Ab initio calculations on ethanol and 2-substituted fluorine and chlorine containing mono-, di- and tri-haloethanols, with reference to conformer interconversions at low temperatures

Abstract

All conformers of ethanol and of several 2-substituted mono-,di- and trihalogenated (halogen = F and/or Cl) ethanols have been optimized, mostly at the 4–31G level. Semirigid torsional energy curves have been constructed for these compounds. For 2-fluoroethanol, calculations were carried out also using polarization functions, as well as on the MP and CID levels. The results are discussed with special reference to conformer interconversion processes in low-temperature matrices, and it is concluded that ab initio calculations help essentially in interpreting the experimental results. It is concluded that 4–31G is the most suitable level for calculating the torsional potential energy barriers for such processes (the minimal basis STO-3G giving much too high CO barriers and too low CC barriers). The CO barriers seem to be quite low for many conformational processes (or even non-existent); thus the only “stable” conformers at the low temperatures in question (10–30 K) seem to be Gg′ and Tt for monohaloethanols, Gg and Tg for 2,2-difluoroethanol and g for 2,2,2-trihaloethanols. The CC barrier between the Gg and Tg conformers of 2,2-difluoroethanol is quite high. The trans-angle rule proposed earlier is found to be independent of the basis set used.