Effects of halogen substituents on the conformations of vinyl alcohol and vinyl thiol : A theoretical study

Abstract

Publisher Summary This chapter focuses on effects of halogen substituents on the conformations of vinyl alcohol and vinyl thiol. Studies on vinyl thiol (or ethene thiol) are scarce. It has been detected experimentally, and its formation from ethylene and singlet sulfur atom has been predicted theoretically. Its conformations have been studied both experimentally and theoretically. The potential energy curves of enols and thiols have been calculated as a function of a single parameter, 8, dihedral angle HOCC (resp. HSCC), all the remaining parameters being independently optimized. The θ angle has been varied from 0 ° (syn conformation) to 180 ° (anti conformation) by 10 ° steps. Structural parameters and relative energies for unsubstituted enol and thiol are reported. The main features in the conformational analysis of these compounds—that is,relative stabilities of syn and anti conformers, rotation energy barriers and eventually out of plane distortions of the anti conformers, result in manyfold weak interactions and it would be tedious and illusive to give their complete qualitative analysis. The chapter displays the most important of them and indicates the trends induced by the change of the nature of the substrate (enol or thioenol) or the position and the nature of substituents. These calculations show that, if vinyl alcohol and vinyl thiol mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than tile syn one.