First-principles prediction of rate coefficients for free-radical cyclization reactions at selenium

High-level ab initio calculations of the barriers, enthalpies, and rate coefficients for the intramolecular cyclization reactions of ·CH ₂CH₂CH₂CH₂SeR (R = Me, w-Bu, s-Bu, t-Bu, Bn, Bz, Ph₂CH) and ·CH₂CH₂CH ₂CH₂CH=CH₂ have been performed at the G3(MP2)-RAD level. The calculated rate coefficients show excellent agreement with experiment (to within a factor of 5 or better), although this might be due, in part, to a systematic cancelation of order-of-magnitude errors in the corresponding Arrhenius parameters. The intramolecular cyclizations at selenium were found to be energetically favorable processes that occur with synthetically accessible rate coefficients on the order of 10⁴-10⁶ s^-1 at 294 K. These values are largely governed by the stabilization energy of the leaving radical, though with contributions from steric and polar effects.