Sulfur-sulfur three-electron bond dissociation enthalpies of dialkyl sulfide dimer radical cations

We have prepared the dimer radical cations (R2S)+·2 of a series of dialkyl sulfides R2S (= Me2S, MeEtS, Et2S, Et-n-PrS, n-Pr2S, i-Pr2S, n-Bu2S) in an FT-ICR cell. These have been found to react with neutral dialkyl sulfides by ligand exchange. By studying the (R2S)+·2 + 2Et2S ⇄ (Et2S)+·2 + 2 R2S equilibria the associated Gibbs energy changes ΔG0 have been determined. From these, the sulfur-sulfur bond dissociation enthalpies (BDE) of the dimers could be estimated using the relationship BDE[(R2S)+·2] = -ΔG0 - TΔS0 + IE[Et2S] - IE[R2S] + BDE[Et2S)+·2] and taking literature values for BDE[(Et2S)+·2] and the ionization energies IE[R2S]. The entropies of reaction ΔS0 have been estimated using statistical thermodynamics and computationally optimized structures. The 298 K sulfur-sulfur BDE values thus obtained are 115, 112, 115, 107, 103, and 97 kJ mol-1 for (Me2S)+·2, (MeEtS)+·2, (Et2S)+·2, (Et-n-PrS)+·2, (n-Pr2S)+·2 and (n-Bu2S)+·2, respectively. The trend in S-S bond strength as well as the various contributions to ΔS0 are discussed. © 1998 Elsevier Science B.V.