An analysis of the N-methyl-2-pyrrolidone: water complex using computational and matrix isolation FTIR methods

Abstract

The weakly bound complexes of N-methyl-2-pyrrolidone (NMP) and water have been analyzed using a combination of computational methods and matrix isolation FTIR spectroscopy. The computational analysis utilized density functional and perturbation theory methods to determine the lowest energy geometries and vibrational frequencies of NMP: H₂O. This analysis yielded four unique structures that could be differentiated by their preferred intermolecular interaction. Two structures formed via relatively strong OH⋯O hydrogen bonds, one structure was stabilized via OH⋯N interactions, and the fourth structure was observed to interact through relatively weak CH⋯O features. The interaction motifs were verified using atoms in molecules analysis and the noncovalent interaction index method. Spectra of NMP with H₂O and its isotopologues showed clear evidence of two unique structures in the cryogenic nitrogen matrix. Both of these structures formed through OH⋯O interactions from the water to the carbonyl oxygen of NMP. This structural assignment was supported by the calculated vibrational shifts seen in NMP: H₂O. A detailed analysis and discussion of this assignment is provided.