Kinetic and ab Initio Theoretical Study of Hydrogen Atom Abstraction from Thiols by Thiyl Radicals: Basis Rate Expressions for Reactions of Sulfur-Centered Radicals

Article abstract of DOI:10.1021/j100196a036

Arrhenius rate expressions were determined for the abstraction of hydrogen atom from thiophenol and hexanethiol by the octanethiyl radical at 25-100 deg 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)/θ, and for thiophenol, log (k_abs/k_t^1/2) = (2.56 +/- 0.19) - (2.88 +/- 0.28)/θ; θ = 2.3 RT 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/θ, which employs experimental diffusion coefficients of octanethiol and a spin selection factor ? = 1, yields, for thiophenol, log (k_abs/M^-1 s^-1) = (8.52 +/- 0.18) - (4.22 +/- 0.27)/θ, and for hexanethiol, log (k_abs/M^-1 s^-1) = (8.90 +/- 0.29) - (5.18 +/- 0.41)/θ (errors are 2?).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 ? = 1 above.Ab initio electronic structure calculations on the reaction HS^. + HSH -> HSH + ^.SH, performed at SCF and correlated levels, predict an activation barrier of ΔH(excit.)₂₉₈ = 4.6 kcal/mol, in close agreement with the experimental barrier for the octanethiyl + hexanethiol reaction.