Hydrogen bonded С–H···Y (Y = O, S, Hal) molecular complexes: A natural bond orbital analysis

Hydrogen bonded C–H···Y complexes formed by H 2 O, H 2 S molecules, hydrogen halides, and halogen-ions with methane, halogen substituted methane as well as with the C 2 H 2 and NCH molecules were studied at the MP2/aug-cc-pVDZ level. The structure of NBOs corresponding to lone pair of acceptor Y, n Y , and vacant anti-σ-bond C–H of proton donor was analyzed and estimates of second order perturbation energy Е (2) characterizing donor–acceptor n Y → σ C-H * charge-transfer interaction were obtained. Computational results for complexes of methane and its halogen substituted derivatives show that for each set of analogous structures, the Е nY→σ*C-H (2) energy tends to grow with an increase in the s-component percentage in the lone pair NBO of acceptor Y. Calculations for different C···Y distances show that the equilibrium geometries of complexes lie in the region where the E (2) energy is highest and it changes symbatically with the length of the covalent С–H bond when the R (C···Y) distance is varied. The performed analysis allows us to divide the hydrogen bonded complexes into two groups, depending on the pattern of overlapping for NBOs of the hydrogen bridge.