Anomeric effect in halogenated methanols: a quantum theory of atoms in molecules study.

The quantum theory of atoms in molecules (QTAIM) has been used to analyze the gauche conformational preference of fluoromethanol and chloromethanol. The analysis of the total atomic population and localization and delocalization indices show trends that are not in line with the hyperconjugative explanation. Energy terms arising from the QTAIM partitioning have been obtained for fluoromethanol, revealing that (i) C-O interaction plays the most significant role in stabilizing the gauche rotamer and (ii) the summation of exchange terms (the only ones that could be related to hyperconjugation) has a smaller weight than electrostatic ones in the energy balance among gauche, anti, and syn conformations; however, they are far from being negligible.