AN INVESTIGATION OF HYDROGEN-BONDING IN AMIDES USING RAMAN-SPECTROSCOPY

Raman and preresonant Raman spectra are reported for epsilon-caprolactam and N,N-dimethylacetamide in both the gas phase and the neat liquid and as a function of concentration in aqueous and other solutions. These spectra are analyzed to determine the influence of hydrogen bonding, particularly amide-amide hydrogen bonding, on amide structure and spectroscopy. A shift in intensity from the Am I (carbonyl stretch) band to the Am II band (C-N stretch) is observed as the extent of intermolecular amide-amide and amide-water hydrogen bonding increases. For epsilon-caprolactam, which can hydrogen bond to itself, a substantial shift in intensity in these amide bands occurs between the gas and the neat liquid. The formation of hydrogen-bonded complexes, for which there is a clear spectral signature in the Raman spectrum, is indicated. In contrast, for N,N-dimethylacetamide the Am I to Am II intensity shift is seen only upon aqueous solvation and is directly proportional to the mole fraction of water present. Shifts in the Am I vibration to lower frequency are also observed upon solvation for both epsilon-caprolactam and N,N-dimethylacetamide whether or not hydrogen bonding is present. However, the magnitudes of these shifts increase with the extent of hydrogen bonding. The Am I carbonyl band in neat epsilon-caprolactam liquid and in acetonitrile solution consists of two peaks, one of which we assign to the unassociated monomer and the other to the cyclic dimer. In aqueous solution the carbonyl band of N,N-dimethylacetamide also consists of two peaks, which appear to be associated with a free and a hydrogen-bonded form.