The Significance of Gas Phase Dynamics

One may at first find it hard to believe that underlying gas phase dynamics can be observed in solution phase reactions. However, a close look at the simulations of A + BC reactions in rare gas solutions shows a time period when the gas phase and solution phase reactant dynamics appear to be quite simi-lar.21 This type of behavior can also be seen in the studies of an Sn2 reaaion in 

A study directed toward understanding when gas phase dynamics closely resembles the dynamics of the same reaction in solution was performed by Li and Wilson. io In this work, they used a model asymmetric A -t- BC reaction. By using an asymmetric reaction, Li and Wilson were able to test the validity in the solution phase of the Evans—Polanyi rule,3n which has proven to be quite useful in understanding gas phase reaction dynamics. The Evans-Polanyi rule states for a collinear A -t- BC reaction, that if the barrier to reaction is located early in the reaction coordinate, then translational excitation of the reactants is necessary to climb this barrier and vibrational excitation of the products will result. Conversely, a late barrier to reaction requires vibrational excitation of the reactants and results in translational excitation of the products. This rule has been validated numerous times in the gas phase and is an ideal example of how a simple rule can explain the dynamics of a large number of reaction systems. 

To test the Evans—Polanyi rule in the solution phase, Li and Wilson performed simulations of the asymmetric A -I- BC solution in the gas phase and in a dense 100 Ar atom solvent. They began the dynamics at the transition state of the reaction and ran 1 ps of dynamics both forward and backward in time from those initial conditions. They calculated over 1700 reactive trajectories in both the solution phase and in the gas phase, monitoring the appropriate vibrational and translational energies. 

A quite different approach to using gas phase dynamics in solution reactions is due to Charutz and Levine. They recast the classical Hamiltonian into an interaction picture that has been used, for example, in the propagation of quantum wavepackets. The picture that Charutz and Levine develop is that just as there are constants of the motion in quantum scattering theory that characterize the reactant and product states, similar constants can be calculated for the classical mechanics of reaction dynamics in solution. While the theoretical treatment is too complicated to present fully here, Charutz and Levine applied this picture to the model Cl -I- CI2 reaction in rare gas solution. They show that momentum of the atom—diatom relative motion is one of the con- 

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