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 ΔHd, ΔSd and Kd. In addition, the spectroscopic parameters of the difference of the monomer and dimer N-H stretching frequencies (Δν) and the ratio of the molar extinction coefficients of the dimer and monomer (ed/em) are experimentally determined. The values of ΔHd, ΔSd, Δν and ed/em 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.
Highlights
Electrostatic potential maps of all three compounds (Figures 11a, 11b and 11c) show where the electron density shifts when going from monomer to dimer
The Isoxindole electrostatic map reveals very little electron shifting from the carbonyl oxygen to the aromatic ring
The primary difference between Oxindole and Isoxindole is the conjugation of the aromatic ring with the amide N or the amide C=O, respectively
Summary
Pauling’s [2] book entitled The Nature of the Chemical Bond related the importance of intra and intermolecular forces that leads to a deeper understanding of the form and function of molecules. His work and the work of Watson and Crick led to understanding the form of protein and DNA structure from a quantum theoretical perspective. The helical structure of proteins and the double helical structure of DNA obtained by X-ray diffraction of their crystal structures were determined to be the product of multiple hydrogen-bonding. The importance of hydrogen-bonding is fundamental in understanding self-assembling structures in biochemistry [3]
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