CONFORMATIONAL COMPOSITION IN THE GAS-PHASE AND IN SOLUTION, STRUCTURE AND INTRAMOLECULAR HYDROGEN-BONDING OF THIIRANEMETHANETHIOL AS STUDIED BY MICROWAVE AND NMR-SPECTROSCOPY AND AB-INITIO CALCULATIONS

The microwave spectrum of thiiranemethanethiol and one deuterated species (mercapto group) has been investigated in the gas phase in the 18.0-39.0 GHz spectral region at -15-degrees-C. Four conformers were assigned. Three of these possess a weak intramolecular hydrogen bond, while the fourth rotamer is not stabilized by this interaction. The most stable rotamer is denoted H bond outer 2. This conformer has an internal hydrogen bond formed between the mercapto group hydrogen atom and the pseudo-pi electrons along the C-C edge of the thiirane ring. The next rotamer, H bond outer 1, is 0.9(3) kJ mol-1 less stable than H bond outer 2. The internal H bond is formed between the mercapto group hydrogen atom and the pseudo-pi electrons of the C-S bond of the ring in H bond outer 1. The third conformer, H bond inner, which is 1.4(3) kJ mol-1 less stable than H bond outer 2, has a S-H...S hydrogen bond. The fourth rotamer assigned is denoted Conformer IV. It is 3.5(4) kJ mol-1 less stable than H bond outer 2 and has no hydrogen bond. Conformer IV is similar to H bond outer 2, with the exception that the H-S-C-C dihedral angle is approximately 180-degrees, compared to 60' in H bond outer 2. The microwave work has been assisted by ab initio computations at the 6-31G* and MP2/6-31G* levels of theory. A detailed NMR analysis of the conformational composition of the title compound in dilute CDCl3 solution has been performed. The ratio of H bond outer 2:H bond outer 1:H bond inner was found to be 43:23:34. This is similar to the composition in the gas phase.