Journal of the Chemical Society. Perkin Transactions 2 (2001) p. 383 - 390 (1996)
Update date:2022-08-03
Topics:
Chaudhri, Shamim A.
Mohan, Hari
Anklam, Elke
Asmus, Klaus-Dieter
The formation of several radical cations ([Ri,Rj]S S[Ri,Rj])+, ([Ri] 2S S[Rj]2])+ and ([R i,Rj]S S[Rj]2)+ with mixed alkyl substitution in aqueous solution has been investigated by means of pulse radiolysis. The following substituents were involved: Rij = H, methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl and octyl. Two methods of generation have been applied: (i) .OH-induced oxidation of a sulfide and (ii) one-electron reduction of the corresponding sulfoxide in very acidic solution. The 2σ/1σ * three-electron bonded (> S S <)+-type species exhibit optical absorptions with maxima ranging from 420 nm for (Me2S SH2)+ to 600 nm for ([Me,But]S S[But]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 λmax and weighted Taft's inductive σ* parameters. For unbranched substituents λmax (in eV) = 1.40 (σ*)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 'σ-lone pair' interaction. The latter becomes apparent by comparing ([Me2]S S[But]2)+ (λmax 545 nm) with ([Me,But]S S[Me,But])+ (λmax 510 nm). Kinetically, a number of rate constants have been determined for the forward and back reactions of the equilibrium > S.+ + S< ? (> S S <)+. They are typically of the order of 109 dm3 mol-1 s -1 and 104-105 s-1, respectively. Equilibrium constants derived from these kinetic data range from 2.0 x 10 5 dm3 mol-1 for (Me2S SMe2)+ (confirming an earlier measurement) to ≤5 x 103 dm3 mol-1 for ([Me,But]S S[But]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 λmax, with shorter lifetimes referring to more red-shifted absorptions.
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