Anomeric effect and rotational barrier in fluoromethanol: A theoretical study

The molecular structure, thermodynamic energy, dipole moment, barrier of the internal rotation about the C-O bond, and the stability of gauche and anti conformations of fluoromethanol have been determined using theoretical methods. It has been found that the gauche conformer of CH2FOH is more stable than the anti one. The effect of charge transfer interactions on the stability of gauche conformer has been investigated using the natural bond orbitals (NBOs) analysis. The NBO analysis shows that the delocalization of the oxygen lone pairs to the C-F and C-H antibonding orbitals play a decisive role in the preference of the gauche conformer of CH2FOH. It is predicted that an approximately pure p-type lone pair orbital of oxygen participates in the electron donation to the σ*(C-F) orbital in the gauche structure, whereas a sp0.98 hybrid-type lone pair orbital participates in the electron donation to the σ*(C-F) orbital in the anti structure. In addition, the electronic delocalization contribution to the anomeric effect (ΔΔEdeloc) has been determined as a computational alternative in the evaluation of the excess of the gauche conformer. Bader's 'Atoms In Molecules' (AIM) theory has been also employed for characterization of the C-O and C-F bonds in the CH2FOH molecule. A good correlation between the structural parameters, NBO results, and the properties of charge density has been found.