KINETIC AND ABINITIO THEORETICAL-STUDY OF HYDROGEN-ATOM ABSTRACTION FROM THIOLS BY THIYL RADICALS - BASIS RATE EXPRESSIONS FOR REACTIONS OF SULFUR-CENTERED RADICALS

Arrhenius rate expressions were determined for the abstraction of hydrogen atom from thiophenol and hexanethiol by the octanethiyl radical at 25-100-degrees-C in nonane. Octanethiyl radicals were produced by steady-state photolysis of octyl thiobenzoate. Analysis of octyl disulfide and octanethiol provided relative rate expressions for abstraction of hydrogen atom from thiols versus self-termination of the octanethiyl radical. For hexanethiol, log (k(abs)/k(t)1/2) = (2.94 +/- 0.29) - (3.84 +/- 0.41)/theta, and for thiophenol, log (k(abs)/k(t)1/2) = (2.56 +/- 0.19) - (2.88 +/- 0.28)/theta; theta = 2.3RT kcal/mol. Combining these expressions with the Smoluchowski expression for self-termination of octanethiyl in nonane, log (k(t)1/2) = 5.96 - 1.335/theta, which employs experimental diffusion coefficients of octanethiol and a spin selection factor sigma = 1, yields, for thiophenol, log (k(abs)/M-1 s-1) = (8.52 +/- 0.18) - (4.22 +/- 0.27)/theta, and for hexanethiol, log (k(abs)/M-1 s-1) = (8.90 +/- 0.29) - (5.18 +/- 0.41)/theta (errors are 2-sigma). The rate of disappearance of octanethiyl/diphenylketyl radical pairs in SDS micelles, determined by nanosecond optical spectroscopy, was found to be unchanged in a 700-G magnetic field, providing evidence for rapid intersystem crossing of sulfur-centered radical pairs and support for the assignment of sigma = 1 above. Ab initio electronic structure calculations on the reaction HS. + HSH --> HSH + .SH, performed at AH and correlated levels, predict an activation barrier of DELTA-H double dagger = 4.6 kcal/mol, in close agreement with the experimental barrier for the octanethiyl + hexanethiol reaction.