Molecular dynamics simulation of the I2(X)•••Ar isomers population in a free-jet expansion:: Thermodynamics versus kinetic control

A molecular dynamics simulation addressing the problem of thermodynamic versus kinetic control of the isomers population of van der Waals complexes in a supersonic expansion is presented. The populations of the linear and T-shaped isomers of I-2(X)...Ar in a supersonic beam expansion were determined by molecular dynamics simulation as a function of the distance to the nozzle and compared to the prediction of thermodynamics. The surprising conclusion is that although there is a barrier equal to half the well depth between the two isomers, their populations are consistent with the existence of thermodynamic equilibrium. This result is rationalized by examining the cooling mechanisms in the Ar+I-2(X)...Ar collisions. In addition to the direct isomerization, a new mechanism (swap cooling), which induces isomerization even for complexes with barriers above the dissociation limit, is evidenced. (C) 2002 American Institute of Physics.