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The mechanism of the reaction OH+CO→CO2+H is studied theoretically. The potential energy surface is constructed by using ab initio multiconfiguration (MC) SCF and configuration interaction (CI) calculations. The reaction surface Hamiltonian is derived to estimate the hydrogen tunneling probability. It is found that the first step is the trans addition of OH to CO and the trans–cis isomerization is the subsequent step to lead the hydrogen elimination from the HOCO. The rate constant is calculated by using the RRKM equation and the curved Arrhenius temperature dependence, experimentally observed, is obtained. We have found here that the effects of both the hydrogen tunneling from the HOCO to products and the backreaction from the HOCO to the reactants are essential to obtain this temperature dependence. [ABSTRACT FROM AUTHOR]

We present a semiglobal potential energy surface for HF dimer that should be especially realistic in the vicinity of the two equivalent hydrogen-bonded-complex geometries and the transition state separating them. It is based on fits to 106 correlated calculations and 378 SCF calculations of the energy of the planar complex plus a separate fit to the empirical out-of-plane bending potential of Barton and Howard at a sequence of geometries along the minimum energy path for the degenerate rearrangement connecting the two minima. [ABSTRACT FROM AUTHOR]

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo\ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F---H--H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict. [ABSTRACT FROM AUTHOR]

Measurements of the proton spin-lattice relaxation time (T1) carried out at 18.775 MHz in two H2–Ne mixtures are presented. Values for T1/ρ)∞lin, accurate to within 2% are deduced for ten temperatures between 99 and 296 K. These data are compared with values obtained from close-coupled scattering calculations for a Hartree–Fock plus damped dispersion (HFD) model potential energy surface, allowing a refined anisotropic contribution to be determined. [ABSTRACT FROM AUTHOR]

Gradient extremals are curves on potential energy surfaces connecting points where the derivative of the gradient norm, subject to the constraint that the energy is constant, is zero. The bifurcation of such curves is analyzed, and exemplified by tracing out some of the gradient extremals on the HF/STO-3G surface for H2CO. It is shown that gradient extremal following provides a semisystematic way of locating stationary points. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A three-dimensional description of vibrationally highly excited linear molecules is formulated in hyperspherical coordinates, based on a successive adiabatic reduction scheme. The method is applied to the low-lying and highly excited vibrational states of FHF-, a prototype of symmetric bihalide anions, which has attracted spectroscopic interest due to its peculiar vibrational anharmonicity. Ab initio potential energy surfaces (PESs) which cover the ground-state potential well of FHF- and/or its dissociation to the F-+HF channel have been obtained by using the coupled electron pair approach (CEPA) method. An hyperspherical calculation using the ab initio PES of the sixth-order Simons–Parr–Finlan analytical form has correctly reproduced the experimental fundamental frequencies. Specifically, the vibrationally highly excited FHF- above the dissociation threshold is proposed as a candidate for transition state spectroscopy (TSS) of unimolecular dissociation reactions without barrier. [ABSTRACT FROM AUTHOR]

A low energy quasiclassical trajectory study has been performed on three potential energy surfaces, obtained by modifying the recent ground state ab initio potential energy hypersurface by Wilhelmsson et al. for the N+(3P)+H2(1Σ+g)→NH+ (2Π)+H(2S) reaction. The importance of explicit consideration of the asymptotic ion–induced dipole and ion–quadrupole interactions in the entrance channel is demonstrated. Reaction cross sections as a function of collision energy (approximately in the range 0.06–0.4 eV in c.m.) are presented. [ABSTRACT FROM AUTHOR]

The relaxation of hydrogen and deuterium nuclei in HD–He gas mixtures is studied both experimentally and theoretically in the temperature range 90–300 K. A rationalization is given for the temperature dependence of the proton and the deuteron relaxations in terms of the relative strengths of the proton and deuteron intramolecular couplings and the role played by those HD molecules in the ground rotational state. Using a recent ab initio potential, quantitative agreement is found between the temperature dependence of the spin–lattice relaxation time of the proton in HD, as calculated theoretically and determined experimentally. A similar comparison between the calculated and experimental temperature dependence of the spin–lattice relaxation time of the deuteron in HD gave only semiquantitative agreement. It is suggested that the difference in quantitative agreement may be attributed to the selectivity of the respective predominant relaxation mechanisms to slightly different aspects of the anisotropic components of the interaction potential. [ABSTRACT FROM AUTHOR]

