Merged- and separate-band behavior of the C═O stretching band in N,N-dimethylformamide isotopic liquid mixtures: DMF/DMF-d1, DMF/DMF-d6, and DMF/DMF-13C═O.

A combined experimental and theoretical analysis is carried out on the polarized (isotropic and anisotropic) Raman spectra in the spectral region of the C═O stretching (amide I) band of three isotopic liquid mixtures of N,N-dimethylformamide (normal/d(1), normal/d(6), and normal/(13)C═O). Two distinct types of spectral behavior are found for the isotropic Raman spectra: the separate-band behavior (for normal/(13)C═O), where two separate bands (one for each species) appear at all concentrations but with significant intensity bias, and the merged-band behavior (for normal/d(6)), where only one band appears at a frequency between those of the two species and with a band shape noticeably different from the simple overlap of their profiles. An intermediate case between these two limits is also found (for normal/d(1)). These main spectral features, as well as the noncoincidence effect (NCE) observed for all the mixtures and neat liquids, are well reproduced by the calculations, meaning that (1) the computational procedure (carried out in the time domain) incorporates all the important factors that determine the main spectral features, and (2) the band merger and the intensity bias are both controlled by the same type of term (resonant intermolecular vibrational coupling) of the vibrational Hamiltonian that gives rise to the NCE. Based on this result, the one- and two-dimensional infrared spectra of the normal/d(1) 1:1 mixture are calculated as theoretical predictions. For this purpose, an eigenstate-free method is developed to increase the efficiency of the time-domain spectral calculations and to do the calculations on a largest possible system. The calculated spectral features are compared with those of the polarized Raman spectra and discussed.