Intramolecular hydrogen bonding in resonance‐stabilized systems

Abstract

The 3-substituted-2-methoxybenzoic acid system exhibits resonance-stabilized 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 and/or resonance stabilization. Full-energy optimizations have been done at HF/D95V (valence double-zeta Dunning–Huzinaga), HF/6-31G* (Pople), HF/D95 (full double-zeta Dunning–Huzinaga), HF/D95V(d, p), and HF/6-31+G(d, p). Further single-point calculations were done at MP2/D95V, MP2/6-31G*, MP2/D95, MP2/D95V(d, p), and MP2/6-31+G(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 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. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 863–875, 1998