The rovibrational spectrum of trideutero-methane has been measured at resolutions mostly close to the Doppler limit on an interferometric Fourier transform spectrometer from the lowest fundamental vibration to high overtones of the CH stretching vibration (wave numbers from 900 to 12 000 cm-1). The CH chromophore spectrum is fully assigned and interpreted by means of the tridiagonal Fermi resonance Hamiltonian for the coupled CH stretching and bending vibrations. The Hamiltonian predicts and also fits the visible spectrum up to 19 000 cm-1 measured by Scherer et al., Perry et al., and Campargue et al. The effective tridiagonal Hamiltonian is derived ab initio by means of MRD-CI and full CI calculations of the potential surface of methane, a variational vibrational calculation in a normal coordinate subspace of the coupled CH stretching and bending motions and an approximate similarity transformation to tridiagonal form. Fits of the experimental results by the tridiagonal and the variational Hamiltonian lead to equivalent spectroscopic constants. A careful experimental estimate of the main Fermi resonance coupling constant gives k’sbb [bar_over_tilde:_approx._equal_to](30±15) cm-1 in agreement with the best current ab initio result (31 cm-1). The ab initio potential in polar normal coordinates agrees with the potential derived from spectroscopic data covering an energy range of about 220 kJ mol-1 (more than half the dissociation energy). Good predictions are obtained for the parameters of the effective Hamiltonian, the spectral patterns, intensity distributions, and rotational constants in the Fermi resonance polyads. Three alternative interpretations of the parameters of the effective Hamiltonian are investigated and rejected on the basis of the available experimental and ab initio data. The results and conclusions are discussed in relation to intramolecular vibrational redistribution on the subpicosecond time scale and the... [ABSTRACT FROM AUTHOR]

The potential energy surface for the H2–I2 system was determined in analytic variational electronic structure calculations with single- and double-substitution configuration interactions for the 16 valence electrons and with effective potentials for the iodine core electrons. The favored pathway for the overall reaction H2+I2→HI+HI was found to pass through the region of the collinear configuration I–H–H–I. The pathway is accessible to bound and unbound iodine atom pairs and it allows the bimolecular and termolecular reactions, H2+I2→HI+HI and H2+I+I→HI+HI, both proposed by Bodenstein 100 years ago. [ABSTRACT FROM AUTHOR]

Values of T1/ρ)∞lin are deduced from previously reported measurements of the proton spin–lattice relaxation time (T1) in two H2–Ar mixtures at ten temperatures between 110 and 296 K. These data are compared with close coupling calculations using the BC3(6.8) potential energy surface of Le Roy and Carley, and the TT3(6,8) potential of Le Roy and Hutson, as well as two minor modifications of the latter potential. Qualitative agreement between theory and experiment is achieved. [ABSTRACT FROM AUTHOR]

Investigates the potential energy surface for the reaction fluorine + hydrogen[sub2] →hydrogen fluorine+hydrogen. Demonstration of a way in which potential energies can be obtained for the system; Application of the random walk method for solving the Schrodinger equation to determine energies; Percentage of recovery of correlation energy.

Artificial neural networks are used to fit a potential energy surface (PES). We demonstrate the benefits of using not only energies but also their first and second derivatives as training data for the neural network. This ensures smooth and accurate Hessian surfaces, which are required for rate constant calculations using instanton theory. Our aim was a local, accurate fit rather than a global PES because instanton theory requires information on the potential only in the close vicinity of the main tunneling path. Elongations along vibrational normal modes at the transition state are used as coordinates for the neural network. The method is applied to the hydrogen abstraction reaction from methanol, calculated on a coupled-cluster level of theory. The reaction is essential in astrochemistry to explain the deuteration of methanol in the interstellar medium. [ABSTRACT FROM AUTHOR]

The 87GTO/corrected H3+ potential energy surface of Meyer, Botschwina, and Burton [J. Chem. Phys. 84, 891 (1986)] has been widely used for studies of the H3+ system. It transpires that two slightly different implementations of this surface are in common use. This observation accounts for a number of small discrepancies noted in studies of the higher vibrational states of the ion. [ABSTRACT FROM AUTHOR]

Accurate 3D quantum reaction probabilities for the F+H2→HF+H reaction are presented for the new 5SEC potential energy surface (PES) of Truhlar and co-workers. This PES was found to be more reactive, on the average, than the closely related 5A PES of Truhlar and co-workers. The resonance structure in the probabilities also differs between the two PESs. [ABSTRACT FROM AUTHOR]

Presents accurate collinear quantum reaction probabilities phosphorus[subn--m](E) for vibrational quantum numbers n≤7 and m≤7 for hydrogen (H) + H[sub2] on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) potential energy surface. Use of the Delves' coordinates in the calculations; Results for the LSTH surface.

Presents a study which examined rovibrational spectrum of the excited potential energy surface of helium and hydrogen. Method of the study; Results and discussion; Conclusion.

A six-dimensional ab initio potential energy surface (PES) for H2-N2O which explicitly includes the symmetric and asymmetric vibrational coordinates Q1 and Q3 of N2O is calculated at the coupled-cluster singles and doubles with noniterative inclusion of connected triple level using an augmented correlation-consistent polarized-valence quadruple-zeta basis set together with midpoint bond functions. Four-dimensional intermolecular PESs are then obtained by fitting the vibrationally averaged interactions energies for υ3(N2O) = 0 and 1 to the Morse/long-range analytical form. In the fits, fixing the long-range parameters at theoretical values smoothes over the numerical noise in the ab initio points in the long-range region of the potential. Using the adiabatic hindered-rotor approximation, two-dimensional PESs for hydrogen-N2O complexes with different isotopomers of hydrogen are generated by averaging the 4D PES over the rotation of the hydrogen molecule within the complex. The band-origin shifts for the hydrogen-N2O dimers calculated using both the 4D PESs and the angle-averaged 2D PESs are all in good agreement with each other and with the available experimental observations. The predicted infrared transition frequencies for para-H2-N2O and ortho-D2-N2O are also consistent with the observed spectra. [ABSTRACT FROM AUTHOR]

