Electrophilic Attack on Sulfur−Sulfur Bonds. 1. Protonation of Various Isomers of the Homoatomic Sulfur Molecules Sn (n = 2−8)

Abstract

The structures and thermodynamic properties of 32 cations of composition [HSn]+ with n = 2−8 have been studied by ab initio methods at the G3X(MP2) level of theory. In the global minimum structures, the proton binds to a single sulfur atom and, except for n = 2, this also holds for the less stable isomers. The binding energies at 0 K range from 619.4 kJ mol-1 in [HS2]+ to 835.6 kJ mol-1 in [S5−SH]+, which represents the global minimum structure of composition [HS6]+. The protonated branched rings are also the global minimum structures in the case of S7 and S8, whereas [HS5]+ is most stable with a five-membered homocycle. The smaller cations [HSn]+ (n = 2−4) are all chainlike with the hydrogen atom at a chain end. Unexpectedly, singlet chainlike structures of [HSn]+ (n = 2−8) with the hydrogen terminating the chain at one end are more stable than the corresponding triplet chains. The protonation of neutral sulfur molecules always takes place at the atom of highest negative charge and the charge transfer to the proton is between 0.77 and 0.86 electrons. The S−H bond lengths are calculated as in the range 135−139 pm. The effect of protonation on the sulfur−sulfur bonds is dramatic, as can be seen in the strong bond length alternation within the cyclic or chainlike structures with the weakest SS bonds originating from the three-coordinate atoms. Hence, there is a pronounced bond activation by protonation. The calculated gas-phase basicities of S6 and S8 are in good accord with the experimental values.