Conformation of Aromatic Carbonyl Derivatives: An Infrared Study

Abstract

Infrared spectra of 14 methyl- and polymethyl-substituted acetophenones and of 14 equally substituted methyl benzoates were investigated in the carbonyl region in tetrachloromethane solution and correlated with their conformation. The ν(C=O) frequency depends moderately on the inductive effect of substituents and more sensitively on the steric effect of ortho substituents. Steric effects are not simply related to the torsion angle ϕ (calculated by the density functional theory) but are better described by dividing the methyl substituents into three classes: nonplanar derivatives with methyl groups in the two ortho positions, slightly nonplanar derivatives with two methyl groups in the positions 2 and 3 (buttressing effect), and the remaining derivatives existing in two equilibrium planar conformations. Eleven various physical properties of methyl-substituted acetophenones, measured or calculated previously, were now correlated by principal component analysis (PCA). Two components are necessary to explain 93% of variance; they relate roughly to the steric and inductive effects. Steric effect affects all properties while inductive effect is of importance mainly for the gas-phase basicities and wavelengths in the UV spectra. Both can be separated very effectively using the classic comparison of ortho and para derivatives; in this way, the steric effect is isolated and in a subsequent PCA only one component is sufficient to explain 94% of variance.