Non-classical buttressing effect: gas-phase ionization of some methyl-substituted benzoic acids

Abstract

The classical concept of the buttressing effect (BE) was recently redefined by us in a general way as the excess energy of a trisubstituted benzene (or another derivative) with respect to the value anticipated from pair-wise interaction energies. This new non-classical definition is applied here to methyl-substituted benzoic acids without ortho substitution: on these compounds BE would be undisturbed by other effects but could not be detected until now. Acidities of the benzoic acids and basicities of their methyl esters were measured in the gas phase by the equilibrium method using Fourier transform ion cyclotron resonance (FT-ICR). By combining these quantities with the enthalpies of formation, it was possible to calculate BE separately for the neutral acid molecules, and for their deprotonated and protonated forms. In the case of uncharged molecules, BE is positive even for non-adjoining substituents, so a kind of steric interaction between distant groups must be accepted as an experimental fact. However, BE is also of comparable magnitude in the deprotonated and protonated forms. As a consequence, the relative ΔBE is strongly reduced in the acidity or basicity values, and is observable only for adjacent substituents. In aqueous solution, it is further attenuated. For this reason, BE has not been revealed previously beyond the classical definition: in all other cases, BE was effectively zero, and the substituent effects in solution were found to be additive.