Diffusion Monte Carlo (DMC) and path-integral Monte Carlo computations of the vibrational ground state and 10 K equilibrium state properties of the H7+/D7+ cations are presented, using an ab initio full-dimensional potential energy surface. The DMC zero-point energies of dissociated fragments H5+(D5+)+H2(D2) are also calculated and from these results and the electronic dissociation energy, dissociation energies, D0, of 752 ± 15 and 980 ± 14 cm-1 are reported for H7+ and D7+, respectively. Due to the known error in the electronic dissociation energy of the potential surface, these quantities are underestimated by roughly 65 cm-1. These values are rigorously determined for first time, and compared with previous theoretical estimates from electronic structure calculations using standard harmonic analysis, and available experimental measurements. Probability density distributions are also computed for the ground vibrational and 10 K state of H7+ and D7+. These are qualitatively described as a central H3+/D3+ core surrounded by 'solvent' H2/D2 molecules that nearly freely rotate. [ABSTRACT FROM AUTHOR]

A detailed quasi-classical trajectory study of the H + CO2 → HO + CO reaction is reported on an accurate potential energy surface based on ab initio data. The influence of the vibrational and rotational excitations of CO2 was investigated up to the collision energy of 2.35 eV. It was found that the total reaction integral cross section increases monotonically with the collision energy, consistent with experimental results. The excitation of the CO2 bending vibration enhances the reaction, while the excitation in its asymmetric stretching vibration inhibits the reaction. The calculated thermal rate constants are in excellent agreement with experiment. At the state-to-state level, the rotational state distributions of the HO product are in good agreement with experimental results, while those for the CO product are much hotter than measurements. The calculated differential cross sections are dominated by forward scattering, suggesting that the lifetime of the HOCO intermediate may not be sufficiently long to render the reaction completely statistical. [ABSTRACT FROM AUTHOR]

The adiabatic global potential energy surface of the H[sub 3][sup +] system for the lowest triplet excited state of A[sup ′] symmetry was computed for an extensive grid of conformations around the minimum region at full configuration interaction ab initio level, using a much more extended basis set than in a previous paper from the same authors. An accurate global fit (rms error lower than 27 cm[sup -1] for energies lower than dissociation into separated atoms and lower than 5 cm[sup -1] for energies lower than the dissociation channel) to these ab initio points and also to part of the previous calculated points (for a total of 7689 energies in the data set) of the lowest triplet excited state of A[sup ′] symmetry is obtained using a diatomics-in-molecules approach corrected by one symmetrized three-body term with a total of 109 linear parameters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H[sub 3][sup +] including the minima and dissociation limits, satisfying the correct symmetry properties of the system. The vibrational eigenstates have been calculated using hyperspherical coordinates with symmetry adapted basis functions with the proper regular behavior at the Eckart singularities. The accuracy of the vibrational levels thus obtained is expected to be better than 2 cm[sup -1] with respect to unknown experimental values. Due to the presence of three equivalent minima at collinear geometries (D[sub ∞h]) the lower vibrational levels are close to triple degenerate. Since the interconversion barrier between the three minima is about 2640 cm[sup -1], these states split for the upper excited vibrational levels. Such splitting can provide a key feature to identifying the unassigned transitions amongst the many H[sub 3][sup +] lines that have been observed in hydrogen plasmas. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Combined-Hyperbolic-Inverse-Power-Representation method, which treats evenly both short- and long-range interactions, is used to fit an extensive set of ab initio points for HO2 previously utilized [Xu et al., J. Chem. Phys. 122, 244305 (2005)] to develop a spline interpolant. The novel form is shown to perform accurately when compared with others, while quasiclassical trajectory calculations of the O + OH reaction clearly pinpoint the role of long-range forces at low temperatures. [ABSTRACT FROM AUTHOR]

The second virial coefficient, B12(T), of the H2O-H2 system has been calculated ab initio over the temperature range 200-700 K. A semi-classical method was employed with two recent accurate potential energy surfaces. The agreement with experimental data is good, although experimental error bars are much larger than the theoretical uncertainties. We show that highly correlated potentials are required for an accuracy better than 30%. [ABSTRACT FROM AUTHOR]

