Theoretical Study of the CH···X- Interaction of Fluoromethanes and Chloromethanes with Fluoride, Chloride, and Hydroxide Anions

The present work focuses on the analysis of the CH···X- interactions between Y n H3 - n CH (n = 0−3, Y = F, Cl as proton donors and X- (X = F, Cl, OH) anions as proton acceptors using the MP2/6-31+G(d,p) method. The optimized geometries of these complexes and their vibrational frequencies and intensities are discussed. The changes in the populations of the relevant molecular orbitals are calculated using the natural bond orbital (NBO) analysis. The interaction energies range from 2.4 to 32.4 kcal mol-1. The Y n H3 - n CH (n = 0, 2, 3)·X- systems are stabilized by CH···X- hydrogen bonds. It is demonstrated that the interaction results in an elongation of the incipient CH bond, a red shift of the ν(CH) vibration, and an increase of its intensity. The NBO analysis shows that the charge transfers go mainly to the σ*(CH) antibonding molecular orbital (MO) and, to a lesser extent, to the lone pairs of Y. It is also shown that the lengthening of the CH bond increases with the intermolecular distance and the increase of the population of the σ*(CH) MO. The interaction energies are correlated to the frequency shifts of the ν(CH) vibration. The CH3Y·X- complexes show contrasting behavior. The intermolecular distances and angles indicate that the CH3Y molecules and the X- anions are not stabilized by CH···X- hydrogen bonds. In contrast with the other systems, complex formation causes a contraction of the CH bond, a blue shift of the ν(CH) vibration, a small decrease of the population of the σ*(CH) MO, and a marked increase of the population of the σ*(CY) MO. These features indicate that the CH3Y·X- systems are stabilized by electrostatic interactions and by charge transfer taking place in the remote part of the CH3Y molecule.