Three-electron bonded sigma/sigma* radical cations from mixedly substituted dialkyl sulfides in aqueous solution studied by pulse radiolysis

The formation of several radical cations ([R(i),R(j)]S therefore S[R(i),R(j)])(+), ([R(i)]S-2 therefore S[R(j)](2)])(+) and ([R(i),R(j)]S therefore S[R(j)](2))(+) with mixed alkyl substitution in aqueous solution has been investigated by means of pulse radiolysis, The following substituents were involved: R(ij) = H, methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl and octyl, Two methods of generation have been applied: (i) (OH)-O-.-induced oxidation of a sulfide and (ii) one-electron reduction of the corresponding sulfoxide in very acidic solution, The 2 sigma/1 sigma* three-electron bonded(> S therefore S<)(+)-type species exhibit optical absorptions with maxima ranging from 420 mn for(Me(2)S therefore SH2)(+) to 600 mn for ([Me,Bu(t)]S therefore S[Bu(t)](2))(+). The actual transition energy can be related to the electron induction by the substituents as concluded from a linear free energy correlation between the respective lambda(max) and weighted Taft's inductive sigma* parameters. For unbranched substituents lambda(max)(in eV) 1.40 (sigma*)(w) + 2.65. Evidence is also provided for the destabilization of the three-electron bond by steric demands of bulky substituents and by the effect of the substitution pattern on the 'sigma-lone pair' interaction. The latter becomes apparent by comparing([Me(2)]S therefore S[Bu(t)](2))(+) (lambda(max)545 nm) with ([Me,Bu(t)]S therefore S[Me,Bu(t)])(+)(lambda(max)510 nm). Kinetically, a number of rate constants have been determined for the forward and back reactions of the equilibrium >S-.+ + < S reversible arrow(>S therefore S<)(+). They are typically of the order of 10(9) dm(3) mol(-1) s(-1) and 10(4)- 10(5) s(-1), respectively. Equilibrium constants derived from these kinetic data range from 2.0 x 10(5) dm(3) mol(-1) for (Me(2)S therefore SMe(2))(+) (confirming an earlier measurement) to less than or equal to 5 x 10(3) dm(3) mol(-1) for ([Me,Bu(t)]S therefore S[Bu(t)](2))(+). Their decrease parallels the total electron-releasing power of the substituents and the steric constraints exerted by them, The decay of the three-electron bonded radical cations includes first-order release of protons, possibly in association with the dissociation of the three-electron bond and second-order processes, presumably disproportionation, By and large, the kinetic stabilities are reflected in the trend in lambda(max) with shorter lifetimes referring to more red-shifted absorptions.