Blue-Shifting C−H···X (X = O, Halogen) Hydrogen Bonds in the Dimers of Formaldehyde Derivatives

Abstract

The potential energy surface (PES) of the dimers of formaldehyde derivatives CH(O)Y (Y = H, CH3, F, Cl, Br, I) has been investigated by means of quantum chemical calculations at the MP2/6-311++G** level. Several minima have been found on the PES characterized by various combinations of C−H···X (X = O, halogen) contacts. The computed dimerization energies revealed the importance of dispersion forces in the formation of [CH(O)Y]2 dimers, while only a lesser role of the intermolecular H···X interactions. The most characteristic geometrical properties of the dimers are the H···X distances being near the sum of the van der Waals radii of the contacting atoms, the lengthening of the contacting C−X bonds, and the general shortening of the C−H bonds by 0.001−0.004 Å with respect to the monomers. The latter bond shortening is responsible for the characteristic blue-shift of the CH stretching frequencies in the dimers. A natural bond orbital (NBO) analysis revealed a slight decrease in the population of the contacting σ*CH orbitals and alterations in the intramolecular charge-transfer effects as the primary reason of the C−H contraction.