Interpolated ab initio potential energy surfaces which describe abstraction and exchange reactions in collisions of hydrogen and water are reported. The electronic structure calculations are performed at the QCISD(T) level of theory, with an additivity approximation. A sufficiently large basis set is required to describe the Rydberg character of the electronic state for molecular configurations which are important for the exchange process. Classical and quantum dynamics calculations on the surfaces are presented. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An infrared-ultraviolet (IR-UV) double-resonance technique has been implemented to obtain the infrared spectrum of ArOH in the vicinity of the fundamental OH stretch at 2.8 μm. A rotationally resolved spectrum of the fundamental OH stretching band of ArOH is observed at 3567.85(1) cm-1 (origin). A combination band, involving both OH stretch and intermolecular bending excitation, is identified at 3577.00(1) cm-1 (origin). The intermolecular energy of the excited bending state provides a direct measure of the anisotropy of the Ar+OH X[sup 2]Π interaction potential. The rotational structure of the combination band reveals a large splitting between parity components with the same total angular momentum in the excited bending state [0.69(1) cm-1 for J=[FRACTION SHAPE="CASE"][NUM]3[/NUM][DEN]2[/DEN][/FRACTION]]. The experimentally derived parity splitting is compared with previous experimental and theoretical determinations of this parameter. The parity splitting associated with the excited bending state reflects the change in the intermolecular potential when the pπ orbital containing the unpaired electron of OH lies in or out of the ArOH plane. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Phosphorus is of particular interest in astrochemistry because it is a biogenic element together with hydrogen, carbon, nitrogen, oxygen, and sulfur. However, the chemical evolution of such element in the interstellar medium (ISM) is still far from an accurate characterization, with the chemistry of P-bearing molecules being poorly understood. To provide a contribution in this direction, we have carried out an accurate investigation of the potential energy surface for the reaction between the CP radical and methanimine (CH2NH), two species already detected in the ISM. In analogy to similar systems, i.e., CH2NH + X, with X = OH, CN, and CCH, this reaction can occur—from an energetic point of view—under the harsh conditions of the ISM. Furthermore, since the major products of the aforementioned reaction, namely, E- and Z-2-phosphanylidyneethan-1-imine (HN=CHCP) and N-(phosphaneylidynemethyl)methanimine (H2C=NCP), have not been spectroscopically characterized yet, some effort has been made for filling this gap by means of accurate computational approaches. [ABSTRACT FROM AUTHOR]

The gas-phase kinetics of S(³P) atoms with H2 and D2 have been studied via the laser flash photolysis—resonance fluorescence technique. S atoms were generated by pulsed photolysis of CS2 at 193 nm and monitored by time-resolved fluorescence at 181 nm. The rate coefficients for H2 (k1) and D2 (k2), respectively, are summarized as k1(600-1110 K) = 3.0 × 109 exp (-1.317×105-2.703×107K=T/8.314 T=K) cm³ molecule-1 s-1 and k2(770-1110 K) = 2.2 × 10-14 (T/298 K)3.55 exp(-5420 K/T) cm³ molecule-1 s-1. Error limits are discussed in the text. The rate coefficients for formation of SH(SD) + H(D) on a newly developed triplet potential energy surface were characterized via ring polymer molecular dynamics and canonical variational transition-state theory. There is excellent agreement above about 1000 K between theory and experiment. At lower temperatures, the experimental rate coefficient is substantially larger than the results computed for the adiabatic reaction, suggesting a significant role for intersystem crossing to the singlet potential energy surface at lower temperatures. [ABSTRACT FROM AUTHOR]

Presents a correction to the article 'Dissociation of H[sub2] on Cu(100): Dynamics on a new two-dimensional potential energy surface,' by G. Wiesenekker et al., from a 1995 issue of the 'Journal of Chemical Physics.'

The Diffusion Monte Carlo (DMC) method was applied to anionic hydrogen clusters H − ( H 2 ) n (n = 1–16, 32) and their deuterated analogs using a polarizable all-atom potential energy surface (PES) developed by Calvo and Yurtsever. For the hydrogen clusters, the binding energy ΔEn appears to be a smooth function of the cluster size n, thus contradicting the previous claim that n = 12 is a "magic number" cluster. The structures of the low energy minima of the PES for these clusters belong to the icosahedral motif with the H2 molecules aligned toward the central H− ion. However, their ground state wavefunctions are highly delocalized and resemble neither the structures of the global nor local minima. Moreover, the strong nuclear quantum effects result in a nearly complete orientational disordering of the H2 molecules. For the deuterium clusters, the ground state wavefunctions are localized and the D2 molecules are aligned toward the central D− ion. However, their structures are still characterized as disordered and, as such, do not display size sensitivity. In addition, DMC simulations were performed on the mixed H − ( H 2 ) n ( D 2 ) p clusters with (n, p) = (6, 6) and (16, 16). Again, in contradiction to the previous claim, we found that the "more quantum" H2 molecules prefer to reside farther from the central H− ion than the D2 molecules. [ABSTRACT FROM AUTHOR]

We report state-to-state quasi-classical trajectory rate constants for the D + H2 reactive collision, using the accurate H3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. High relative collision energies (up to ≈56 000 K) and high rovibrational levels of H2 (up to ≈50 000 K), relevant to various astrophysical media, are considered. The HD product molecule is formed in highly excited rovibrational states, over a wide collision energy range. The collision-induced dissociation channel (often overlooked in fully quantum reaction dynamics calculations) is found to be significantly populated, even at collision energies as low as 1500 K. [ABSTRACT FROM AUTHOR]

