The largest angle generalization of the rotating bond order potential: Three different atom reactions

Closer versus Long Range Interaction Effects on the Non-Arrhenius Behavior of Quasi-Resonant O2 + N2 Collisions

We report in this paper an investigation on energy transfer processes from vibration to vibration and/or translation in thermal and subthermal regimes for the O₂ + N₂ system performed using quantum-classical calculations on different empirical, semiempirical, and ab initio potential energy surfaces. In particular, the paper focuses on the rationalization of the non-Arrhenius behavior (inversion of the temperature dependence) of the quasi-resonant vibration-to-vibration energy transfer transition rate coefficients at threshold. To better understand the microscopic nature of the involved processes, we pushed the calculations to the detail of the related cross sections and analyzed the impact of the medium and long-range components of the interaction on them. Furthermore, the variation with temperature of the dependence of the quasi-resonant rate coefficient on the vibrational energy gap between initial and final vibrational states and the effectiveness of quantum-classical calculations to overcome the limitations of the purely classical treatments were also investigated. These treatments, handled in an open molecular science fashion by chaining data and competencies of the various laboratories using a grid empowered molecular simulator, have allowed a rationalization of the dependence of the computed rate coefficients in terms of the distortion of the O₂-N₂ configuration during the diatom-diatom collisions. A way of relating such distortions to a smooth and continuous progress variable, allowing a proper evolution from both long to closer range formulation of the interaction and from its entrance to exit channel (through the strong interaction region) relaxed graphical representations, is also discussed in the paper.

An Efficient and Accurate Formalism for the Treatment of Large Amplitude Intramolecular Motion

We propose a general approach to describe large amplitude motions (LAM) with multiple degrees of freedom (DOF) in molecules or reaction intermediates, which is useful for the computation of thermochemical or kinetic data. The kinetic part of the LAM Lagrangian is derived using a Z-matrix internal coordinate representation within a new numerical procedure. This derivation is exact for a classical system, and the uncertainties on the prediction of observable quantities largely arise from uncertainties on the LAM potential energy surface (PES) itself. In order to rigorously account for these uncertainties, we present an approach based on Bayesian theory to infer a parametrized physical model of the PES using ab initio calculations. This framework allows for quantification of uncertainties associated with a PES model as well as the forward propagation of these uncertainties to the quantity of interest. A selection and generalization of some treatments accounting for the coupling of the LAM with other internal or external DOF are also presented. Finally, we discuss and validate the approach with two applications: the calculation of the partition function of 1,3-butadiene and the calculation of the high-pressure reaction rate of the CH₃ + H → CH₄ recombination.

ACCURATE AND HIGHLY EFFICIENT CALCULATION OF THE O(1D)HCl VIBRATIONAL BOUND STATES, USING A COMBINATION OF METHODS

Hypochlorous acid, HOCl, is an important intermediate in the O (1D) HCl reactive system. Due in part to a large number of vibrational bound states (over 800), extremely large direct product basis sets (around 300,000) are required to compute the energy levels just below the dissociation threshold. This situation, combined with a very high density of states, results in difficult convergence for iterative methods — e.g. Lanczos requires 50,000 iterations, and filter diagonalization uses 60,000 iterations. In contrast, using new methodologies, we are able to compute the highest-lying bound states with only 271 iterations, although the CPU cost per iteration is substantially greater. Lower lying states are also computed, for a fraction of the CPU cost of the highest energy calculation.

The shape of the potential energy surface and the thermal rate coefficients of the N + N2 reaction

Full-dimensional quantum time-dependent calculations of the detailed probabilities of the N + N2 reaction have been performed on different potential energy surfaces, initial quantum states, and total angular momentum quantum numbers. The calculations allowed a rationalization of the effect of both moving the saddle to reaction out of collinearity and lowering its height. On some of these surfaces, more extended studies of the reactive dynamics of the system were performed. On one of them also, thermal rate coefficients were computed using J = 0 quantum probabilities and the J-shift model after testing the applicability of such a model against centrifugal sudden results. A comparison of the calculated thermal rate coefficients with theoretical and experimental data available from the literature is also made, and possible effects of inserting an intermediate well at the top of the saddle are argued.

