The effect of hydroxyl groups on the stability and thermodynamic properties of polyhydroxylated xanthones as calculated by density functional theory

There are three types of intramolecular hydrogen bonds with bond energy about 52 kJ mol−1, 12 kJ mol−1, 20 kJ mol−1, respectively in PHOXTHs which were determined by computation on B3LYP/6-311G** level. The internal rotational potentials of the hydroxy group of 1-MHOXTH and 4′-MHOXTH are evaluated, and the influences of the spatial orientation of the hydroxy groups on the intramolecular hydrogen bonds and molecular stability are illustrated. The standard enthalpy of formation (ΔfHθ) and standard Gibbs energy of formation (ΔfGθ) for the most stable conformation of 135 PHOXTHs are calculated by the combination of Gaussian 03 and isodesmic reactions and the theoretical order of relative stability is proposed according to the relative magnitude of calculated ΔfGθ values. In addition, the values of molar heat capacities at constant pressure (Cp,m) from 200 to 1000 K for PHOXTH congeners are calculated.