Two-dimensional correlation spectroscopy study of temperature-dependent spectral variations of N-methylacetamide in the pure liquid state .1. Two-dimensional infrared analysis

This paper reports two-dimensional Fourier-transform infrared (2D FT IR) correlation spectroscopy study of temperature-dependent spectral variations of N-methylacetamide (NMA) in the pure liquid. The 2D correlation spectroscopic analysis has demonstrated that the amide I band of NMA consists of at least four distinct bands at 1685, 1665, 1650, and 1635 cm(-1). The signs of asynchronous cross peaks corresponding to these four bands have shown that the sequence of spectral intensity change in the ascending order of temperature is given by 1635 cm(-1) < 1650 cm(-1) < 1665 cm(-1) < 1685 cm(-1). These bands at 1635, 1650, 1665, and 1685 cm(-1) may be due to the amide I modes of chain oligomers of various sizes and dimer of NMA. The longer the chain, the lower the frequency. Similarly, it has been found that the amide A, II, and III bands consist of at least two bands. For example, the amide II is split into two bands at 1570 and 1545 cm(-1). The 2D correlation analysis between different spectral regions has enabled to make correlations among each band of the amide A, I, II, and III. The amide I band at 1635 cm(-1) is correlated with the amide A at 3275 cm(-1), amide II at 1570 cm(-1), and amide III at 1300 cm(-1), while the amide I band at 1665 cm(-1) is correlated with the amide A at 3335 cm(-1), amide II at 1545 cm(-1), and amide III at 1285 cm(-1). The asynchronous correlation peaks have provided new information for bands due to asymmetric and symmetric bending and rocking modes of the methyl groups. The intensities of the bands at 1455 and 1159 cm(-1) due to the asymmetric methyl bending mode and methyl (-N) rocking mode, respectively, decrease at a lower temperature than those of other bands assignable to amide modes. The result indicates that the conformational changes of the methyl groups is a precursor of the full dissociation of amide hydrogen bondings.