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Vibrational energy transfer from hot pyrazine (E‘ = 40 322 cm-1) to cold CO is modeled using classical trajectories. Collisional energy transfer properties are studied as a function of the initial rotational state J‘ of the CO, the length of the CO, the energy E‘ in pyrazine, the relative kinetic energy, the temperature, isotopic substitution on pyrazine, and the intermolecular potential. The energy transfer probability function P(E,E‘) exhibits distinct deviation from single exponential behavior. Collisions that transfer particularly large energy are associated with large amplitude out-of-plane motion of a C−H bond, imparting a kick to the departing CO. Slower collisions are particularly effective in relaxing pyrazine; faster collisions can add energy as often as they remove it. Energy transfer properties also depend on the initial rotational state of the CO. Temperature effects on 〈E − E‘〉 are weak in the 200−500 K range; this results from an increase in the magnitudes of both 〈ΔE〉down and 〈ΔE〉up. The f...

Classical trajectory calculations for collisions between vibrationally hot pyrazine (E = 40332 cm-1) and room-temperature CO are compared with recent diode laser experiments. Ab initio calculations were carried out to determine parameters for the pairwise Lennard-Jones intermolecular potential. The trajectory results appear to show good qualitative agreement with preliminary experimental results. The highest ΔE collisions occur when the CO happens to lie above the pyrazine plane just as the underlying CH wag executes an unusually high amplitude motion. Artificially doubling the length of the CO produces results for the angular momentum transfer to the CO molecule that look quite similar to those for angular momentum transfer to CO2 in analogous pyrazine−CO2 experiments.

Quasiclassical trajectories have been used to investigate the heavy−light−heavy ion−molecule reaction, O- + HF → OH + F-, using approximate potential energy surfaces. Multiple surface effects are neglected. 1D and 3D dynamics on the same surface produce very different vibrational distributions. OH product vibrational distributions are relatively insensitive to details of the potential. Product rotational energy distributions, on the other hand, are quite sensitive to features of the potential surface, particularly the steepness of the bend. Long-range ion−dipole forces play an important role in the dynamics, particularly in the angular distributions. Product vibrational energy and angular distributions are compared to two sets of experiments.

A series of model potential energy surfaces for the reaction O3+NO→O2+NO2 is described. Nuclear geometry is unconstrained: The surfaces are nine dimensional. However, the possible interaction of more than one electronic surface has not been included. The model surfaces include various exoergicities, transition state geometries, and overall reaction topologies. Classical trajectories have been used to describe the dynamics on these surfaces. A variety of initial conditions, including selective excitation of vibrational modes of the ozone molecule, has been included. While translational and vibrational energy have distinctive effects on the reaction, no mode‐specific enhancement of the reaction cross section was observed for any of the model functions. This is interpreted to result from the high degree of coupling of the internal coordinates to the reaction path and to the multidimensional curvature of the reaction coordinate. The results suggest caution in using prior studies of triatomic exchange reaction...

The three adiabatic surfaces needed to describe Ar+ + H2 in the nonrelativistic limit are calculated using the diatomics‐in‐molecules with zero overlap approach. Approximate nonadiabatic interaction terms which couple these surfaces are computed. Using these terms, a first‐order semiclassical calculation of the electronic transition probabilities is carried out. The results imply that all the important nonadiabatic transitions are localized in the reactant channel around an avoided crossing between the surfaces formed from combination of Ar(1S)+H2+(2Σg) and of Ar+(2P)+H2(1Σ). Because the results show that the transitions are induced by vibrational motion of the diatomic, any realistic study of the dynamics must include the full three dimensionality of the problem. We investigated the dynamics using the 3D trajectory surface hopping model proposed by Tully and Preston. Ar+ + H2 (v=0) was studied at a relative energy of 3.36 eV. The predictions of the model for cross sections, average final translational en...

Rate constants for rotational excitation of CO by collisions with Ar atoms have been computed within the infinite order sudden (IOS) approximation and compared with values from exact classical trajectories. For CO-Ar, as for COHe, accurate values for R (O → j ) - at least for small and intermediate j - may be obtained by taking the geometric average of rates computed assuming the IOS energy is the initial energy for upward O → j transitions and the initial energy for downward j → O transitions. however, especially for higher j , it appears preferable to interpret the IOS energy as the final kinetic energy. The best overall results are probably obtained by averaging rates computed assuming the IOS energy is the final energy for upward transitions and the final enery for downward transitions.

