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21 Jan 2012

Volume 136, Issue 3, Articles (03xxxx)

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J. Chem. Phys. 136, 035101 (2012); http://dx.doi.org/10.1063/1.3671986 (13 pages)

L. Dupuis and Normand Mousseau
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Communication: The rotational excitation of D2 by H: On the importance of the reactive channels

François Lique and Alexandre Faure

J. Chem. Phys. 136, 031101 (2012); http://dx.doi.org/10.1063/1.3678310 (4 pages) | Cited 1 time

Online Publication Date: 18 January 2012

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We report fully-quantum time-independent calculations of cross sections and rate coefficients for the collisional excitation and dissociation of D2 by H, two astrophysically relevant processes. Our calculations are based on the recent H3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)10.1063/1.1432319]. Results of exact three-dimensional calculations, i.e., including the reactive channels, are compared to pure inelastic two-dimensional calculations based on the rigid rotor approximation. A reasonable agreement is found between the two sets of inelastic cross sections over the whole energy range 10–9000 cm−1. At the highest collisional energies, where the reactive channels are significant, the rigid rotor approach slightly overestimates the cross sections, as expected. At moderate collisional energies, however, the opposite behaviour is observed. The rigid rotor approach is found to be reliable at temperatures below ∼500 K, with a significant but moderate contribution from reactive channels
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
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Communication: Quantum mechanics without wavefunctions

Jeremy Schiff and Bill Poirier

J. Chem. Phys. 136, 031102 (2012); http://dx.doi.org/10.1063/1.3680558 (4 pages) | Cited 2 times

Online Publication Date: 19 January 2012

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We present a self-contained formulation of spin-free non-relativistic quantum mechanics that makes no use of wavefunctions or complex amplitudes of any kind. Quantum states are represented as ensembles of real-valued quantum trajectories, obtained by extremizing an action and satisfying energy conservation. The theory applies for arbitrary configuration spaces and system dimensionalities. Various beneficial ramifications—theoretical, computational, and interpretational—are discussed.
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03.65.Ge Solutions of wave equations: bound states
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Effective homogeneity of the exchange–correlation and non-interacting kinetic energy functionals under density scaling

Alex Borgoo, Andrew M. Teale, and David J. Tozer

J. Chem. Phys. 136, 034101 (2012); http://dx.doi.org/10.1063/1.3676722 (6 pages) | Cited 1 time

Online Publication Date: 17 January 2012

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Correlated electron densities, experimental ionisation potentials, and experimental electron affinities are used to investigate the homogeneity of the exchange–correlation and non-interacting kinetic energy functionals of Kohn–Sham density functional theory under density scaling. Results are presented for atoms and small molecules, paying attention to the influence of the integer discontinuity and the choice of the electron affinity. For the exchange–correlation functional, effective homogeneities are highly system-dependent on either side of the integer discontinuity. By contrast, the average homogeneity—associated with the potential that averages over the discontinuity—is generally close to 4/3 when the discontinuity is computed using positive affinities for systems that do bind an excess electron and negative affinities for those that do not. The proximity to 4/3 becomes increasingly pronounced with increasing atomic number. Evaluating the discontinuity using a zero affinity in systems that do not bind an excess electron instead leads to effective homogeneities on the electron abundant side that are close to 4/3. For the non-interacting kinetic energy functional, the effective homogeneities are less system-dependent and the effect of the integer discontinuity is less pronounced. Average values are uniformly below 5/3. The study provides information that may aid the development of improved exchange–correlation and non-interacting kinetic energy functionals.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.V- Electron correlation calculations for atoms, ions and molecules
32.50.+d Fluorescence, phosphorescence (including quenching)
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.15.ec Hohenberg-Kohn theorem and formal mathematical properties, completeness theorems

Improved self-consistent and resolution-of-identity approximated Becke'05 density functional model of nondynamic electron correlation

