DFT study of the monomers and dimers of 2-pyrrolidone: equilibrium structures, vibrational, orbital, topological, and NBO analysis of hydrogen-bonded interactions.

A computational study of the monomers and hydrogen-bonded dimers of 2-pyrrolidone was executed at different DFT levels and basis sets. The above dimeric complexes were treated theoretically to elucidate the nature of the intermolecular hydrogen bonds, geometry, thermodynamic parameters, interaction energies, and charge transfer. The processes of dimer formation from monomers and concerted reactions of double proton transfer were considered. The evolution of geometry, vibrational frequencies, charge distribution, and AIM properties in going from monomers to dimers was systematically followed. The solvent effects upon dimer formation were investigated in terms of the self-consistent reaction field (SCRF Onsager model). For the monomers and three dimers, vibrational frequencies were calculated and the changes in frequencies of the vibrations most sensitive to complexation were discussed. The orbital interactions were shown to lengthen the X-H (X = N, O) bond and lower its vibrational frequency (a red shift). To better understand the nature of the corresponding intermolecular interactions, we performed natural bond orbital (NBO) analysis. Topological analysis of electron density at bond critical points (BCP) was executed for complex molecules using the Bader's atoms in molecules (AIM) theory. The interaction energies were calculated, and the basis set superposition errors (BSSE) were estimated systematically. Satisfactory correlations between the structural parameters, interaction energies, and electron density characteristics at BCP were found.