An explicit formulation of the rotational relaxation time in terms of state-to-state rate coefficients associated to inelastic collisions is reported. The state-to-state rates needed for the detailed interpretation of relaxation in H2 and D2, including isotopic variant mixtures, have been calculated by solving the close-coupling Schrödinger equations using the H2–H2 potential energy surface by Diep and Johnson [J. Chem. Phys.112, 4465 (2000)]. Relaxation related quantities (rotational effective cross section, bulk viscosity, relaxation time, and collision number) calculated from first principles agree reasonably well with acoustic absorption experimental data on H2 and D2 between 30 and 293 K. This result confirms at once the proposed formulation, and the validation of the H2–H2 potential energy surface employed, since no approximations have been introduced in the dynamics. Accordingly, the state-to-state rates derived from Diep and Johnson potential energy surface appear to be overestimated by up to 10% for H2, and up to 30% for D2 at T = 300 K, showing a better agreement at lower temperatures. [ABSTRACT FROM AUTHOR]

Rotationally resolved reaction probabilities, integral cross sections, and rate constant for the H+ + H2 (v = 0, j = 0 or 1) → H2 (v' = 0, j') + H + reaction are calculated using a timeindependent quantum mechanical method and the potential energy surface of Kamisaka et al. [J. Chem. Phys. 116,654 (2002)] (say KBNN PES). All partial wave contributions of the total angular momentum, J, are included to obtain converged cross sections at low collision energies and rate constants at low temperatures. In order to test the accuracy of the KBNN PES, the results obtained here are compared with those obtained in our earlier work [P. Honvault et al., Phys. Rev. Lett. 107,023201 (2011)] using the accurate potential energy surface of Velilla et al. [J. Chem. Phys. 129, 084307 (2008)]. Integral cross sections and rate constants obtained on the two potential energy surfaces considered here show remarkable differences in terms of magnitude and dependence on collision energy (or temperature) which can be attributed to the differences observed in the topography of the surfaces near to the entrance channel. This clearly shows the inadequacy of the KBNN PES for calculations at low collision energies. [ABSTRACT FROM AUTHOR]

Progress in the hydrogen fuel field requires a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen nuclei, any theoretical studies of these systems must be treated quantum dynamically. One class of material that has been examined in this context are dihydrogen complexes. Since their discovery by Kubas in 1984, many such complexes have been studied both experimentally and theoretically. This particular study examines the rotational dynamics of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 complex, allowing for full motion in both the rotational degrees of freedom and treating the quantum dynamics (QD) explicitly. A "gas-phase" global potential energy surface is first constructed using density functional theory with the Becke, 3-parameter, Lee-Yang-Parr functional; this is followed by an exact QD calculation of the corresponding rotation/libration states. The results provide insight into the dynamical correlation of the two rotation angles as well as a comprehensive analysis of both ground- and excited-state librational tunneling splittings. The latter was computed to be 6.914 cm-1--in excellent agreement with the experimental value of 6.4 cm-1. This work represents the first full-dimensional ab initio exact QD calculation ever performed for dihydrogen ligand rotation in a coordination complex. [ABSTRACT FROM AUTHOR]

A quasi-classical study of the endoergic Au+(1S) + H2(X1Σg+) → AuH+ (2Σ+) + H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au++H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion. [ABSTRACT FROM AUTHOR]

Seven-dimensional time-dependent wave packet calculations were carried out to study the title reaction on the potential energy surface constructed recently by the group [Y. Zhou, B. Fu, C. Wang, M. A. Collins, and D. H. Zhang, J. Chem. Phys. 134, 064323 (2011)]. Total reaction probabilities and integral cross sections were calculated for a number of initial vibration states. It is found that the fundamental C-H stretching excitation can promote the reaction dramatically. At E = 1.53 eV, it enhances the integral cross section by a factor of 13.2, which is in good agreement with the quasiclassical trajectory result on the ZBB1 potential energy surface, but larger than that on the the EG-2002 potential energy surface. The thermal rate constants were obtained for the title reaction by taking into account the contributions from all relevant initial vibration states. It is found that the ground initial state has a dominant contribution to the thermal rate constant at low temperature region. As the temperature increases, the relative contribution to the thermal rate constant from the ground initial state decreases quickly, and those from the stretching and bending excited states increases substantially. It is estimated that the C-H stretching mode contributes about 40% of the thermal rate constant at T = 1000 K despite the fact that the overall population for stretching excited states is tiny. [ABSTRACT FROM AUTHOR]

In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH4→ H2 + CH3. The present calculations were carried out using a modified and recalibrated version of the Jordan-Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom-diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. [ABSTRACT FROM AUTHOR]

