MNDO study of catenated sulfur: The molecules and ions S3 to S8

Semiempirical molecular orbital calculations by the MNDO method are reported for sulfur rings and chains S₃ through S₈, for the corresponding dications, S through S, and for S. The MNDO method seems quite successful in predicting the geometries of neutral catenated sulfur molecules, even the unusual bond-length alternation and extent of coplanarity in cyclo-S₇. In contrast to hydrocarbon rings, for which its prediction of strain is erratic, MNDO is consistent in its calculated strain energies in small cyclosulfur rings; unfortunately all the strain energies are overestimated by 70%. As a consequence of this error, the method must be considered unreliable in its predictions of structures for the dications S, since many of these ions could potentially exist as strained bicyclic systems. In addition, MNDO appears to have difficulty handling long, partial SS σ bonds, as are found to occur in S. It may be for this reason that MNDO predicts, apparently incorrectly, that the open-chain isomers of S are more stable than are any of the cyclic forms, at least for S to S. With respect to neutral Sⁿ molecules, however, the MNDO predictions appear more reliable than ab initio molecular orbital (MO ) calculations using small basis sets without polarization functions and without configuration interaction (CI ). However, MNDO apparently underestimates by about a factor of two the strength of the three-electron π bonds present in the terminal links of sulfur diradical chains.