Emil Proynov, Fenglai Liu, Yihan Shao, and Jing Kong

J. Chem. Phys. 136, 034102 (2012); http://dx.doi.org/10.1063/1.3676726 (14 pages)

Online Publication Date: 17 January 2012

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In a recent letter [E. Proynov, Y. Shao, and J. Kong, Chem. Phys. Lett. 493, 381 (2010)10.1016/j.cplett.2010.05.029], Becke's B05 model of nondynamic electron correlation in density functional theory was implemented self-consistently with computational efficiency (the “SCF-RI-B05” scheme). Important modifications of the algorithm were done in order to make the self-consistency feasible. In the present work, we give a complete account of the SCF-RI-B05 algorithm, including all the formulae for the analytical representation of the B05 functional and for its self-consistent field (SCF) potential. The average performance of the SCF-RI-B05 method reported in the above letter was somewhat less accurate, compared to the original B05 implementation, mainly because the parameters of the original B05 model were optimized with post-local-spin-density calculations. In this work, we report improved atomization energies with SCF-RI-B05, based on a SCF re-optimization of its four linear parameters. The re-optimized SCF-RI-B05 scheme is validated also on reaction barriers, and on the subtle energetics of NO dimer, an exemplary system of strong nondynamic correlation. It yields both the binding energy and the singlet-triplet splitting of the NO dimer correctly, and close to the benchmarks reported in the literature.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xr Self-consistent-field methods
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
34.20.Cf Interatomic potentials and forces

A new method to generate spin-orbit coupled potential energy surfaces: Effective relativistic coupling by asymptotic representation

Hameth Ndome, Ralph Welsch, and Wolfgang Eisfeld

J. Chem. Phys. 136, 034103 (2012); http://dx.doi.org/10.1063/1.3675846 (8 pages)

Online Publication Date: 17 January 2012

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A new method has been developed to generate fully coupled potential energy surfaces including derivative and spin-orbit coupling. The method is based on an asymptotic (atomic) representation of the molecular fine structure states and a corresponding diabatization. The effective relativistic coupling is described by a constant spin-orbit coupling matrix and the geometry dependence of the coupling is accounted for by the diabatization. This approach is very efficient, particularly for certain systems containing a very heavy atom, and yields consistent results throughout nuclear configuration space. A first application to a diatomic system is presented as proof of principle and is compared to accurate ab initio calculations. However, the method is widely applicable to general polyatomic systems in full dimensionality, containing several relativistic atoms and treating higher order relativistic couplings as well.
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31.50.-x Potential energy surfaces
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Pw Fine and hyperfine structure
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Higher order alchemical derivatives from coupled perturbed self-consistent field theory

Michał Lesiuk, Robert Balawender, and Janusz Zachara

J. Chem. Phys. 136, 034104 (2012); http://dx.doi.org/10.1063/1.3674163 (11 pages)

Online Publication Date: 17 January 2012

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We present an analytical approach to treat higher order derivatives of Hartree-Fock (HF) and Kohn-Sham (KS) density functional theory energy in the Born-Oppenheimer approximation with respect to the nuclear charge distribution (so-called alchemical derivatives). Modified coupled perturbed self-consistent field theory is used to calculate molecular systems response to the applied perturbation. Working equations for the second and the third derivatives of HF/KS energy are derived. Similarly, analytical forms of the first and second derivatives of orbital energies are reported. The second derivative of Kohn-Sham energy and up to the third derivative of Hartree-Fock energy with respect to the nuclear charge distribution were calculated. Some issues of practical calculations, in particular the dependence of the basis set and Becke weighting functions on the perturbation, are considered. For selected series of isoelectronic molecules values of available alchemical derivatives were computed and Taylor series expansion was used to predict energies of the “surrounding” molecules. Predicted values of energies are in unexpectedly good agreement with the ones computed using HF/KS methods. Presented method allows one to predict orbital energies with the error less than 1% or even smaller for valence orbitals.
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31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory
31.15.E- Density-functional theory
31.15.B- Approximate calculations

Hybrid modeling and simulation of stochastic effects on progression through the eukaryotic cell cycle

Zhen Liu, Yang Pu, Fei Li, Clifford A. Shaffer, Stefan Hoops, John J. Tyson, and Yang Cao

