is discussed. The experimental spectrum is compared with the simulation and an assignment of the spectrum is given. © 2001 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.80.Eh Autoionization, photoionization, and photodetachment
34.50.Gb Electronic excitation and ionization of molecules

Photoelectron spectroscopy of C3Si and C4Si2 anions

Gustavo E. Davico, Rebecca L. Schwartz, and W. Carl Lineberger

J. Chem. Phys. 115, 1789 (2001); http://dx.doi.org/10.1063/1.1380713 (6 pages) | Cited 6 times

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The 364 nm photoelectron spectra of the linear C3Si− and C4Si2− anions are reported. Accurate adiabatic electron affinities are determined: EA(3Σ C3Si)=2.827±0.007 eV and EA(C4Si2)=2.543±0.006 eV. Several vibrational frequencies for both neutral molecules are also obtained. The term energy for the first linear excited state of C3Si (either 1Δ or 1Σ) is 0.274±0.015 eV. For C4Si2, the term energy is substantially lower than in C3Si and vibronic interactions between the two states become stronger. Experimental results are compared with high-level ab initio calculations for C3Si (see Rintelman and Gordon, following paper) and with our own calculations for C4Si2 and its anion. © 2001 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Structure and energetics of the silicon carbide clusters SiC3 and Si2C2

Jamie M. Rintelman and Mark S. Gordon

J. Chem. Phys. 115, 1795 (2001); http://dx.doi.org/10.1063/1.1380714 (9 pages) | Cited 20 times

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A comprehensive ab initio study of the four atom silicon carbide clusters SiC3 and Si2C2 using multiconfigurational self-consistent-field wave functions is presented. In contrast to previous studies the global minimum isomer for SiC3 is predicted to be a C∞v linear triplet with a terminal silicon atom. For Si2C2 the global minimum is a rhombic structure, in accordance with previous studies, while the linear triplet Si–C–C–Si is just 1.0 kcal mol−1 higher in energy. © 2001 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods

The treatment of classically forbidden electronic transitions in semiclassical trajectory surface hopping calculations

Ahren W. Jasper, Michael D. Hack, and Donald G. Truhlar

J. Chem. Phys. 115, 1804 (2001); http://dx.doi.org/10.1063/1.1377891 (13 pages) | Cited 38 times

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A family of four weakly coupled electronically nonadiabatic bimolecular model photochemical systems is presented. Fully converged quantum mechanical calculations with up to 25 269 basis functions were performed for full-dimensional atom–diatom collisions to determine the accurate scattering dynamics for each of the four systems. The quantum mechanical probabilities for electronically nonadiabatic reaction and for nonreactive electronic deexcitation vary from 10−1 to 10−5. Tully’s fewest-switches (TFS) semiclassical trajectory surface-hopping method (also called molecular dynamics with quantum transitions or MDQT) is tested against the accurate quantal results. The nonadiabatic reaction and nonreactive deexcitation events are found to be highly classically forbidden for these systems, which were specifically designed to model classically forbidden electronic transitions (also called frustrated hops). The TFS method is shown to systematically overestimate the nonadiabatic transition probabilities due to the high occurrence of frustrated hops. In order to better understand this problem and learn how to best minimize the errors, we test several variants of the TFS method on the four new weakly coupled systems and also on a set of three more strongly coupled model systems that have been presented previously. The methods tested here differ from one another in their treatment of the classical trajectory during and after a frustrated hopping event. During the hopping event we find that using a rotated hopping vector results in the best agreement of semiclassical and quantal results for the nonadiabatic transition probabilities. After the hopping event, we find that ignoring frustrated hops instead of reversing the momentum along the nonadiabatic coupling vector results in the best agreement with the accurate quantum results for the final vibrational and rotational moments. We also test the use of symmetrized probabilities in the equations for the TFS hopping probabilities. These methods systematically lead to increased error for systems with weakly coupled electronic states unless the hopping probabilities are symmetrized according to the electronic state populations. © 2001 American Institute of Physics.
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82.20.Fd Collision theories; trajectory models
82.20.Ln Semiclassical theory of reactions and/or energy transfer
82.50.-m Photochemistry
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
34.50.-s Scattering of atoms and molecules
82.20.Kh Potential energy surfaces for chemical reactions

