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Infrared spectra and conformation of methyl‐substituted benzoic acids
Author(s) -
Fiedler Pavel,
Exner Otto
Publication year - 1998
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199802)11:2<141::aid-poc982>3.0.co;2-t
Subject(s) - chemistry , conformational isomerism , substituent , monomer , computational chemistry , spectral line , infrared , infrared spectroscopy , benzoic acid , degenerate energy levels , equilibrium constant , crystallography , stereochemistry , molecule , organic chemistry , polymer , physics , astronomy , optics , quantum mechanics
Infrared spectra of all isomers of polymethyl‐substituted benzoic acids were recorded in the carbonyl and hydroxyl regions in tetrachloromethane at various concentrations and interpreted in terms of conformation. According to a plot of ν(C=O) of the monomeric form vs Hammett substituent constants σ, these compounds may be classified into two classes. Derivatives with none or only one methyl group in the ortho position are concluded to exist in an equilibrium of two planar conformations, unless the equilibrium is degenerate. Derivatives with two ortho ‐situated methyl groups are concluded to take up one non‐planar conformation. These findings are supported by the shape of the hydroxyl and carbonyl bands, which are unsymmetrical in the former class, although they could not be reliably separated into bands pertinent to the individual conformers. This conclusion is at variance with the common interpretation which has invariably ascribed to these and similar ortho derivatives non‐planar conformations with a variable torsional angle. © 1998 John Wiley & Sons, Ltd.