We present a quasiclassical trajectory study of the H+CO2 reaction dynamics, with emphasis on product angular and translational distributions, and OH angular momentum alignment. A new potential surface has been developed for this study, based on modifications of a previously developed full dimensional empirical HCO2 potential surface. The modifications include correcting errors that cause the HO⋅⋅⋅CO dissociation barrier to be too loose, and adjusting the depth of the HCO2 minimum and the heights of several barriers, in order to bring them into agreement with their best estimates determined from ab initio calculations. We compare cross sections, energy partitioning, and mechanistic information calculated using the unmodified and modified surfaces with experimental results. Results from the modified surface improve the comparison with experiment for the product OH energy partitioning, but the product CO internal excitation is still high. The translational distributions have the same shape as measured distributions, and the average translational excitation matches some experiments but is lower than others. The angular distribution calculated at 2.6 eV on the modified surface is in good agreement with experimental results, showing both forward and backward scattered peaks, with a slight preference for backward scattering. By studying the average lifetime of the HOCO collision complex, we find that the lifetime is comparable to the rotational period so that there is considerable forward scattering (half rotation) and backward scattering (full rotation). The OH product angular momentum alignment indicates no preference for polarization of the OH rotational angular momentum vector. This result–an essentially isotropic distribution–agrees within the experimental uncertainty for measurements of OH Π(A′) polarization dependent differential cross sections and center-of-mass frame alignment parameters, but not with OH Π(A″)

A threedimensional quantum mechanical study of the NH+NO reactionsWe present a quasiclassical trajectory study of the NHϩNO reaction using a global potential energy surface that is capable of describing branching to the HϩN 2 O and OHϩN 2 products after initial formation of a HNNO intermediate complex. The surface is based on a many-body expansion wherein fragment potentials for the species N 2 H, HNO, and N 2 O are incorporated, using either previously developed potentials, or in the case of N 2 O, a newly developed potential. The three-body parts of these fragment potentials are damped in the four-body region to provide a zeroth order four-body surface, and then additional four-body terms and mapping transformations are applied to make the final four-body potential match the results of ab initio calculations for eight important HNNO stationary points ͑minima and saddle points͒ and for several reaction paths. In addition to this ''best fit'' surface ͑surface I͒, a second surface ͑surface II͒ is developed in which the ordering of the saddle points leading to formation of HϩN 2 O and OHϩN 2 is reversed, and the energy release during 1,3 hydrogen migration is modified so that the N-N stretch experiences smaller distortions from N 2 equilibrium during the reaction leading to OHϩN 2 . Quasiclassical trajectory results on surface I show generally good correspondence with experiment, with a branching fraction of 13Ϯ3% for the formation of OHϩN 2 at 300 K, and relatively low OH and N 2 vibration/rotation excitation. The results on surface II are similar with respect to both branching and energy partitioning, indicating relatively weak sensitivity of the results of key features of the surface.

We present a detailed theoretical study of the H+H2O reaction dynamics using quasiclassical trajectories and two potential energy surfaces, one from Walch–Dunning–Schatz–Elgersma (WDSE) and one from Isaacson (I5). Collision energies of 1.0, 1.4, and 2.2 eV are considered, and both scalar and vector properties of the product distributions are presented. The vector properties include polarization-dependent differential cross sections (PDDCS) and angular momentum alignment parameters for both OH and H2. The WDSE and I5 scalar and vector results are in most respects very similar. However, we find that they differ noticeably with respect to angular momentum alignment, with I5 predicting weak OH alignment, while WDSE shows much stronger alignment with the OH angular momentum vector preferentially perpendicular to the scattering plane. The I5 surface is a more recent and more accurate surface for H3O, so it is extremely encouraging that the alignment predicted by I5 is in quantitative agreement with a recent measurement from Brouard and co-workers. In addition, the I5 differential cross section matches the Brouard results quantitatively, while WDSE does not. Detailed mechanistic information underlying the angular distributions, alignment, and PDDCS results is presented, and we find that the differences between I5 and WDSE alignments are connected to different energy release characteristics of the surface in the corner cutting region.

We present a quasiclassical trajectory study of state resolved cross sections, rate coefficients, and product energy partitioning for the reaction H + N2O using a potential surface which is based on ab initio calculations. This surface allows for hydrogen attack on either end of N2O, with N-atom attack giving an intermediate complex HNNO, which can then dissociate into NH + NO or undergo 1,3-hydrogen migration to produce OH + N2. O-atom attack, which involves a higher barrier than N-atom attack, leads only to direct reaction to form OH + N2. We find that the dominant mechanism for the production of OH + N2 changes with energy, with N-atom attack being dominant near the threshold and O-atom attack becoming more important when that pathway becomes energetically accessible. At reagent translational energies of 2 eV, the OH + N2 product is dominant by a factor of 15 over NH + NO, in reasonable agreement with experimental results. Our product OH and NH vibrational distributions also match the experiment, but the partitioning of energy between N2 vibration−rotation and relative translation of OH + N2 is seriously different. Factors which control this difference in energy partitioning are examined, and it is concluded that the difference in energy partitioning between O-atom attack and N-atom attack is not sufficient to explain the discrepancy. We also examine cross sections for different N2O initial states, and we find that N−N stretch is more effective than N−O stretch in enhancing overall reactivity near the OH + N2 reactive threshold, while bend excitation is ineffective. At higher energies, N−O excitation becomes more effective as a result of the change in reaction mechanism from N-atom attack to O-atom attack, and N−N stretch excitation is ineffective. These results are in qualitative accord with recent mode-specific rate measurements. Our calculated thermal rate coefficients for formation of OH + N2 are below measured values, but the agreement is much better for a slightly modified surface in which the barriers for O- and N-atom attack are lowered by 0.1 eV. The trajectory results indicate that O-atom attack rather than N-atom attack is the predominant reaction mechanism at temperatures above 700 K.

We present a quasiclassical trajectory study of OH + H2 -H2O + H and its D2 counterpart using the Schatz-Elgersma semiempirical potential surface. Emphasis in the work has been on using an accurate determination of the H2O vibrational actions to calculate product vibrational state distributions. Other aspects of the product energy and angular distributions have also been studied so as to make comparisons with recent molecular beam and laser experiments and with other calculations. The OH + H2 results generally confirm earlier calculations by Schatz and Elgersma, but the vibrational distributions are somewhat different, indicating that the earlier perturbation theory-based calculations were not completely converged. The calculated OH + Hz(D2) integral cross sections are in good agreement at energies close to the reactive threshold with quantum calculations due to Nyman and Clary. Comparisons with recent measurements by Koppe et al. indicate good agreement for OH + Dz but not as good for OH + H2. The OH + D2 product angular distributions are in good agreement with measurements by Casavecchia and co-workers, but the fraction of energy in translation is significantly higher (0.45 vs 0.32). Nyman and Clary's fraction of energy in translation for the same potential energy surface is higher still (0.71). Their calculation leaves out HOD rotation, which may explain thedifferences between the two theoretical estimates. The difference between theory and experiment is probably due to inaccurate energy release behavior in the potential surface, with too little energy going to product vibration.