Focal-point conformational analysis of ethanol, propanol, and isopropanol.

Conformational analysis of three small alcohols--ethanol, propanol, and isopropanol--was carried out by systematically improving the basis set and the level of electron correlation. Correlation energy contributions to conformational energies are strongly basis-set-dependent but accurate energy contributions can be obtained by extrapolation to the basis-set limit. At the basis-set limit, second- and third-order electron correlation effects play a significant role for rotations around the CC-OH, HC-CO, and CC-CO bonds. Specifically, second- and third-order correlation effects strongly stabilize structures in which the hydroxylic hydrogen eclipses with the adjacent carbon; a lesser stabilization is present in structures where the CC-OH moiety is in the gauche form. Fourth-order correlation effects to the CC-OH rotation are small due to a partial cancellation of the singles, doubles, and quadruples contribution by the triples contribution. Electron correlation significantly lowers barriers for methyl-group rotations in ethanol and isopropanol, and in these cases the fourth-order correlation effects are noticeable. The relatively large overall importance of third-order correlation energy contributions raises a concern that the inability to accurately estimate this slowly converging contribution may become a limiting factor when highly accurate conformational energies in larger molecules are sought.