Determination of the conformation of 2-hydroxy- and 2-aminobenzoic acid dimers using 13C NMR and density functional theory/natural bond order analysis:: The central importance of the carboxylic acid carbon

The C-13 chemical shift for the carboxylic acid carbon provides a powerful diagnostic probe to determine the preferred isomeric dimer structures of benzoic acid derivatives undergoing intra- and intermolecular H-bonding in the gas, solution and crystalline phases. We have employed hybrid density functional calculations and natural bond orbital analysis to elucidate the electronic origins of the observed C-13 shieldings and their relationship to isomeric stability. We find that delocalizing interactions from the carbonyl oxygen lone pairs (n(O)) into vicinal carbon-oxygen and carbon-carbon antibonds (sigma*(CO),sigma*(CC)) make critical contributions to the C-13 shieldings, and these n(O) -> sigma*(CO), n(O) -> sigma*(CC) interactions are in turn sensitive to the intramolecular interactions that dictate dimer structure and stability. The carboxyl carbon atom can thus serve as a useful detector of subtle structural and conformational features in this pharmacologically important class of carboxylic acid interactions.