STRAIN ENERGIES IN SULFUR RINGS, SN, N = 3-8

We report strain energies of sulfur monocycles containing three to eig ht atoms. Strain energies are related to energy changes for reactions that convert rings into chains. Those energy changes can be obtained f rom experimentally determined heats of formation of products and react ants, from MNDO calculated heats of formation, and from ab initio calc ulated total energies. We chose model reactions of types that favor th e cancellation of errors involved in the ab initio calculation of mole cular total energies. Such reactions preserve numbers of electron pair s between reactants and products (isogyric), conserve numbers of bonds of different types (isodesmic), maintain atomic valence environments (homodesmotic), and more. The sources of experimental heats of formati on are reviewed. MNDO heats of formation are taken from the published work of Dewar and McKee (J. Comput. Chem. 1983, 4, 84) and Baird (J. C omput. Chem. 1984, 5, 35). We performed additional ab initio calculati ons to supplement the collection of total energies of S(n) and H2Sn pr eviously reported by Dixon and Wasserman (J. Phys. Chem. 1990, 94, 577 2). The availability of experimental data allows the comparison of cal culated and experimental strain energies at several different levels. Comparisons, however, require the reservation that S3 and S4 rings are experimentally unknown. At the most reliable level of theory, the ab initio based strain energies are within a few kilocalories per mole of the experimentally based results. Among cycloalkanes, strain energies are known to decline from a maximum at n = 3 to zero at n = 6 with su bsequent increases for n = 7 and 8. Among sulfur rings, we find the st rain energy Of S4 to be greater than that of either S3 or S5 with a de cline to near zero for S8.