Two-dimensional correlation spectroscopy study of temperature-dependent spectral variations of N-methylacetamide in the pure liquid state .2. Two-dimensional Raman and infrared-Raman heterospectral analysis

This paper demonstrates the potential of two-dimensional infrared-Raman (2D IR-Raman) heterospectral correlation spectroscopy in analyses of IR and Raman spectra of complex systems and of correlations between IR and Raman bands. Temperature-dependent spectral variations of N-methylacetamide (NMA) in the pure liquid state have been investigated as the first example of 2D IR-Raman correlation spectroscopy study, IR and Raman spectra of NMA in the pure liquid state have been measured over a temperature range of 30-65 degrees C, Generalized 2D Raman correlation spectroscopy has been also performed to assist the 2D IR-Raman correlation analysis, The 2D Raman correlation spectroscopic analysis,reveals that the amide I Raman band of NMA consists of at least three distinct bands at 1665, 1650, acid 1635 cm(-1), and the 2D IR-Raman study shows that thy are correlated with the amide I IR bands at the same frequencies, respectively. In the preceding paper, these IR bands at 1635, 1650, and 1665 cm(-1) have been ascribed to large and medium-size oligomers and dimer of NMA, respectively. Therefore, the above Raman bands may also be assignable to them, Comparison of the IR spectra in the 3500-2700 cm(-1) region and the Raman spectra in the 1750-1050 cm(-1) region gives the following correlation: 3275 cm(-1) (IR, amide A)-1650 and 1635 cm(-1) (Raman, amide I)-1305 cm(-1) (Raman, amido III), Combining this correlation with the correlation found for the IR amide bands in the preceding paper, the following correlation can be proposed: 3275 cm(-1) (IR)-1635 cm(-1) (IR)-1570 cm(-1) (IR)-1300 cm(-1) (IR)-1650 and 1635 cm(-1) (Raman)-1305 cm(-1) (Raman). All the bands involved in this correlation decrease in the low-temperature range and thus may be due to the large or medium-size oligomers of NMA. Similar correlation has been obtained for the dimer.