Effect of multiple & cooperative intramolecular hydrogen bonding on polyhydroxylated thiopyrans acidities: Detailed view from AIM & NBO analyses

Abstract

Gas-phase acidity (GPA) and pKa calculations were computed using MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) method to predict the acidity of polyhydroxylated thiopyrans. To realize the character of hydrogen bonding interactions, the quantum theory of atoms in molecules (QTAIM) and natural bonding orbital (NBO) analyses have also been used. Results exhibit that by adding each hydroxyl group, ΔHacid, ΔGacid and pKa values decrease. Therefore, the intramolecular hydrogen bonds lead to enhance the acidity strength. In both gas and solution phases (DMSO & H2O), the β-5-thio-d-glucopyranose-OH1 (4-OH1) is found to be the most acidic compound with the GPA of 348.1 kcal.mol−1 and pKa of 18.10 & 9.5 kcal.mol−1 (in DMSO & H2O, respectively). Moreover, a linear correlation between the GPA and pKa was obtained and can be used for pKa prediction of the similar compounds in the solution phase.