J. Chem. Phys. 136, 034105 (2012); http://dx.doi.org/10.1063/1.3677190 (11 pages)

Online Publication Date: 17 January 2012

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The eukaryotic cell cycle is regulated by a complicated chemical reaction network. Although many deterministic models have been proposed, stochastic models are desired to capture noise in the cell resulting from low numbers of critical species. However, converting a deterministic model into one that accurately captures stochastic effects can result in a complex model that is hard to build and expensive to simulate. In this paper, we first apply a hybrid (mixed deterministic and stochastic) simulation method to such a stochastic model. With proper partitioning of reactions between deterministic and stochastic simulation methods, the hybrid method generates the same primary characteristics and the same level of noise as Gillespie's stochastic simulation algorithm, but with better efficiency. By studying the results generated by various partitionings of reactions, we developed a new strategy for hybrid stochastic modeling of the cell cycle. The new approach is not limited to using mass-action rate laws. Numerical experiments demonstrate that our approach is consistent with characteristics of noisy cell cycle progression, and yields cell cycle statistics in accord with experimental observations.
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87.15.R- Reactions and kinetics
87.10.Mn Stochastic modeling
87.17.Aa Modeling, computer simulation of cell processes

Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation

Maria Hellgren, Daniel R. Rohr, and E. K. U. Gross

J. Chem. Phys. 136, 034106 (2012); http://dx.doi.org/10.1063/1.3676174 (9 pages)

Online Publication Date: 17 January 2012

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Self-consistent correlation potentials for H2 and LiH for various inter-atomic separations are obtained within the random phase approximation (RPA) of density functional theory. The RPA correlation potential shows a peak at the bond midpoint, which is an exact feature of the true correlation potential, but lacks another exact feature: the step important to preserve integer charge on the atomic fragments in the dissociation limit. An analysis of the RPA energy functional in terms of fractional charge is given which confirms these observations. We find that the RPA misses the derivative discontinuity at odd integer particle numbers but explicitly eliminates the fractional spin error in the exact-exchange functional. The latter finding explains the improved total energy in the dissociation limit.
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34.20.Cf Interatomic potentials and forces
31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies

Automatic computer procedure for generating exact and analytical kinetic energy operators based on the polyspherical approach

Mamadou Ndong, Loïc Joubert-Doriol, Hans-Dieter Meyer, André Nauts, Fabien Gatti, and David Lauvergnat

J. Chem. Phys. 136, 034107 (2012); http://dx.doi.org/10.1063/1.3675163 (16 pages)

Online Publication Date: 17 January 2012

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We develop a new general code to automatically derive exact analytical kinetic energy operators in terms of polyspherical coordinates. Computer procedures based on symbolic calculations are implemented. Sets of orthogonal or non-orthogonal vectors are used to parametrize the molecular systems in space. For each set of vectors, and whatever the size of the system, the exact analytical kinetic energy operator (including the overall rotation and the Coriolis coupling) can be derived by the program. The correctness of the implementation is tested for different sets of vectors and for several systems of various sizes.
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82.20.Wt Computational modeling; simulation

Coupling between internal dynamics and rotational diffusion in the presence of exchange between discrete molecular conformations

Yaroslav Ryabov, G. Marius Clore, and Charles D. Schwieters

J. Chem. Phys. 136, 034108 (2012); http://dx.doi.org/10.1063/1.3675602 (5 pages)

Online Publication Date: 17 January 2012

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We present a general formalism for the computation of orientation correlation functions involving a molecular system undergoing rotational diffusion in the presence of transitions between discrete conformational states. In this formalism, there are no proscriptions on the time scales of conformational rearrangement relative to that for rotational diffusion, and the rotational diffusion tensors of the different states can be completely arbitrary. Although closed-form results are limited to the frequency domain, this is generally useful for many spectroscopic observables as the result allows the computation of the spectral density function. We specialize the results for the computation of the frequency-domain correlation function associated with the NMR relaxation.
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33.25.+k Nuclear resonance and relaxation
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Sn Rotational analysis

Atomic volumes and polarizabilities in density-functional theory

Felix O. Kannemann and Axel D. Becke

J. Chem. Phys. 136, 034109 (2012); http://dx.doi.org/10.1063/1.3676064 (5 pages) | Cited 2 times

Online Publication Date: 18 January 2012

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Becke and Johnson introduced an ad hoc definition of atomic volume [J. Chem. Phys. 124, 014204 (2006)] in order to obtain atom-in-molecule polarizabilities from free-atom polarizabilities in their nonempirical exchange-hole dipole moment model of dispersion interactions. Here we explore the dependence of Becke-Johnson atomic volumes on basis sets and density-functional approximations and provide reference data for all atoms H–Lr. A persuasive theoretical foundation for the Becke-Johnson definition is also provided.
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31.15.E- Density-functional theory
32.30.-r Atomic spectra

