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 °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, and 1635 cm -1, and the 2D IR-Raman study shows that they 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, amide 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.