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146 Ingo Krossing Fig. 6 Solvent dependence of the calculated free energies of the dissociation of selected equilibria of S82+ giving S62+ and radical cations Sn·+ (n=4–7) [in kJ mol1 at 298 K. [3] of S42+ in solution is no longer kinetically hindered and, therefore, S42+ is in equilibrium with S82+ and the most favored equilibria include the formation of the hitherto unknown S4·+ radical cation in addition to the established S5·+ (Eqs. c, d, and j in Table 2). This is in agreement with preliminary tem- perature and concentration dependent ESR and magnetic measurements [27]. The quantity of radical cations formed by dissociation of S82+ is depen- dent on the polarity of the solvent with less polar solvents such as SO2 or SO2ClF favoring the formation of radical (mono-)cations if compared to po- lar solvents like HF or oleum. This can be understood by quantum chemical calculations with inclusion of solvation energies that approximate solvents of different polarities. A measure for the polarity of a solvent is the dielectric constant (DC) and a graphic representation of the calculated Gibbs energies of selected dissociation equilibria of S82+ in solvents with dielectric constants between 1 and 30 is given in Fig. 6 [3]. For comparison: the DC of SO2 at 298 K is 14 and that of HF is 83. Analysis of the data in Fig. 6 shows that, in agreement with the experi- ment, non-polar solvents favor the dissociation of S82+ to sulfur monocations as the overall solvation energies decrease with decreasing dielectric con- stant. 7 Bonding The structure and bonding of the sulfur cations has been the focus of recent work and revealed the presence of non-classical 3pp-3pp, p*-p*, and 3p2!3s* bonding interactions as a means to delocalize the unfavorable localized pos- itive charges expected from classical electron precise structures [1a].PDF Image | Topics in Current Chemistry
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