Nuclear dynamics for a three-state Jahn–Teller model system

Pascal Krause and Spiridoula Matsika

J. Chem. Phys. 136, 034110 (2012); http://dx.doi.org/10.1063/1.3677273 (13 pages)

Online Publication Date: 18 January 2012

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We report wavepacket dynamics on a model system with a three-state conical intersection. Quantum wavepacket dynamics using the multiconfigurational time-dependent Hartree method have been carried out for the T ⊗ (e + t2) Jahn–Teller problem, using a Jahn–Teller vibronic model Hamiltonian. The effects of the magnitude of the coupling parameters and of the initial position of the wavepacket on the dynamics around the three-state conical intersection have been considered. It was found that the effect of the coupling strength is not dramatic for the population transfer in most cases, but the details of the dynamics and the involvement of the different modes are affected by it.
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31.15.xr Self-consistent-field methods
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
82.20.Kh Potential energy surfaces for chemical reactions

A block variational procedure for the iterative diagonalization of non-Hermitian random-phase approximation matrices

Dario Rocca, Zhaojun Bai, Ren-Cang Li, and Giulia Galli

J. Chem. Phys. 136, 034111 (2012); http://dx.doi.org/10.1063/1.3677667 (8 pages)

Online Publication Date: 18 January 2012

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We present a technique for the iterative diagonalization of random-phase approximation (RPA) matrices, which are encountered in the framework of time-dependent density-functional theory (TDDFT) and the Bethe-Salpeter equation. The non-Hermitian character of these matrices does not permit a straightforward application of standard iterative techniques used, i.e., for the diagonalization of ground state Hamiltonians. We first introduce a new block variational principle for RPA matrices. We then develop an algorithm for the simultaneous calculation of multiple eigenvalues and eigenvectors, with convergence and stability properties similar to techniques used to iteratively diagonalize Hermitian matrices. The algorithm is validated for simple systems (Na2 and Na4) and then used to compute multiple low-lying TDDFT excitation energies of the benzene molecule.
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31.15.ee Time-dependent density functional theory
31.15.xt Variational techniques
02.10.Yn Matrix theory
02.60.-x Numerical approximation and analysis

Strongly correlated barriers to rotation from parametric two-electron reduced-density-matrix methods in application to the isomerization of diazene

Andrew M. Sand, Christine A. Schwerdtfeger, and David A. Mazziotti

J. Chem. Phys. 136, 034112 (2012); http://dx.doi.org/10.1063/1.3675683 (7 pages) | Cited 1 time

Online Publication Date: 19 January 2012

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Recently, parameterization of the two-electron reduced density matrix (2-RDM) has made possible the determination of electronic energies with greater accuracy and lower cost than traditional electron-pair theories including coupled cluster with single and double excitations [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)]. We examine the method's performance for strongly correlated barriers to rotation; in particular, we study two distinct pathways in the isomerization of diazene (N2H2) from cis to trans: (i) a strongly correlated rotational pathway and (ii) a moderately correlated inversion pathway. While single reference wavefunction methods predict that the rotational barrier is higher than the inversional barrier, the parametric 2-RDM method predicts that the rotational barrier is lower than the inversional barrier by 3.1 kcal/mol in the extrapolated basis set limit. The parametric 2-RDM results are in agreement with those from multireference methods including multireference perturbation theory and the solution to the anti-Hermitian contracted Schrödinger equation. We report energies, optimized structures, and natural orbital occupation numbers for three diazene minima and two transition states.
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33.20.Sn Rotational analysis
82.30.Qt Isomerization and rearrangement
31.15.eg Exchange-correlation functionals (in current density functional theory)

Reduced density matrix hybrid approach: An efficient and accurate method for adiabatic and non-adiabatic quantum dynamics

