Two-dimensional Fourier-transform near-infrared correlation spectroscopy study of dissociation of hydrogen-bonded N-methylacetamide in the pure liquid state

Generalized two-dimensional (2D) correlation spectroscopy has been applied to the near-infrared (NIR) region to investigate temperature-dependent spectral variations of N-methylacetamide (NMA) in the pure liquid state. The 2D NIR approach clearly reveals that there are at least five bands in the 6800-6050 cm(-1) region. These five bands at 6790, 6650, 6510, 6440, and 6250 cm(-1) are assigned, respectively, to the first overtones of stretching modes of the free NH group of the monomer, terminal-free (free-end) NH groups of the dimer and oligomers, and hydrogen-bonded NH groups of the dimer and oligomers of NMA. To monitor the complex sequence of events arising from the dissociation of hydrogen-bonded NMA oligomeric association complexes of various sizes during the entire temperature range from 30 to 90 degrees C, we have focused on a relatively small number of spectra representing a narrow window of temperature change. On the basis of this approach, the following mechanism for the dissociation of polymeric species of NMA may be proposed. (1) The dissociation of the oligomers is coupled with the formation of the monomer as well as the creation of smaller oligomers. (2) The production of medium-size oligomers from the dissociation of large oligomers occurs in the lower temperature region. As the temperature goes up, such medium-size oligomers are also consumed by the dissociation of hydrogen bonds. (3) The production of oligomers and dimers continues well into the higher temperature range. The increase of these populations eventually slows down at a higher temperature, however, as they start dissociating to the monomers.