Steric effects in crowded molecules in the gas phase: Polymethyl-substituted benzoic acids

The gas-phase acidities of seven polymethyl benzoic acids have been measured by Fourier transform ion cyclotron resonance IFT-ICR) spectrometry, and estimated from semiempirical AMI and PM3 calculations. Together with the derivatives investigated previously, complete data are now available for all methyl-substituted benzoic acids. AMI calculations reproduce reasonably well the relative enthalpies of formation provided ortho derivatives are considered separately, and can be used to determine conformations and approximate conformational equilibria. However, only the trends in the gas-phase acidities are reproduced by the AMI calculations. Empirical correlations have been carried out using two approaches. The additivity scheme approach distributes the errors equally among all derivatives but requires special correction terms that account for the steric crowding. The substituent effect approach starts from monomethyl derivatives and attempts to separate polar and steric effects. In a further step, the buttressing effect (BE) is defined as the excess energy in dimethyl and polymethyl derivatives. Both the steric effect and BE have been obtained independently, either from enthalpies of formation or from gas-phase acidities, with concordant results. The final conclusion is that the global steric effect of two substituents is always larger than the sum calculated from effects in mono derivatives. Nevertheless, in more substituted compounds, this excess of steric energy tends to approach a limit.