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 S 3 through S 8 , 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 ‐S 7 . 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 S n 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.