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

The energetics of dissociation reactions of S(8)(2+) into stoichiometric mixtures of S(n)(+), n = 2-7, and S(m)(2+), 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 S(4)(2+) and S(6)(2+) also at the TD-DFT level (BP86/SV(P)). Standard enthalpies of formation at 298 K were derived for S(3)(2+) (2538 kJ/mol), S(6)(2+) (2238 kJ/mol), and S(10)(2+) (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 Br(2)), S(8)(2+) dissociates in dilute SO(2) solution giving an equilibrium mixture of ca. 0.5S(6)(2+) and S(5)(+) (K approximately 8.0) while in the more polar HSO(3)F some S(8)(2+) remains (K approximately 0.4). According to our calculations, the blue color of this solution is likely due to the pi-pi transition of the previously unknown 10 pi S(6)(2+) dication, and the previously assigned S(5)(+) is a less important contributor. Although not strictly planar, S(6)(2+) may be viewed as a 10 pi electron Hückel-aromatic ring containing a thermodynamically stable 3p(pi)-3p(pi) bond [d(S-S) = 2.028 A; tau(S-S-S-S) = 47.6 degrees ]. The computations imply that the new radical cation S(4)(+) may be present in sulfur dioxide solutions given on reaction of sulfur oxidized by AsF(5) in the presence of a facilitating agent. The standard enthalpy of formation of S(6)(AsF(6))(2)(s) was estimated as -3103 kJ/mol, and the disproportionation enthalpy of 2S(6)(AsF(6))(2)(s) to S(8)(AsF(6))(2)(s) and S(4)(AsF(6))(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., AsF(3), that additionally favors the disproportionation of 2S(6)(AsF(6))(2)(s) into S(8)(AsF(6))(2)(s) and S(4)(AsF(6))(2)(AsF(3))(s) by 144 kJ/mol.

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