Hydrogen bonding characteristics of 2-pyrrolidinone: a joint experimental and theoretical study

Abstract

The mid-IR spectrum of the hydrogen bonding lactam 2-pyrrolidinone in CCl₄ was studied using FT-IR spectroscopy accompanied by a quantum chemical anharmonic normal mode analysis combined with a Monte Carlo approach based on semi-empirical harmonic frequencies. We characterized and assigned the spectroscopic features in the range from 1500 cm⁻¹ to 3600 cm⁻¹ covering the respective amide bands related to hydrogen bonding CO and NH groups as well as the backbone CH vibrational band. By comparing theory and experiment we are able to assign all mid-IR features to a variety of distinct structures of 2-pyrrolidinone, ranging from monomers, singly and doubly hydrogen bonded dimers to longer hydrogen-bonded chains. These chains account for the most prominent features in the IR spectrum. Furthermore, two peaks in the CH band have been identified as highly localized axial and equatorial C–H stretch vibrations qualified for probes of conformational dynamics. The results provide a detailed microscopic picture of 2-pyrrolidinone in solution, helpful for further dynamical structure studies of this amide.