A theoretical study of spectroscopy and predissociation dynamics in nitrosoalkanes

Alessandro Toniolo and Maurizio Persico

J. Chem. Phys. 115, 1817 (2001); http://dx.doi.org/10.1063/1.1379329 (11 pages) | Cited 4 times

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We have computed ab initio transition energies, equilibrium geometries, force constants and potential energy curves for the dissociation of S0, T1, and S1 of two nitrosoalkanes, CH3NO and t-BuNO. A normal coordinate analysis has been performed for the three states, and the harmonic wave function for the C–N bond torsional coordinate has been replaced by hindered rotor eigenfunctions. The n→π∗ absorption spectra have been simulated by computing the appropriate Franck–Condon factors in order to assign the vibrational sub-bands. The predissociation lifetimes of several vibrational states of S1 have been evaluated by computing nonadiabatic and spin-orbit couplings, which determine the Internal Conversion and Intersystem Crossing rates. For t-BuNO the computed lifetimes (10–160 ns) are in the same range as those measured by Noble et al. [J. Chem. Phys. 85, 5763 (1986)]. The lifetimes of CH3NO, for which no experimental data are available, are longer (50–330 ns). Both the IC to S0 and the ISC to T1 are important. © 2001 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Tp Vibrational analysis
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)

Variational transition state theory and quasiclassical trajectory studies of the H2+OH→H+H2O reaction and some isotopic variants

Diego Troya, Matthew J. Lakin, George C. Schatz, and Miguel González

J. Chem. Phys. 115, 1828 (2001); http://dx.doi.org/10.1063/1.1382646 (15 pages) | Cited 20 times

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Variational transition state theory (VTST) methods and quasiclassical trajectories (QCT) have been used to study the dynamics of the OH+H2 reaction, along with the isotopic counterparts OD+H2, OH+HD, OD+H2, OD+D2, and the reverse H+H2O→H2+OH reaction. Two new global analytical potential energy surfaces (PES) for H3O are employed, Wu, Schatz, Lendvay, Fang, Harding (WSLFH) and Ochoa, Clary (OC), both of which are based on high quality electronic structure calculations. Extensive comparisons with earlier results based on the Walch, Dunning, Schatz, Elgersma (WDSE) PES are also presented. The WSLFH PES surface, in combination with our best VTST estimate (ICVT/μOMT), yields rate constants for OH+H2 in quantitative agreement with experiment, while the OC PES yields somewhat less accurate results. The agreement with the OH+D2 experimental rate constants is less quantitative, but the WSLFH PES rate constant agrees with experiment to within a factor of 2 at all temperatures for which there are measurements. The OH+HD, OD+H2, and OD+D2 WSLFH PES rate constants calculated at the ICVT/μOMT level are in very good agreement with the less detailed experimental information that is available for these isotopes. The two surfaces give comparable predictions for the reverse H+H2O reaction at high temperatures, with deviations of less than 30%. This good agreement is maintained by the WSLFH PES at room temperature, while the OC PES predicts rate constants one order of magnitude larger than experiment. The QCT excitation functions for OH+H2, OH+D2, and OH+HD are well below experiment for both potentials, as was the case for earlier accurate quantum mechanical calculations that employed the WDSE PES. The WSLFH PES improves the agreement with the experimental vibrational state selected rate constants for the OH+H2 reaction compared to the WDSE PES. OC is also less accurate and presents antithreshold behavior for H2(v = 1)+OH. H2 and OH rotational excitation have opposing effects: while rotation in H2 promotes reactivity, OH rotation impedes it. This impeding effect applies likewise to HD for high rotational excitation, explaining the selectivity toward HOH+D products in the OH+HD reaction. © 2001 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
02.30.Xx Calculus of variations
31.30.Gs Hyperfine interactions and isotope effects
82.20.Tr Kinetic isotope effects including muonium