The reaction Be+HF(v,J)→BeF(v′,J′)+H in three dimensions is studied using the quasiclassical trajectory method. The surface was recently calculated using ab initio techniques. The surface has a high barrier and a noncollinear transition state. The angular dependence of the surface is weak over a fairly wide range of angles. There is a deep potential well representing the stable molecule HBeF. We have explored the effects of reagent translation, vibration, and rotation on the reaction. The surface exhibits a strong preference for product translation, particularly near threshold. Collisions which pass near the deep potential well make a significant contribution to the reaction only when the HF molecule is internally excited. The dynamics of these collisions are in sharp contrast to the more direct ones. These results are related to recent theoretical work on the LiFH system and to experimental work on alkaline earth‐hydrogen halide reactions.

The potential energy surface for collinear Be+FH→BeF+H has been studied at various levels of ab initio approximation. A final surface was obtained from a first order configuration interaction wavefunction, using the iterative natural orbital method and a medium‐sized basis set of Slater atomic functions; this is expected to give a semiquantitative description of the reactive process. The exothermicity is computed to be 6 kcal/mole which can be compared with the best experimental value of 2±4 kcal/mole. The barrier height is predicted to be 28 kcal/mole at a geometry where both internuclear separations are extended by about 0.4 bohr from their asymptotic equilibrium values. This surface differs qualitatively from simple LEPS models. The curvature of the reaction path is much more abrupt, the atom effecting little distortion of the partner molecule until quite close approach in both entrance and exit channels. The surface was fit with bicubic splines and dynamics was studied by the quasiclassical trajectory...

Both charge transfer and chemical reaction are studied for the reactants Ar++H2, Ar+H+2, and Ar+D+2, using the trajectory‐surface‐hopping model with diatomics‐in‐molecules 2A’ surfaces for ArH+2. Results are compared with a number of recent experiments. Agreement with experiment is generally satisfactory. The reactions are direct. The Ar++H2 → ArH++H reaction is well characterized as a stripping process. Charge transfer occurs predominantly by long‐range electron jump. The Ar+H+2 and Ar+D+2 cross sections depend sensitively on reactant vibration, rising sharply from v=0 to v=1, and falling gradually for v≥2. The ArH+ product is rotationally hot. Points of disagreement with experiment are discussed in the light of the approximations in the surface and the TSH model.

Energy distributions in the products of the ion-molecule reaction Cl − + HBr → HC1 + Br − have been studied using quasiclassical trajectories on a semi-empirical collinear potential energy surface. Vibrational energy is favored in the products. While some trajectories are long-lived, the kinematic factor of the light central atom prevents effective energy redistribution.

An empirically calibrated potential energy surface, obtained previously [J. Chem. Phys. 61, 21 (1974)], was used in a trajectory study of the effect of reactant energy partitioning on cross section for CH3 + H2 → CH4 + H. Artificial variations in the barrier position, and artificial and naturally occuring changes in the H masses, were also made. For natural CH3 + H2, H2 vibration enhances and CH3 out‐of‐plane bending depresses the cross section, at constant (25 kcal) translational energy of approach. The isotope substitution and barrier position effects are more complex and not predictable from triatomic A + BC generalizations. The relationship of these results to experimental ones is discussed.

Three body classical trajectory calculations are used to investigate bond energy effects in the hot atom reactions of tritium atom with a series of hydrocarbons. The surface for T + CH3–H is a modification of that used by Polanyi and co‐workers. The hydrocarbon series is represented by changing the bond dissociation energy, bond length, and mass only. Other potential parameters are fixed. It is found that the experimental ratios of abstraction yields are reproduced quite well theoretically at the single tritium atom energy of 2.8 eV. Of the parameters varied, only the bond dissociation energy had a significant effect on the abstraction ratios. These results lead us to support the high energy model for these effects.

A semiclassical vibrationally adiabatic theory is applied to the collinear gas phase proton transfer reaction Cl − + HCl → ClH + Cl − . It is found that tunneling is negligible in the reaction and that the principal dynamic features are governed by the deep potential for the reactants. Evidence is presented that quasiclassical trajectory dynamics is unreliable for this collinear system, in that the trajectories exhibit marked nonadiabaticity - i.e. complex formation in which there is significant energy transfer between the proton motion and the relative motion of the two chloride ions. It is argued that, by contrast, the quantum collinear reaction is adiabatic and direct in character. Comparison with recent experimental results suggests that development of a full three-dimensional theory may be necessary for an adequate picture of this reaction.

Six-particle T+CD/sub 4/ trajectory studies show that bond energy effects on H abstraction, previously explored with a 3-particle LEPS potential, arise from correlation of barrier height with exoergicity. (AIP)

Rates for excitation of CO, CS, OCS, and HC3N by collisions with He atoms and H2 molecules are presented. These have been obtained from extensive theoretical calculations using methods of known reliability. In general, rates necessary for modeling interstellar molecular clouds with kinetic temperatures to 100 K have been included. Methods for fitting these rates in more compact form are discussed.

Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO–He, CS–H2, OCS–H2, HCl–He, and HCl–Ar at collision energies up to 500 cm−1. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.