Vibrational Spectroscopic Signature Indicating the Mixing of the Cis Conformers in Liquid N-Methylformamide

For the purpose of seeking vibrational spectroscopic signatures of the mixing of the cis conformers in liquid N-methylformamide (NMF), infrared (IR) and Raman spectra are calculated by the ab initio molecular orbital method for three clusters of NMF: linear and cyclic hexamers of the trans conformers, and a cyclic trimer of the cis conformers. It is found that a small feature at ∼250 cm−1 in the observed Raman spectrum of liquid NMF corresponds to the strongly Raman-active modes calculated at 263 cm−1 for the cis cyclic trimer. Considering that there is no vibrational mode around this frequency position for the clusters of the trans conformers, it is suggested that the Raman band at ∼250 cm−1 is a marker band of hydrogen-bonded cis conformers, and the existence of a small feature at this position in the observed Raman spectrum of liquid NMF is a vibrational spectroscopic signature indicating the mixing of the cis conformers in this liquid. However, the main features of the observed low-frequency Raman spectrum (10–400 cm−1) of liquid NMF are explained by the strongly Raman-active modes of the clusters of the trans conformers, in contradiction to the suggestion in a recent x-ray diffraction study that liquid NMF is dominated by the cis conformers. The features of the IR and Raman spectra in the amide I (C=O stretching) region are also discussed.