Energy of crowded molecules: 3,4,5-trimethoxybenzoic acid

The enthalpy of combustion and enthalpy of sublimation of 3,4,5-trimethoxyb enzoic acid (2) were measured, and the gas-phase enthalpy of formation at 2 98.15 K was calculated. Ab initio energies at the B3LYP/ 6-311+G(d,p) level were calculated for this compound, some mono- and dimethoxybenzoic acids, and a similar series of methyl-substituted benzoic acids. Intramolecular in teraction (called the buttressing effect) of the groups COOH and OCH3 (or C H3) was defined in terms of isodesmic homodesmotic reactions. Steric crowdi ng raises the energy of 2 in relation to monomethoxybenzoic acids, but no f urther effect is observed in comparison with 3.,4-dimethoxybenzoic acid, on either calculated or experimental values. Similar results were obtained wi th 3,4,5-trimethylbenzoic acid (1); a previous claim of interaction of remo te groups in the meta position was based on inaccurate enthalpies of format ion of reference compounds.