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Evidence for the Blue 10π S62+ Dication in Solutions of S8(AsF6)2: A Computational Study Including Solvation Energies

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journal contribution
posted on 09.02.2004, 00:00 by Ingo Krossing, Jack Passmore
The energetics of dissociation reactions of S82+ into stoichiometric mixtures of Sn+, n = 2−7, and Sm2+, m = 3, 4, 6, 10, were investigated by the B3PW91 method [6-311+G(3df)//6-311+G*] in the gas phase and in solution, with solvation energies calculated using the SCIPCM model and in some cases also the COSMO model [B3PW91/6-311+G*, dielectric constants 2−30, 83, 110]. UV−vis spectra of all species were calculated at the CIS/6-311G(2df) level and for S42+ and S62+ also at the TD-DFT level (BP86/SV(P)). Standard enthalpies of formation at 298 K were derived for S32+ (2538 kJ/mol), S62+ (2238 kJ/mol), and S102+ (2146 kJ/mol). A comparison of the observed and calculated UV−vis spectra based on our calculated thermochemical data in solution suggests that, in the absence of traces of facilitating agent (such as dibromine Br2), S82+ dissociates in dilute SO2 solution giving an equilibrium mixture of ca. 0.5S62+ and S5+ (K ≈ 8.0) while in the more polar HSO3F some S82+ remains (K ≈ 0.4). According to our calculations, the blue color of this solution is likely due to the π*−π* transition of the previously unknown 10π S62+ dication, and the previously assigned S5+ is a less important contributor. Although not strictly planar, S62+ may be viewed as a 10π electron Hückel-aromatic ring containing a thermodynamically stable 3pπ−3pπ bond [d(S−S) = 2.028 Å; τ(S−S−S−S) = 47.6°]. The computations imply that the new radical cation S4+ may be present in sulfur dioxide solutions given on reaction of sulfur oxidized by AsF5 in the presence of a facilitating agent. The standard enthalpy of formation of S6(AsF6)2(s) was estimated as −3103 kJ/mol, and the disproportionation enthalpy of 2S6(AsF6)2(s) to S8(AsF6)2(s) and S4(AsF6)2(s) as exothermic by 6−17 kJ/mol. The final preference of the observed disproportionation products is due to the inclusion of solvent molecules, e.g., AsF3, that additionally favors the disproportionation of 2S6(AsF6)2(s) into S8(AsF6)2(s) and S4(AsF6)2(AsF3)(s) by 144 kJ/mol.