Journal of Physical Chemistry, Vol.98, No.21, 5460-5470, 1994
Solvent Barriers in Unimolecular Ionizations .2. Electron-Transfer Perspective for Alkyl Iodide Ionizations
The analysis of the alkyl iodide ionizations RI-->R(+) + I- begun in I (preceding paper in this issue) is continued by focusing on a nuclear (r) coordinate-dependent electron transfer (ET) perspective. Considerations presented within provide an understanding of the factors that determine, e.g., whether or not charge transfer plus bond breakage occurs in a stepwise or concerted fashion. Two different ET rate constants are investigated : a conventional r-dependent ET rate k(1)(ET)-generalized to these S(N)1 ionizations-and a new ET rate k(2)(ET). k(2)(ET) accounts for the r-dependence of the solvent barrier location and thus incorporates into the rate analysis dividing surface curvature or, equivalently, the strong transition state electronic structure variation along the nuclear coordinate. This added component-which is not present in the standard ET rate-is essential in providing a reasonable estimate of the ionization rate while still attempting to retain an ET perspective. k(1)(ET) can be more than an order of magnitude smaller than k(2)(ET) and does not provide a reasonable estimate of the ionization rate. In the absence of electronic structure variation effects, k(2)(ET) reduces to k(1)(ET). Comparison of k(2)(ET) with the variationally optimized result k(v) from I shows that they are comparable, but with k(2)(ET) being slightly larger. Trajectory calculations, carried out to test the no-recrossing assumption for the k(2)(ET) dividing surface, show that there is no recrossing. This recrossing correction brings k(2)(ET) into accord with k(v), so that k(v) is the best estimate of the actual rate. The results are applicable to other reaction classes, including the S(RN)1 reaction mechanism and inner sphere electron transfer reactions.
Keywords:S(N)1 IONIC DISSOCIATION;TRANSITION-STATE THEORY;RADICAL CHAIN PROCESSES;NONEQUILIBRIUM SOLVATION;REACTION-PATH;CHEMICAL-REACTIONS;THEORETICAL-MODEL;POLAR-SOLVENTS;SN2 REACTIONS;MOLECULAR-DYNAMICS