Demianenko E. Quantum chemical study on the destruction of acetylsalicylic acid, ascorbic acid, and histamine adsorbed on silica surface

The thesis is devoted to the theoretical study on the mechanisms of degradation of organic substances adsorbed on silica surface by ab initio Hartree-Fock and density functional theory methods using various basis sets and functionals (USGAMESS, PCGAMESS and GAUSSIAN 09 program packages). The dissociation of orthosilicic acid molecule in water clusters has been studied. Thermodynamic properties of the reactions of proton transfer from orthosilicic acid molecule to water molecules and the equilibrium constant of this process have been calculated. It has been shown that the values of dissociation constants of orthosilicic acid in water closest to the experimental findings are those given by DFT calculations involving the effect of solvent. The mechanisms of hydrolytic degradation of acetylsalicylic acid in aqueous solution at the pH values within 4 to 8.5 as well as those involving silica surface have been considered by quantum chemical methods. It has been shown that due to specific interaction of silica surface with water molecules and acetylsalicylic acid ones, the activation energy of its hydrolysis is lower than that in aqueous solution. The oxidation mechanisms of ascorbic acid molecule and its monoanion in aqueous medium by molecular oxygen have been studied. It has been shown that under normal conditions oxygen molecule favourably interacts with partially dissociated ascorbic acid molecule rather than with its non-dissociated form. Silica surface hinders oxidation of ascorbic acid with air oxygen due to its proton-donor properties and dissociation of silanol groups; in aqueous solution within the surface layer, the equilibrium dissociation of ascorbic acid is shifted toward the formation of its molecular forms. The mechanisms of thermal degradation have been examined for both free histamine and chemisorbed one on silica surface, occuring in a temperature-programmed desorption mass spectrometry experiment. It has been shown that adsorption of histamine on silica surface promotes its thermal degradation via the concerted mechanism of pyrolytic elimination with formation of fragments m/z 82 and 94.