Theoretical Study of the CH···O Interaction in Fluoromethanes·H2O and Chloromethanes·H2O Complexes

MP2/6-31+G(d,p) calculations are used to analyze the CH···O interaction between X n H 3 - n CH (n = 1, 2, 3, X = F, Cl) as proton donors and H2O as acceptor. Only the constrained linear structures are considered, although they do not correspond, for the CH3X and CH2X2 complexes, to any global minima on the potential energy surface. The interaction energies range from 0.29 to 3.68 kcal mol-1 and are correlated to the intermolecular H···O distances and the acidities of the proton donors, similar to conventional OH···O hydrogen bonds. Interaction with H2O results in a contraction of the CH bond and an elongation of the external CH and CX ones. The CH stretching vibrations are analyzed for the fully or partially deuterated isotopomers in order to decouple them from the other vibrational modes. The CH(D) stretching vibrations are blue-shifted and the CH(D) stretching vibrations of the external CH bonds are red-shifted. Linear correlations are established between the shifts of the CH(D) stretching vibrations and the variations of the distances in the corresponding CH bonds. Complex formation results in a substantial decrease in intensity of the CH(D) stretching vibrations in the methane or fluoromethanes complexes and an increase in intensity in the chloromethanes complexes. In all the studied complexes, the charge transfer from the proton acceptor to the proton donor goes mainly to the lone pair of the X atom(s). By analogy with the conventional hydrogen bonds, there is also an increase in the population of the recipient antibonding molecular orbital, in the present case the σ*(CH) orbital. The interaction energies are correlated to the percentage of s character of the CH bonds in the isolated monomers. The results suggest that the interaction in the CH···O and OH···O systems is characterized by several similar properties, in relatively good agreement with the statements of Gu, Kar and Scheiner (J. Am. Chem. Soc. 1999, 121, 9411). In many aspects, the CHCl3·H2O complex appears as a precursor of the standard hydrogen bonds.