Timothy C. Berkelbach, David R. Reichman, and Thomas E. Markland

J. Chem. Phys. 136, 034113 (2012); http://dx.doi.org/10.1063/1.3671372 (10 pages) | Cited 1 time

Online Publication Date: 19 January 2012

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We present a new approach to calculate real-time quantum dynamics in complex systems. The formalism is based on the partitioning of a system's environment into “core” and “reservoir” modes with the former to be treated quantum mechanically and the latter classically. The presented method only requires the calculation of the system's reduced density matrix averaged over the quantum core degrees of freedom which is then coupled to a classically evolved reservoir to treat the remaining modes. We demonstrate our approach by applying it to the spin-boson problem using the noninteracting blip approximation to treat the system and core, and Ehrenfest dynamics to treat the reservoir. The resulting hybrid methodology is accurate for both fast and slow baths, since it naturally reduces to its composite methods in their respective regimes of validity. In addition, our combined method is shown to yield good results in intermediate regimes where neither approximation alone is accurate and to perform equally well for both strong and weak system-bath coupling. Our approach therefore provides an accurate and efficient methodology for calculating quantum dynamics in complex systems.
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31.15.E- Density-functional theory

Dimensional scaling treatment with relativistic corrections for stable multiply charged atomic ions in high-frequency super-intense laser fields

Ross D. Hoehn, Jiaxiang Wang, and Sabre Kais

J. Chem. Phys. 136, 034114 (2012); http://dx.doi.org/10.1063/1.3673317 (14 pages)

Online Publication Date: 19 January 2012

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We present a theoretical framework which describes multiply charged atomic ions, their stability within super-intense laser fields, and also lay corrections to the systems due to relativistic effects. Dimensional scaling calculations with relativistic corrections for systems: H, H−, H2 −, He, He−, He2 −, He3 − within super-intense laser fields were completed. Also completed were three-dimensional self consistent field calculations to verify the dimensionally scaled quantities. With the aforementioned methods the system's ability to stably bind “additional” electrons through the development of multiple isolated regions of high potential energy leading to nodes of high electron density is shown. These nodes are spaced far enough from each other to minimize the electronic repulsion of the electrons, while still providing adequate enough attraction so as to bind the excess electrons into orbitals. We have found that even with relativistic considerations these species are stably bound within the field. It was also found that performing the dimensional scaling calculations for systems within the confines of laser fields to be a much simpler and more cost-effective method than the supporting D = 3 SCF method. The dimensional scaling method is general and can be extended to include relativistic corrections to describe the stability of simple molecular systems in super-intense laser fields.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.xr Self-consistent-field methods

A pathwise derivative approach to the computation of parameter sensitivities in discrete stochastic chemical systems

Patrick W. Sheppard, Muruhan Rathinam, and Mustafa Khammash

J. Chem. Phys. 136, 034115 (2012); http://dx.doi.org/10.1063/1.3677230 (13 pages)

Online Publication Date: 20 January 2012

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Characterizing the sensitivity to infinitesimally small perturbations in parameters is a powerful tool for the analysis, modeling, and design of chemical reaction networks. Sensitivity analysis of networks modeled using stochastic chemical kinetics, in which a probabilistic description is used to characterize the inherent randomness of the system, is commonly performed using Monte Carlo methods. Monte Carlo methods require large numbers of stochastic simulations in order to generate accurate statistics, which is usually computationally demanding or in some cases altogether impractical due to the overwhelming computational cost. In this work, we address this problem by presenting the regularized pathwise derivative method for efficient sensitivity analysis. By considering a regularized sensitivity problem and using the random time change description for Markov processes, we are able to construct a sensitivity estimator based on pathwise differentiation (also known as infinitesimal perturbation analysis) that is valid for many problems in stochastic chemical kinetics. The theoretical justification for the method is discussed, and a numerical algorithm is provided to permit straightforward implementation of the method. We show using numerical examples that the new regularized pathwise derivative method (1) is able to accurately estimate the sensitivities for many realistic problems and path functionals, and (2) in many cases outperforms alternative sensitivity methods, including the Girsanov likelihood ratio estimator and common reaction path finite difference method. In fact, we observe that the variance reduction using the regularized pathwise derivative method can be as large as ten orders of magnitude in certain cases, permitting much more efficient sensitivity analysis than is possible using other methods.
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82.20.Uv Stochastic theories of rate constants
82.20.Wt Computational modeling; simulation
02.50.Cw Probability theory
02.50.Ey Stochastic processes
02.50.Ga Markov processes

