Synergistic Effects of Conjugation and Polarization on Dual Amide Hydrogen-bonding in CCl₄: Theory and Experiment

Abstract

The doubly hydrogen-bonded dimer of pyrrolidinone (γ-lactam) is used as a benchmark for the effect of conjugation on the strength and extent of hydrogen-bonding of the compounds Oxindole and Isoxindole. The experimental portion of this project consists of collecting the FTIR spectrum of γ-lactam, Oxindole and Isoxindole in CCl4. The concentrations and temperatures were varied to determine the thermodynamic properties of ∆Hd, ∆Sd and Kd. In addition, the spectroscopic parameters of the difference of the monomer and dimer N-H stretching frequencies (∆ν) and the ratio of the molar extinction coefficients of the dimer and monomer (εd/εm) are experimentally determined. The values of ∆Hd, ∆Sd, ∆ν and εd/εm are computed using MP2 and B3LYP methods with 6-31G** and 6-31+G** basis sets. The experimental values are best described by B3LYP/6-31G**with the incorporation of the dielectric effect (PCM) of CCl4. The experimental and computational results support the results of increased or decreased hydrogen-bond strength due to conjugation effects on the polarization of the monomers in forming dimers.