Focal-Point Conformational Analysis of Ethanol, Propanol, and Isopropanol

Graphical Abstract

Correlation matters for molecular shape: The conformational energies of ethanol, propanol, and isopropanol strongly depend on electron correlation effects. Accurate results can be obtained by extrapolation to the basis-set limit. Calculations beyond the MP2 level may be necessary for conformational analysis (see figure).

Abstract

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 CCOH, HCCO, and CCCO 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 CCOH moiety is in the gauche form. Fourth-order correlation effects to the CCOH 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.