Theoretical study on the adsorption and relative stability of conformers of L-ascorbic acid on γ - alumina (100) surface

The adsorption energies (E-ads) and relative stabilities of selected conformers of the most stable tautomer of L-ascorbic acid (vitamin C) on the dehydroxylated gamma-alumina (100) surface were calculated in both gas phase and solvent (water) using the density functional theory (DFT) method. The selected conformers were related to the different rotational angles of OH groups of L-ascorbic acid. The conformational analysis of bare tautomer in both gas and water showed that the conformer No.20 (conf. 20) and 13 (conf. 13) with the dihedral angles of H-15-O-10-C-11-C-9 (-73 degrees) and H-20-O-19-C-9-C-11 (-135 degrees) were the most stable and unstable conformers, respectively. The performed calculations in the presence of surface showed that the interaction of the conformers with the surface changes their relative stabilities and structures in both gas phase and water. The E-ad of each conformer was calculated and it was determined that conf. 8 and conf. 16 have the highest value of E-ad in the gas phase (-62.56 kcal/mol) and water (-54.44 kcal/mol), respectively. The optimized structure of each conformer on the surface and the number of hydrogen bonds between it and surface along with their bond lengths were determined. (C) 2017 Elsevier B.V. All rights reserved.