Dynamics of Bimolecular Reactions of Vibrationally Highly Excited Molecules: Quasiclassical Trajectory Studies

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.

Heavy atom tunneling in chemical reactions: Study of H+LiF collisions

The H+LiF(X (1)sigma(+),upsilon=0-2,j=0)-->HF(X (1)sigma(+),upsilon',j')+Li(2S) bimolecular process is investigated by means of quantum scattering calculations on the chemically accurate X 2A' LiHF potential energy surface of Aguado et al. [A. Aguado, M. Paniagua, C. Sanz, and J. Roncero, J. Chem. Phys. 119, 10088 (2003)]. Calculations have been performed for zero total angular momentum for translational energies from 10(-7) to 10(-1) eV. Initial-state selected reaction probabilities and cross sections are characterized by resonances originating from the decay of metastable states of the H...F-Li and Li...F-H van der Waals complexes. Extensive assignment of the resonances has been carried out by performing quasibound states calculations in the entrance and exit channel wells. Chemical reactivity is found to be significantly enhanced by vibrational excitation at low temperatures, although reactivity appears much less favorable than nonreactive processes due to the inefficient tunneling of the relatively heavy fluorine atom strongly bound in van der Waals complexes.

Quantum dynamics of the Li+HF→H+LiF reaction at ultralow temperatures

Quantum-mechanical calculations are reported for the Li+HF(v=0,1,j=0)-->H+LiF(v',j') bimolecular scattering process at low and ultralow temperatures. Calculations have been performed for zero total angular momentum using a recent high-accuracy potential-energy surface for the X2A' electronic ground state. For Li+HF(v=0,j=0), the reaction is dominated by resonances due to the decay of metastable states of the Li cdots,...F-H van der Waals complex. Assignment of these resonances has been carried out by calculating the eigenenergies of the quasibound states. We also find that while chemical reactivity is greatly enhanced by vibrational excitation, the resonances get mostly washed out in the reaction of vibrationally excited HF with Li atoms. In addition, we find that at low energies, the reaction is significantly suppressed due to the less-efficient tunneling of the relatively heavy fluorine atom.

Accurate and highly efficient calculation of the highly excited pure OH stretching resonances of O(1D)HCl, using a combination of methods

Accurate calculation of the energies and widths of the resonances of HOCl--an important intermediate in the O(1D)HCl reactive system--poses a challenging benchmark for computational methods. The need for very large direct product basis sets, combined with an extremely high density of states, results in difficult convergence for iterative methods. A recent calculation of the highly excited OH stretch mode resonances using the filter diagonalization method, for example, required 462,000 basis functions, and 180,000 iterations. In contrast, using a combination of new methods, we are able to compute the same resonance states to higher accuracy with a basis less than half the size, using only a few hundred iterations-although the CPU cost per iteration is substantially greater. Similar performance enhancements are observed for calculations of the high-lying bound states, as reported in a previous paper [J. Theo. Comput. Chem. 2, 583 (2003)].

EXPERIMENTS ON THE DYNAMICS OF MOLECULAR PROCESSES: A Chronicle of Fifty Years

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▪ Abstract  This paper reviews the way in which, in the Italy of the years immediately after World War II, interest in the dynamics of molecular processes was awakened. The narrative begins with the work of a small number of chemists and physicists who, in the initial stage, interacted closely. In the course of the years, their interests diverged and younger people joined the newly formed groups. Even now, after half a century, a common approach can still to be seen regarding how to attack problems and perform experiments. Experimental work is discussed, bringing out the common viewpoint of fields as diverse as mass spectrometry, isotope effects, chemical kinetics, molecular beams, molecule-molecule interactions, molecule-ion interactions, molecule-surface interactions, and plasma chemistry.