C-H⋯X interactions of fluoroform with ammonia, water, hydrogen cyanide, and hydrogen fluoride: Conventional and improper hydrogen bonds

The structural changes, dipole moments, and interaction energies for a series of C-H⋯X hydrogen bonded complexes involving F₃CH as proton donor and NH₃, OH₂, NCH, and FH as proton acceptors have been studied using ab initio calculations including electron correlation effect for various basis sets. The NH₃ shows a conventional hydrogen bond, while the other molecules show improper hydrogen bonds as evidenced by the C-H bond length shortening and the blue-shift of C-H stretching vibrational frequency. In the complex of F₃CH⋯NCH, both C-H and N-C bonds are contracted, and their corresponding stretching vibrational frequencies are blue-shifted compared with monomers. The blue-shifted stretching vibration in the proton acceptor molecule has not been much addressed until now. Both conventional and improper hydrogen bonds are noted in the nonlinear complex of F₃CH⋯FH, where FH molecule plays a dual role of proton acceptor and donor at the same time. The interaction energies are almost equivalent for HF and MP2 calculations, which does not support a recent proposition that the dispersive interaction is the origin of improper H-bonding.