Ab initio conformational analysis of 1,5-dithiacyclooctane, 1,5-diselenacyclooctane, and 1,5-ditelluracyclooctane

Energy-minimum structures of 1,5-dithiacyclooctane (1,5-DTCO), 1,5-diselenacyclooctane (1, 5-DSeCO), and 1, 5-ditelluracyclooctane (1,5-DTeCO) were calculated by the ab initio molecular orbital method. Nine energy-minimum structures were obtained for each compound. A twist-boat-chair (TBC) structure is the most stable for 1,5-DTCO and 1,5-DSeCO, whereas a boat-boat (BB) structure is the most stable in 1,5-DTeCO. The TBC conformer of 1,5-DTCO has received little attention so far The energy gap between HOMO and NHOMO in the TBC conformer of 1,5-DTCO is in good agreement with the experimental data (photoelectron spectrum). For 1,5-DTCO and 1,5-DSeCO, the boat-chair (BC) conformer in which two chalcogen atoms face each other has the highest HOMO energy among the nine conformers, and the energy barriers between the TBC and BC conformers were calculated to be relatively low for these compounds. Therefore, a conformational change from the TBC to the BC is predicted to occur before these compounds ave oxidized in solution. (C) 1999 John Wiley & Sons, Inc.