Steric inhibition of resonance: Revision of the principle on the electronic spectra of methyl-substituted acetophenones

Abstract

The principle of steric hindrance to resonance (SIR) was revised on the well-known example of the electronic spectra of methyl-substituted acetophenones. The spectra were reinvestigated on an extended series 1-14, and computer separation of bands was undertaken. The conformation of these molecules, total energy, rotational barrier, geometry of the transition state and electronic spectra were calculated within the framework of the density functional theory. The classical literature was correct that some molecules of the series are twisted from the planar arrangement by a variable angle φ. However, these angles do not agree with the values anticipated; the main difference is that all derivatives with only one ortho-methyl group are planar (φ = 0). Our main objection is that the rotational barrier is in some cases very low; hence Δ4E does not have the same importance with all derivatives. A better measure of steric hindrance is the calculated energy Δ4E of the isodesmic reaction Eqn (4), in which the molecule of the substituted acetophenone is formed from acetophenone and a methyl-substituted benzene. In contrast to the angles φ, there was good agreement between calculation and experiment in electronic spectra, viz. between the extinction coefficients of the 1La band and the calculated oscillator strengths, at least in the relative values. Summarizing, SIR is valid as a qualitative principle but the reported angles φ based on several experimental quantities are often wrong. Each case must be examined separately as to the actual shape of the molecule. The classical paper by Braude et al. is a particularly bad example: the basic hypothesis of a planar excited state was not confirmed by our calculations. Copyright © 2004 John Wiley & Sons, Ltd.