Structural Stabilities of Small Cationic Sulfur Clusters

The cations of sulfur clusters exhibit intensities of significance on the mass spectra of sulfur clusters generated in direct laser vaporization. To our knowledge, theoretical investigations on cationic sulfur clusters are rare. Forty-nine isomers of sulfur clusters were acquired by means of the molecular model design, molecular mechanics and semi-empirical PM3 and the recently proposed and widely used B3LYPhybrid, non-local, DFT method with basis set 6-31G*. To look for stable configurations, full geometry optimizations at the 6-31G* level for all sulfur atoms were performed using Gaussian 98 program packages. The initial models were constructed by means of breaking bond(s), making bond(s), adding atom(s), deleting atom(s), rotating fragment(s), translating fragment(s) and merging fragment(s). The one-fold, twofold and three-fold modes for modeling were considered, according to the bonding characteristics of the sulfur atom. Finally a total of eleven structures forS3+, S4+and S5+; ten isomers of S6+and S7+; fifteen isomers of S8+ and S9+ and thirteen isomers of S10+, S11+, S12+and S13+are acquired, respectively. A large number of structural possibilities of non-minimum structures were not covered. According to total energies, the most stable Sn+ (n=3~13) isomers are predicted. The geometry, relative stability and structural rules of these cationic sulfur clusters were described. Some structures of neutral sulfur clusters with true minima transform upon ionization into non-minimum cationic structures and vice versa. The results of a large amount of calculations show that the two-fold coordination is generally favored in sulfur cationic clusters, though some sulfur atoms coordinated with the others in one-fold or three-fold mode are higher in total energy. The most stable isomer of some cationic clusters shows a structure completely different from that of the corresponding neutral cluster. In chain structures, the atoms at the two ends adopt the one-fold mode and the others are in two-fold mode. Theoretical studies on sulfur cationic structures with a three-fold atom are rare. It is unlikely to have a sulfur cationic cluster in cage structure, for the bonding of a three-fold atom are not strong enough. The calculation results can serve as guiding factors for future theoretical studies on large sulfur clusters.