Exploring quantum non-locality with de Broglie-Bohm trajectories

Ivan P. Christov

J. Chem. Phys. 136, 034116 (2012); http://dx.doi.org/10.1063/1.3677372 (5 pages)

Online Publication Date: 20 January 2012

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Here in this paper, it is shown how the quantum nonlocality reshapes probability distributions of quantum trajectories in configuration space. By variationally minimizing the ground state energy of helium atom, we show that there exists an optimal nonlocal quantum correlation length which also minimizes the mean integrated square error of the smooth trajectory ensemble with respect to the exact many-body wave function. The nonlocal quantum correlation length can be used for studies of both static and driven many-body quantum systems.
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03.65.Ca Formalism
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Reaction dynamics of Mo + O2 → MoO + O studied by a crossed-beam velocity map imaging technique

Kenji Honma and Yoshiteru Matsumoto

J. Chem. Phys. 136, 034301 (2012); http://dx.doi.org/10.1063/1.3676724 (7 pages)

Online Publication Date: 17 January 2012

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The oxidation reaction dynamics of gas-phase molybdenum atoms by oxygen molecules was studied under a crossed-beam condition. The product MoO was detected by a time-of-flight mass spectrometer combined with laser multi-photon ionization. An acceleration lens system designed for the ion-velocity mapping condition, a two-dimensional (2D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products at three collision energies: 10.0, 17.8, and 50.0 kJ/mol. The angular distributions showed forward and backward peaks, whose relative intensities changed by the collision energy. While two peaks had similar intensities at low collision energies, the forward peak became dominant at the highest collision energy, 50 kJ/mol. The product kinetic energy distributions showed a good correlation with the initial collision energies, i.e., almost the same energy as the collision energy appeared as the product kinetic energy. These results suggested that the reaction proceeds via an intermediate complex, and the lifetime of the complex becomes shorter than its rotational period at high collision energy.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

Bin Jiang, Daiqian Xie, and Hua Guo

J. Chem. Phys. 136, 034302 (2012); http://dx.doi.org/10.1063/1.3676725 (13 pages)

Online Publication Date: 17 January 2012

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State-to-state photodissociation dynamics of H2O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two 1A′ states of H2O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(math) products. On the other hand, the adiabatic channel on the excited state leading to the OH(math) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.
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82.50.-m Photochemistry
82.20.Fd Collision theories; trajectory models
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

On the molecular structure of HOOO

Michael C. McCarthy, Valerio Lattanzi, Damian Kokkin, Oscar Martinez, Jr., and John F. Stanton

J. Chem. Phys. 136, 034303 (2012); http://dx.doi.org/10.1063/1.3673875 (10 pages)

Online Publication Date: 17 January 2012

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The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H2O and O2 heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H2 and O2 as precursor gases. Using equal mixtures of normal and 18O2, it has been possible to detect three new isotopic species, H18OOO, HO18O18O, and H18O18O18O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O2 → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r0) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O–O bond (1.684 Å), in agreement with earlier work, a markedly shorter O–H bond distance (0.913 Å), and a more acute ∠HOO angle (92.4°) when compared to equilibrium (re) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (reemp) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen–oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.
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31.15.bw Coupled-cluster theory
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Bx Radio-frequency and microwave spectra

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Xiaofeng Tang, Xiaoguo Zhou, Manman Wu, Shilin Liu, Fuyi Liu, Xiaobin Shan, and Liusi Sheng