We extend to full dimensionality a recently developed wave packet method [M. T. Cvitasˇ and S. C. Althorpe, J. Phys. Chem. A 113, 4557 (2009)] for computing the state-to-state quantum dynamics of AB + CD → ABC + D reactions and also increase the computational efficiency of the method. This is done by introducing a new set of product coordinates, by applying the Crank-Nicholson approximation to the angular kinetic energy part of the split-operator propagator and by using a symmetry-adapted basis-to-grid transformation to evaluate integrals over the potential energy surface. The newly extended method is tested on the benchmark OH + H2 → H2O + H reaction, where it allows us to obtain accurately converged state-to-state reaction probabilities (on the Wu-Schatz-Fang-Lendvay-Harding potential energy surface) with modest computational effort. These methodological advances will make possible efficient calculations of state-to-state differential cross sections on this system in the near future. [ABSTRACT FROM AUTHOR]

We report quantum dynamics calculations of rotational and vibrational energy transfer in collisions between two para-H2 molecules over collision energies spanning from the ultracold limit to thermal energies. Results obtained using a recent full-dimensional H2-H2 potential energy surface (PES) developed by Hinde [J. Chem. Phys. 128, 154308 (2008)] are compared with those derived from the Boothroyd, Martin, Keogh, and Peterson (BMKP) PES [J. Chem. Phys. 116, 666 (2002)]. For vibrational relaxation of H2(v=1,j=0) by collisions with H2(v=0,j=0) as well as rotational excitations in collisions between ground state H2 molecules, the PES of Hinde is found to yield results in better agreement with available experimental data. A highly efficient near-resonant energy transfer mechanism that conserves internal rotational angular momentum and was identified in our previous study of the H2-H2 system [Phys. Rev. A 77, 030704(R) (2008)] using the BMKP PES is also found to be reproduced by the Hinde PES, demonstrating that the process is largely insensitive to the details of the PES. In the absence of the near-resonance mechanism, vibrational relaxation is driven by the anisotropy of the potential energy surface. Based on a comparison of results obtained using the Hinde and BMKP PESs with available experimental data, it appears that the Hinde PES provides a more accurate description of rotational and vibrational transitions in H2-H2 collisions, at least for vibrational quantum numbers v <= 1. [ABSTRACT FROM AUTHOR]

We report the first six-dimensional quantum dynamical study of the dissociative adsorption of H[sub 2] on a (110) surface. We have performed quantum coupled-channel calculations for the system H[sub 2]/Pd(110) based on a potential energy surface (PES) that was derived from ab initio electronic structure calculations. In particular, we have focused on the effects of the corrugation and anisotropy of the PES on the H[sub 2] dissociation probability. Our results agree well with the available experimental data for the sticking probability as a function of the initial kinetic energy and the angle of incidence. Because of the coupling between the anisotropy and corrugation of the potential energy surface our calculations predict an unusual rotational heating and a rather small rotational alignment in desorption. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper presents fully converged integral cross sections for the D[sub 2](j[sub 1]=0,2,4)+OH(j[sub 2]=0,2,4)→HOD+D reaction on the YZCL2 potential energy surface. It is found that the influence of reagent rotation excitation on reactivity is relatively mild compared with that of the D[sub 2] vibration excitation, but is fairly important to the thermal rate constant at low temperature. In general, the OH rotation excitation tends to hinder the reaction, and the D[sub 2] rotation excitation tends to enhance the reactivity. The simultaneous reagent rotation has some correlated effect, with the influence of the D[sub 2] rotation excitation considerably dependent on the OH rotation excitation. It is also found that the reaction is strongly dependent on j[sub 12] (the coupled angular momentum between j[sub 1] and j[sub 2]), K[sub 0] (the projection of the reagent rotation on the body fixed axis), and the parity of the system, in particular for either the OH or D[sub 2] rotationally excited initial states, but this dependence becomes relatively weaker as the collision energy increases. Excellent agreement achieved between theory and experiment on the thermal rate coefficients for temperatures up to 400 K clearly shows that the YZCL2 potential energy surface is capable of producing “quantitatively” accurate results for the title reaction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report an investigation of the translation-rotation (TR) level structure of H2 entrapped in C60, in the rigid-monomer approximation, by means of a low-order perturbation theory (PT). We focus in particular on the degree to which PT can accurately account for that level structure, by comparison with the variational quantum five-dimensional calculations. To apply PT to the system, the interaction potential of H2@C60 is decomposed into a sum over bipolar spherical tensors. A zeroth-order Hamiltonian, Ħ 0, is then constructed as the sum of the TR kinetic-energy operator and the one term in the tensor decomposition of the potential that depends solely on the radial displacement of the H2 center of mass (c.m.) from the cage center. The remaining terms in the potential are treated as perturbations. The eigenstates of Ħ0, constructed to also account for the coupling of the angular momentum of the H2 c.m. about the cage center with the rotational angular momentum of the H2 about the c.m., are taken as the PT zeroth-order states. This zeroth-order level structure is shown to be an excellent approximation to the true one except for two types of TR-level splittings present in the latter. We then show that first-order PT accounts very well for these splittings, with respect to both their patterns and magnitudes. This allows one to connect specific features of the level structure with specific features of the potential-energy surface, and provides important new physical insight into the characteristics of the TR level structure. [ABSTRACT FROM AUTHOR]

