Theoretical study on salicylic acid and its analogues: intramolecular hydrogen bonding

The molecular structures and the intramolecular hydrogen bonding for salicylic acid, 2-hydroxythiobenzoic acid, 2-hydroxythionobenzoic acid and 2-hydroxydithiobenzoic acid have been precisely investigated by an ab initio method and density functional theory (DFT). We have considered two possible conformations for each molecule in order to understand the nature of the intramolecular hydrogen bonding between these conformers. The optimized geometrical parameters and the computed H-1 NMR chemical shifts for the syn conformers 1a, 2a, and 4a at the B3LYP levels are in good agreement with previous experimental results. It is confirmed from these results that the inclusion of electron correlation is crucial to elucidate the molecular properties of the intramolecular hydrogen bonding systems. The hydrogen bonding energies for salicylic acid, 2-hydroxythiobenzoic acid, 2-hydroxythionobenzoic acid, and 2-hydroxydithiobenzoic acid are computed to be 10.5, 8.4, 6.4, and 4.2 kcal/mol, respectively, at the MP2/6-31+(d) level. Moreover, it is found that the computed proton NMR chemical shifts for the O-H proton (H-11) in the syn conformers 1a, 2a, 3a, and 4a are well correlated with the strength of the hydrogen bonding in these molecules. (C) 2000 Elsevier Science B.V. All rights reserved.