An effective method for studying intermolecular interactions in binary liquids with hydrogen bonds; FTIR spectra and ab initio calculations in the N-methylformamide-methanol system.

Molecular complexes in methanol (MeOH)-N-methylformamide (NMF) mixtures were studied based on their FTIR-ATR spectra, to which two methods of analysis were applied: factor analysis and a quantitative version of the difference-spectra method. The mean composition of a complex between NMF and MeOH molecules over the whole range of mixture compositions was determined. Absorbing species differentiated with regard to the interaction energies of the carbonyl oxygen with methanol molecules were recognized in both compositional regions with a marked excess of one component. Possible structures for complexes of various stoichiometries were optimized by ab initio calculations in the gas phase and both liquid NMF and MeOH using the polarizable continuum model (PCM). Thermodynamic functions calculated for the optimized structures were used to find the most stable structure for each stoichiometry. Individuals distinguished by the spectral analysis were assigned to the complexes of definite composition, and a linear correlation between the positions of the carbonyl group absorption and the total interaction energies of the complexes was found. The results of the spectral analysis of the NMF-MeOH mixtures were compared to those we obtained previously for similar binary systems, i.e., mixtures of methanol and formamide (FA) or N,N-dimethylformamide (DMF). It was shown that the factor analysis applied to the infrared spectra is an effective method for distinguishing molecular complexes with different polarizations of component molecules and allows for the detection of even weak intermolecular interactions and low-concentration species. Combined with the difference-spectra method, factor analysis provides a comprehensive picture of intermolecular interactions in binary mixtures.