Fundamental details concerning the interaction between H2 and CH3-Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H2 and D2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H2 compared to the strong RID features observed for D2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH3-Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H2 and D2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H2 (D2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H2(D2)/CH3-Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems. [ABSTRACT FROM AUTHOR]

We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling were applied using a fitted potential energy surface [J. Chen et al., J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval 4·10-20 to 4·10-17 cm³ s-1, demonstrating that even deuterated versions of the title reaction are possibly relevant to astrochemical processes in molecular clouds. The transferred hydrogen atom dominates the kinetic isotope effect at all temperatures. [ABSTRACT FROM AUTHOR]

To fast and accurately compute rate coefficients of the H/D + CH4→H2/HD + CH3 reactions, we propose a segmented strategy for fitting suitable potential energy surface (PES), on which ring-polymer molecular dynamics (RPMD) simulations are performed. On the basis of recently developed permutation invariant polynomial neural-network approach [J. Li et al., J. Chem. Phys. 142, 204302 (2015)], PESs in local configuration spaces are constructed. In this strategy, global PES is divided into three parts, including asymptotic, intermediate, and interaction parts, along the reaction coordinate. Since less fitting parameters are involved in the local PESs, the computational efficiency for operating the PES routine is largely enhanced by a factor of ~20, comparing with that for global PES. On interaction part, the RPMD computational time for the transmission coefficient can be further efficiently reduced by cutting off the redundant part of the child trajectories. For H + CH4, good agreements among the present RPMD rates and those from previous simulations as well as experimental results are found. For D + CH4, on the other hand, qualitative agreement between present RPMD and experimental results is predicted. [ABSTRACT FROM AUTHOR]

The correct description of nondynamic correlation by electronic structure methods not belonging to the multireference family is a challenging issue. The transition of D2h to D4h symmetry in H4 molecule is among the most simple archetypal examples to illustrate the consequences of missing nondynamic correlation effects. The resurgence of interest in density matrix functional methods has brought several new methods including the family of Piris Natural Orbital Functionals (PNOF). In this work, we compare PNOF5 and PNOF6, which include nondynamic electron correlation effects to some extent, with other standard ab initio methods in the H4 D4h/D2h potential energy surface (PES). Thus far, the wrongful behavior of single-reference methods at the D2h-D4h transition of H4 has been attributed to wrong account of nondynamic correlation effects, whereas in geminal-based approaches, it has been assigned to a wrong coupling of spins and the localized nature of the orbitals. We will show that actually interpair nondynamic correlation is the key to a cusp-free qualitatively correct description of H4 PES. By introducing interpair nondynamic correlation, PNOF6 is shown to avoid cusps and provide the correct smooth PES features at distances close to the equilibrium, total and local spin properties along with the correct electron delocalization, as reflected by natural orbitals and multicenter delocalization indices. [ABSTRACT FROM AUTHOR]

We have measured the temperature-dependent kinetics for the reactions of OH+ with H2 and D2 using a selected ion flow tube apparatus. Reaction occurs via atom abstraction to result in H2O+/HDO+ + H/D. Room temperature rate coefficients are in agreement with prior measurements and resulting temperature dependences are T0.11 for the hydrogen and T0.25 for the deuterated reactions. This work is prompted in part by recent theoretical work that mapped a full-dimensional global potential energy surface of H3O+ for the OH+ + H2 → H + H2O+ reaction [A. Li and H. Guo, J. Phys. Chem. A 118, 11168 (2014)], and reported results of quasi-classical trajectory calculations, which are extended to a wider temperature range and initial rotational state specification here. Our experimental results are in excellent agreement with these calculations which accurately predict the isotope effect in addition to an enhancement of the reaction rate constant due to the molecular rotation of OH+. The title reaction is of high importance to astrophysical models, and the temperature dependence of the rate coefficients determined here should now allow for better understanding of this reaction at temperatures more relevant to the interstellar medium. [ABSTRACT FROM AUTHOR]