J. Chem. Phys. 136, 034304 (2012); http://dx.doi.org/10.1063/1.3676411 (8 pages)

Online Publication Date: 17 January 2012

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Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH3Cl+ ions was investigated in the excitation energy range of 11.0–18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH3+ dissociated from CH3Cl+(A2A1 and B2E) ions were recorded. CH3+ was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH2Cl+ fragment was very low. For dissociation of CH3Cl+(A2A1) ions, a series of homocentric rings was clearly observed in the CH3+ image, which was assigned as the excitation of umbrella vibration of CH3+ ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH3+(11A′) provided a direct experimental evidence of a shallow potential well along the C–Cl bond rupture. For CH3Cl+(B2E) ions, total kinetic energy released distribution for CH3+ fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B2E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH3Cl+, CH3+ formation from CH3Cl+(A2A1) ions was a rapid direct fragmentation, while CH3Cl+(B2E) ions statistically dissociated to CH3+ + Cl via internal conversion to the high vibrational states of X2E.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ta Mass spectra
33.50.Hv Radiationless transitions, quenching

Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism

Amit R. Sharma, Joel M. Bowman, and David J. Nesbitt

J. Chem. Phys. 136, 034305 (2012); http://dx.doi.org/10.1063/1.3666987 (8 pages)

Online Publication Date: 18 January 2012

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We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated “reaction paths.” The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm−1. Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Qt Isomerization and rearrangement
31.15.ae Electronic structure and bonding characteristics
82.20.Kh Potential energy surfaces for chemical reactions
31.15.bw Coupled-cluster theory
33.15.Fm Bond strengths, dissociation energies

Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal

Prasenjit Seal, Ewa Papajak, Tao Yu, and Donald G. Truhlar

J. Chem. Phys. 136, 034306 (2012); http://dx.doi.org/10.1063/1.3674995 (10 pages)

Online Publication Date: 18 January 2012

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The purpose of the present investigation is to calculate partition functions and thermodynamic quantities, viz., entropy, enthalpy, heat capacity, and Gibbs free energies, for 1-butanol, 2-methyl-1-propanol, and butanal in the vapor phase. We employed the multi-structural (MS) anharmonicity method and electronic structure calculations including both explicitly correlated coupled cluster theory and density functional theory. The calculations are performed using all structures for each molecule and employing both the local harmonic approximation (MS-LH) and the inclusion of torsional anharmonicity (MS-T). The results obtained from the MS-T calculations are in excellent agreement with experimental data taken from the Thermodynamics Research Center data series and the CRC Handbook of Chemistry and Physics, where available. They are also compared with Benson's empirical group additivity values, where available; in most cases, the present results are more accurate than the group additivity values. In other cases, where experimental data (but not group additivity values) are available, we also obtain good agreement with experiment. This validates the accuracy of the electronic structure calculations when combined with the MS-T method for estimating the thermodynamic properties of systems with multiple torsions, and it increases our confidence in the predictions made with this method for molecules and temperatures where experimental or empirical data are not available.
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31.15.bw Coupled-cluster theory
02.70.Rr General statistical methods

Variation of radiative lifetimes of NH2(Ã2A1) with rotational levels in the (0, 8, 0) and (0, 9, 0) vibration bands

Marc N’Doumi and Joshua B. Halpern

J. Chem. Phys. 136, 034307 (2012); http://dx.doi.org/10.1063/1.3676782 (7 pages)

Online Publication Date: 19 January 2012

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Radiative lifetimes from the first electronically excited state of the amidogen free radical, NH2(Ã2A1), are reported for rotational states in selected vibrational levels ν2′ using laser-induced fluorescence. Thermal collision of argon, Ar*(3P0, 3P2) metastable atoms in a microwave discharge-flow system with ammonia (NH3) molecules produced ground state NH2(math2B1). The radiative lifetimes for the deactivation of NH2(Ã2A1) were determined by measuring the decay profiles of NH2(Ã2A1 → math2B1). In addition to the Fermi resonances with the ground state that lengthen the radiative lifetimes, a systematic increase in the radiative lifetimes with rotational quantum number was observed. Furthermore, the average radiative lifetimes of the (0, 9, 0) Γ, τ1 = 18.65 ± 0.47 μs and (0, 8, 0) Φ, τ2 = 23.72 ± 0.65 μs levels were much longer than those of the (0, 9, 0) Σ, τ3 = 10.62 ± 0.47 μs, and (0, 8, 0) Π, τ4 = 13.55 ± 0.55 μs states suggesting increased mixing of the first electronic excited and the ground states.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.-b Photon interactions with molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.50.Dq Fluorescence and phosphorescence spectra
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