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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.

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Infrared and Raman spectra have been obtained of crystalline glutathione and its deuterated derivative and interpreted by normal mode analysis. The force field consisted of our empirical force fields for the peptide group and NH3+ and CO2- end groups, plus our ab initio force fields for the CH2SH and CH2COOH moieties. Observed bands are reproduced with an average error of 5 cm-1, demonstrating that the vibrational spectrum of such a complex molecule can be understood in great depth.