[17] The strength of hydrogen bonding: Infrared spectroscopy

This chapter describes the use of infrared spectroscopy for the detection of H-bonding, estimating H-bond characteristics from the frequency shifts of specific infrared bands, and determining the changes of thermodynamic parameters associated with H-bond formation. In biological macromolecules—including enzymes—H-bonding usually involves O–H or N–H groups as proton donors. In the past few years, it has been suggested that C–H–O hydrogen bonding might also occur in some instances. Careful infrared studies indicate that the stretching and bending vibrations of H-bonded C–H linkages undergo changes that are quite analogous (although smaller) than corresponding changes in O–H and N–H bands. In general, some practical rules have been proposed for infrared measurements for obtaining information on hydrogen bonding. The solvent should be selected with attention to its H-bonding properties. The concentration should be varied to determine the sensitivity of the spectrum to this variable. If possible, sufficiently low concentrations should be used to permit the control of intermolecular association. The temperature should be rigorously controlled. The cell temperature should be varied to detect and identify H-bonded species. Assignments should be verified by deuterium substitution. The solvent should be rigorously dried, alcohol (added as an antioxidant) should be removed from chloroform solutions, solid samples should be annealed and care should be exercised in using KBr pellet spectra.