Structures and vibrational spectra of the sulfur-rich oxides SnO (n = 4-9): the importance of pi*-pi* interactions

The structures of a large number of isomers of the sulfur oxides S(n)O with n = 4-9 have been calculated at the G3X(MP2) level of theory. In most cases, homocyclic molecules with exocyclic oxygen atoms in an axial position are the global minimum structures. Perfect agreement is obtained with experimentally determined structures of S(7)O and S(8)O. The most stable S(4)O isomer as well as some less stable isomers of S(5)O and S(6)O are characterized by a strong pi*-pi* interaction between S==O and S==S groups, which results in relatively long S--S bonds with internuclear distances of 244-262 pm. Heterocyclic isomers are less stable than the global minimum structures, and this energy difference approximately increases with the ring size: 17 (S(4)O), 40 (S(5)O), 32 (S(6)O), 28 (S(7)O), 45 (S(8)O), and 54 kJ mol(-1) (S(9)O). Owing to a favorable pi*-pi* interaction, preference for an axial (or endo) conformation is calculated for the global energy minima of S(7)O, S(8)O, and S(9)O. Vapor-phase decomposition of S(n)O molecules to SO(2) and S(8) is strongly exothermic, whereas the formation of S(2)O and S(8) is exothermic if n<7, but slightly endothermic for S(7)O, S(8)O, and S(9)O. The calculated vibrational spectra of the most stable isomers of S(6)O, S(7)O, and S(8)O are in excellent agreement with the observed data.