Steric inhibition of resonance: a revision and quantitative estimation on the basis of aromatic carboxylic acids

Abstract

The classical concepts of steric inhibition of resonance (SIR) and primary steric effect (van der Waals interactions) were revised with the aid of methyl-substituted benzoic acids. The quantum chemical model was based on the energies of various conformations, calculated at the RHF/6-31 +G(d,p) and B3LYP/6-311 + G(3df,2pd)//RHF/6-31 +G(d,p) levels. The molecule of 2-methylbenzoic acid is planar: no SIR is possible, and van der Waals interaction is practically equal in the acid molecule and in its anion. Therefore, the increased strength of this acid is not due to any steric effect but can be described in terms of electrostatic interaction pole/induced dipole, which lowers the energy of the anion. The molecule of 2,6-dimethylbenzoic acid is nonplanar. SIR is significant in the acid molecule but equal or even greater in the anion. The higher acidity cannot be connected with SIR and can be explained also by electrostatic interaction. In 2,3,5,6-tetramethylbenzoic acid, SIR is greater and may be responsible for one third of the acid-strengthening effect. The concept of SIR is to be applied with caution; even when the nonplanar conformation is proven, SIR need not be responsible for any observable quantity, and particularly not for the acidic and basic properties.