STATISTICAL AND DYNAMICAL BEHAVIOR IN THE UNIMOLECULAR REACTION DYNAMICS OF POLYATOMIC-MOLECULES

The dominant theories of unimolecular reaction are statistical. A fund amental assumption is that the timescale on which energy moves about a reactant molecule is much shorter than the timescale for reaction. It is assumed that intramolecular vibrational energy redistribution (IVR ) is globally rapid throughout the molecular phase space. It has been widely thought that the assumption of rapid IVR referred to above is v alid for sufficiently large polyatomics. Much of the supporting eviden ce for this view comes from indirect experimental studies of IVR and c omparisons of statistical and dynamical calculations. However, the pre sence of a fast IVR rate, as derived from some experiments, does not a utomatically ensure the reaction dynamics will be statistical. In fact , in recent studies, we have shown that even in the presence of fast I VR rates between some modes the reaction dynamics can be extremely non -statistical. Secondly, most comparisons of statistical and dynamical calculations have made simplifying assumptions which render the compar isons ambiguous. In the present paper, we investigate results of recen t statistical and dynamical calculations performed on identical potent ial energy surfaces for a range of polyatomic molecules. Our ultimate goal is to determine how the extent and timescale of IVR plays a role in determining the statistical or non-statistical behaviour in the sub sequent unimolecular reaction dynamics of locally and microcanonically excited polyatomic molecules.