Low-frequency vibrational spectra of some hydrogen-bonded amides

Low-frequency infrared and Raman spectra at 10–400 cm−1 are presented for some simple amides. Both the Stokes and the anti-Stokes sides are given for N-methylformamide to ensure that no fluorescence bands are present in the spectrum. Spectra are presented for compounds in the liquid state and that for N-methylacetamide is compared with spectra of the crystalline compound at −15°C−20°C. The liquid state spectra are all given in the R(v̄)-representation. A complex is formed between lithium ions and formamide molecules and the vibrational spectra are discussed in terms of this complex. Hydrogen bonding in these simple amides is used as a model for more complicated hydrogen-bonded biological systems. Some synthetic polymers such as nylon, are also very useful in this context. The spectrum of nylon-6 is presented and a low-frequency band with a maximum at 103 cm−1 is assigned to a vibration involving H-bonded atoms.