Product energy distribution trajectory studies

This chapter concentrates on the experimental determination of product energy distributions and their interpretation in terms of the dynamics of collisions. In a volume concerned with excited states, it seemed appropriate to bias the article in favor of the spectroscopic methods and results, at the expense of the crossed-beam studies. Electronically adiabatic reactions, which pass smoothly from reactants to products in their electronic ground states, are emphasized, since the results of such processes may be compared with trajectories computed with the equations of classical, rather than wave, mechanics, and the effects of kinematic factors on the sharing of energy can be explored. 

Classical trajectories are the only feasible means to explicitly treat all atoms in a dynamical study of a unimolecular reaction. Trajectories have been used extensively to interpret A + BC bimole-cular reactions and a considerable amount of literature exists with respect to these studies. Excitation functions, scattering angles, product energy distributions, and other dynamical properties are usually quantitatively determined by the trajectory calculations. The semiclassical studies of Marcus and Miller have in general confirmed the accuracy of classical trajectories in calculating dynamical properties for bimolecular reactions. However, the trajectories do not describe quantum mechanical effects such as interferences, tunneling, and nonadiabatic electronic transitions. 

The use of apparent non-RRKM behavior in studying unimolecular dynamics should continue. It would be beautiful if experiments could be designed to measure unimolecular rate constants, branching ratios, and product energy distributions as a function of time. These properties can be easily calculated by classical trajectories and it would be wonderful to be able to directly compare experiment and theory. 

Quasiclassical direct dynamics trajectories at the various levels of theory were later calculated to study the central barrier dynamics for the C1 I CH3C1, Cr + C2H5C1, C1- + CH3I, F +CH3C1, OH +CH3C1, and other Sn2 reactions.31,32,47,97 108 The effect of initial reaction conditions, such as energy injection, substrate orientations, and the mode of collision, on the fate of the reaction, product, and energy distribution, was analyzed. Some of these trajectory calculations required serious modification in RRKM and TST for... 

Quantum mechanical and classical calculations have been performed [245] for H + Cl2 on a recently optimised extended LEPS surface [204]. The quantum mechanical results were transformed to three dimensions by the information theoretic procedure and are in good agreement with the distributions determined in the chemiluminescence experiments. However, three-dimensional trajectory calculations on the surface consistently underestimate (FR) at thermal energies and it is concluded that the LEPS surface which was optimised using one-dimensional calculations does not possess the angular dependence of the true three-dimensional surface. This appears to result from the lack of flexibility of the LEPS form. Trajectory studies [196] for H + Cl2 on another LEPS surface find a similar disposal of the enhanced reagent energy as was found for H + F2. The effect of vibrational excitation of the Cl2 on the detailed form of the product vibrational and rotational state distributions was described in Sect. 2.3. 

Product energy partitioning in the dissociation of ketene in the Sq state, CH2CO CH2(Mi)-t-CO, has also been studied by direct dynamics [376]. Here the trajectories axe initialized at the variational transition state separating reactants and products. The rotational energy distributions of the CH2 and CO products are in good agreement with experiment and different... 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

The modified spectator stripping model (polarization model) thus appears to be a satisfactory one which explains the experimental velocity distribution from very low to moderately high energies. The model emphasizes that the long-range polarization force has the dominant effect on the dynamics of some ion—molecule reactions. However, a quite different direct mechanism based on short-range chemical forces has been shown to explain the experimental results equally satisfactorily [107, 108]. This model is named direct interaction with product repulsion model (DIPR model) and was originally introduced by Kuntz et al. [109] in the classical mechanical trajectory study of the neutral reaction of the type... 

Smith and Wood s work also showed that both non-reactive and reactive processes, e.g. chaniwls (a) and (b) in equation (41), could remove vil ationally exdted molecules in electronically adiabatic collisions. However, the non-reactive contribution came almost entirely from trajectories which crossed the surface an even number of times, so that the motions of the three-atom system became strongly coupled. The product vibrational distributions both from these non-reactive trajectories and from reactive collisions were broad, showing that multiquantum transfers, i.e. (v - v ) > 1, are probable. The m ority of trajectories did not, of course, cross = bc °d the transfer of a substantial amount of vibrational energy in these collisions was extremely rare. These general findings have been confirmed in a study modelled on the system Br + HBr with the potential chosen to have a barrier to H atom transfer of 16 kJ mol and one... 

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