The 3-substituted-2-methoxybenzoic acid system exhibits resonancestabilized intramolecular hydrogen bonding between the 2-methoxy oxygen and the adjacent carboxylic acid. This intramolecular hydrogen bond can be disrupted by adding another substituent with variable size on the neighboring 3-position of the ring. To relieve steric strain, the system must sacrifice hydrogen bonding andror resonance Ž stabilization. Full-energy optimizations have been done at HFrD95V valence double-zeta . U Ž . Ž Dunning᎐Huzinaga , HFr6-31G Pople , HFrD95 full double-zeta . Ž . Ž . Dunning᎐Huzinaga , HFrD95V d, p , and HFr6-31 q G d, p . Further single-point U Ž . calculations were done at MP2rD95V, MP2r6-31G , MP2rD95, MP2rD95V d, p , and Ž . MP2r6-31qG d, p . The thermal populations of various conformational states including the hydrogen-bonding conformation are presented. The computational results were compared with the experimental thermal population of hydrogen bonding determined by Ž .
Ž . nuclear magnetic resonance NMR and infrared IR spectroscopies. Results indicate that polarization of the second-row elements in intramolecular hydrogen bonding and perturbation-theory calculations that correct for electron correlations are very important for intramolecular hydrogen bonding. Adding polarization and diffuse functions to the hydrogens, while useful, are quite costly for these systems and do not seem to be as important.