Vibrational Spectroscopy of hydrogen bonding:: Origin of the different behavior of the C-H•••O hydrogen bond

The spectroscopic behavior of the C-H...O hydrogen bond is generally opposite to that of typical (e.g., O-H...O and N-H...O) hydrogen bonds: the C-H stretching frequency is blue-shifted and the infrared band intensity decreases on hydrogen bonding. We show that this can be understood on the basis of the dynamic properties of the donor group, in particular the force on the bond resulting from the interaction of the external electric field created by the acceptor atom with the permanent and induced dipole derivatives of the X-H bond: when the field and dipole derivative are parallel, as in the case of O-H...O, the bond lengthens, and red shift and intensity increases result; when the field and dipole derivative are antiparallel, as in the case of C-H...O, the bond shortens and blue shift results with the possibility of intensity decrease. We demonstrate these properties in an ab initio and perturbation study of cis formic acid in an Onsager reaction field. This analysis provides the basis for a more general understanding of the intermolecular electrical interactions that are the underlying impetus for the formation and properties of hydrogen bonds.