Quantum and anharmonic effects are investigated in (H2)2-Li+-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials, using rigid-body diffusion Monte Carlo (RBDMC) simulations. The potential-energy surface (PES) is calculated as a modified Shepard interpolation of M05-2X/6-311+G(2df,p) electronic structure data. The RBDMC simulations yield zero-point energies (ZPE) and probability density histograms that describe the groundstate nuclear wavefunction. Binding a second H2 molecule to the H2-Li+-benzene complex increases the ZPE of the system by 5.6 kJ mol-1 to 17.6 kJ mol-1. This ZPE is 42% of the total electronic binding energy of (H2)2-Li+-benzene and cannot be neglected. Our best estimate of the 0 K binding enthalpy of the second H2 to H2-Li+-benzene is 7.7 kJ mol-1, compared to 12.4 kJ mol-1 for the first H2 molecule. Anharmonicity is found to be even more important when a second (and subsequent) H2 molecule is adsorbed; use of harmonic ZPEs results in significant error in the 0 K binding enthalpy. Probability density histograms reveal that the two H2 molecules are found at larger distance from the Li+ ion and are more confined in the coordinate than in H2-Li+-benzene. They also show that both H2 molecules are delocalized in the azimuthal coordinate, f. That is, adding a second H2 molecule is insufficient to localize the wavefunction in f. Two fragment-based (H2)2-Li+-benzene PESs are developed. These use a modified Shepard interpolation for the Li+-benzene and H2-Li+-benzene fragments, and either modified Shepard interpolation or a cubic spline to model the H2-H2 interaction. Because of the neglect of three-body H2, H2, Li+ terms, both fragment PESs lead to overbinding of the second H2 molecule by 1.5 kJ mol-1. Probability density histograms, however, indicate that the wavefunctions for the two H2 molecules are effectively identical on the "full" and fragment PESs. This suggests that the 1.5 kJ mol-1 error is systematic over the regions of configuration space explored by our simulations. Notwithstanding this, modified Shepard interpolation of the weak H2-H2 interaction is problematic and we obtain more accurate results, at considerably lower computational cost, using a cubic spline interpolation. Indeed, the ZPE of the fragment-with-spline PES is identical, within error, to the ZPE of the full PES. This fragmentation scheme therefore provides an accurate and inexpensive method to study higher hydrogen loading in this and similar systems. [ABSTRACT FROM AUTHOR]

It has been established from earlier ab initio calculations that in its equilibrium configuration the CH+5 molecular ion consists of an H2 moiety bound to the apex of a pyramidal CH+3 group; the H2 group is approximately perpendicular to the C3 axis of the CH+3 group, and the binding energy is about 15000 cm-1. Two internal motions, the torsion and the flip, provide connections with low barriers between all 120 symmetrically equivalent minima on the potential energy surface so that all proton permutations are feasible. We present the results of new high level ab initio calculations of the parts of the potential energy surface associated with these two motions, and in particular we determine the continually optimized structure, and associated electronic energy, for the CH+5 molecular ion as it undergoes the flip motion. For the flip motion we numerically integrate the one-dimensional Schrödinger equation for the tunneling to determine the splitting. Since this splitting is small (1.4 cm-1) we can incorporate it into a 120×120 matrix treatment of the simultaneous torsion-flip dynamics to determine the energy level splitting pattern in the J=0 and 1 states, in the approximation of neglecting other tunneling pathways, and we calculate the positions of the lines in the J=1←0 millimeter wave spectrum. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Calculations are reported of hydrogen and deuterium tunneling splittings in the ground state S0 (X,1A1) and the first excited singlet state S1 (A,1B2) of tropolone-d0 and -d1. The main focus of the calculations is on the splittings observed in vibrationally excited levels of S1, some of which are larger while others are smaller than the zero-point splitting. To account for these observations, a potential-energy surface is constructed by standard quantum-chemical methods and the dynamics on this surface is treated by a method derived from the instanton approach. The potential-energy surface is a complete multidimensional surface resulting from the combination of a potential-energy curve along the tunneling coordinate with a harmonic force field calculated at the stationary points. The level of calculation adopted is HF/6-31G** for S0 and CIS/6-31G** for S1. A few other, nominally more accurate, methods were tried but proved to be unsatisfactory. To deal with the dynamics, the instanton method, used previously for the calculation of zero-point level splittings, is modified so as to make it applicable to excited levels. As expected, it is found that excitation of the tunneling mode strongly promotes hydrogen transfer. The effects of exciting modes that are symmetric or antisymmetric with respect to the symmetric transition state are evaluated for all such modes with assigned splittings by a straightforward generalization of the correction terms previously derived for zero-point splittings. Of special interest are out-of-plane modes, some of which show up as overtones with splittings smaller than the zero-point splitting, despite the fact that there is no linear coupling between these modes and the tunneling mode. The effect is ascribed to anharmonic coupling and an effort is made to calculate the required anharmonicities quantum-chemically. In general the agreement between theory and experiment is satisfactory for modes that are linearly coupled while the... [ABSTRACT FROM AUTHOR]

We show in this paper a time-dependent (TD) quantum wave packet calculation for the combustion reaction H+O2 using the DMBE IV (double many-body expansion) potential energy surface which has a deep well and supports long-lived resonances. The reaction probabilities from the initial states of H+O2(3Σ-g) (v=0–3, j=1) for total angular momentum J=0 are obtained for scattering energies from threshold up to 2.5 eV, which show numerous resonance features. Our results show that, by carrying out the wave packet propagation to several picoseconds, one can resolve essentially all the resonance features for this reaction. The present TD results are in good agreement with other time-independent calculations. A particular advantage of the time-dependent approach to this reaction is that resonance structures—strong energy dependence of the reaction probability—can be mapped out in a single wave packet propagation without having to repeat scattering calculations for hundreds of energies. We also report calculations of some low-lying vibrational energies of the hydroperoxyl radical HO2(2A‘) and their spectroscopic assignments. The vibrational frequencies of HO2(2A‘) on the DMBE IV potential energy surface are lower than experimental values, indicating the need to further improve the accuracy of the potential energy surface. [ABSTRACT FROM AUTHOR]