NBO and AIM analyses of the anomeric effect in fluoromethanthiol

The origin of generalized anomeric effect in fluoromethanthiol, CH2FSH, has been investigated by means of ab initio calculations and Natural Bond Orbital (NBO) and Atoms In Molecules (AIM) analyses. The calculated energy data at various computational levels show that the electron correlation increases the relative energy of less stable conformer (anti), while augmenting the basis set with diffuse and polarization functions acts in the opposite way. The NBO analysis reveals that a very close to pure p-type lone pair orbital and a sp0.41 hybrid-type lone pair orbital participate in the electron donation to the σ*(C-F) orbital in the gauche and anti structures, respectively. The NBO analysis also shows that the electrostatic and steric contributions included in the Lewis term tend to keep anti conformer, whereas delocalization contribution tends to gauche conformer, as expected for an anomeric effect. The origin of the anomeric effect in CH2FSH has been also compared with CH2FOH. The obtained results of thermodynamic calculations and NBO and AIM analyses show that the anomeric effect in CH2FSH is slightly smaller than CH2FOH.