The effect of molecular orientation in collisions of OH with CO and N2

M. C. van Beek and J. J. ter Meulen

J. Chem. Phys. 115, 1843 (2001); http://dx.doi.org/10.1063/1.1369136 (10 pages) | Cited 14 times

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The effect of OH orientation on rotationally inelastic collisions of OH(X2Π) with CO and N2 has been studied in a crossed molecular beam setup at translational energies of 750 and 690 cm−1, respectively. The OH molecules were prepared in the v = 0,Ω = math,J = math,f state by hexapole state selection and oriented with their O end or H end toward the collision partner by a static electric field in the collision zone. A degree of orientation of 〈cosθ〉 = 0.46 has been obtained. In general the cross sections are larger for collisions at the O end in excitation to low rotational states, whereas the cross sections are larger for H end excitation to higher rotational states. OH+CO and OH+N2 behave quite similarly when compared to OH+Ar. Systematic differences between OH+CO and OH+N2 may be attributed to the influence of complex formation on the inelastic collision process. Reanalysis of state-to-state scattering experiments on unoriented OH+CO and OH+N2 indicate that the interaction potential is more head–tail symmetric with respect to OH for OH+N2 compared to OH+CO. © 2001 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
37.20.+j Atomic and molecular beam sources and techniques
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

A theorem for inhomogeneous systems: The generalization of the nucleation theorem

R. K. Bowles, D. Reguera, Y. Djikaev, and H. Reiss

J. Chem. Phys. 115, 1853 (2001); http://dx.doi.org/10.1063/1.1382818 (14 pages) | Cited 21 times

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We show that the validity of the nucleation theorem transcends the phenomenon of nucleation and extends to all equilibrium systems containing local nonuniform density distributions stabilized by external fields, and that it remains valid down to the molecular level. This result is tested by the application of exact theory at the molecular level and is shown to be valid in all the cases for which we have been able to complete such an exact analysis. These cases include cavities and clusters in hard rod fluids, as well as the molecular excesses associated with the “atmospheres” of molecules in single and multicomponent fluids. We show that, at the molecular level, the theorem can be associated with the compressibility equation of state and, at the macroscopic level, with the Gibbs adsorption equation. It is thus a relation of great power and should be useful in many contexts. © 2001 American Institute of Physics.
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64.60.Q- Nucleation
64.10.+h General theory of equations of state and phase equilibria
68.03.Fg Evaporation and condensation of liquids

Control of thermal photoinduced electron transfer reactions in the activated and activationless regimes

Eli Pollak and Lev Plimak

J. Chem. Phys. 115, 1867 (2001); http://dx.doi.org/10.1063/1.1382815 (8 pages) | Cited 3 times

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Photoinduced electron transfer rates depend on the internal energy distribution of the locally excited donor state. This energy distribution may be hot or cold relative to the temperature of the donor in the ground electronic state and is dependent on the photoexcitation frequency. In the activated regime, the electron transfer rate depends exponentially on the temperature of the locally excited donor state. Therefore, the electron transfer rate is sensitive to the photoexcitation frequency. In the activationless regime, even if the vibrational frequencies of the locally excited donor state and the acceptor state differ, the electron transfer rate is rather insensitive to the internal energy distribution of the locally excited donor state. Therefore, changing the photoexcitation frequency does not lead to a significant change in the transfer rate. Model computations are presented to demonstrate this qualitative difference between the two regimes, as well as to confirm that the photoinduced electron transfer rate is well-approximated as a thermal electron transfer rate, but at an effective temperature of the locally excited donor state that depends on the photoexcitation frequency. © 2001 American Institute of Physics.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
34.70.+e Charge transfer
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.50.Nd Control of photochemical reactions
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