MOLECULAR ORBITAL CALCULATIONS ON THE VIBRATIONAL SPECTRA OF Q3 T-(OH) SPECIES AND THE HYDROLYSIS OF A THREE-MEMBERED ALUMINOSILICATE RING

Abstract

Force constant analyses of the molecules [(H3SiO)3Al-(OH)]1-, Na+ [(H3SiO)3Al-(OH)]1-, [((OH)3SiO)3Al-(OH)]1-, [(H3SiO)3Si-(OH)], [((OH)3SiO)3Si-(OH)], [((OH)3SiO)2((OH)3AlO)Si-(OH)]1-, and Na +[(H3SiO)2(H3AlO)Si-(OH)]1- have been performed with ab initio molecular orbital calculations to determine the frequencies and H-D shifts of T-(OH) vibrations in Q3 T-(OH) species where T is a Si4+ or Al3+. Calculated Q3 Si-(OH) vibrations are close to the observed value in H2O-bearing SiO2 glass. The theoretical Al-(OH) stretching frequency in Na +[(H3SiO)3Al-(OH)]1- matches the 880 cm-1 shoulder in the vibrational spectra of hydrous albitic glasses within experimental error. Isotopic substitution of deuterium for hydrogen in the T-(OH) bond results in frequency shifts of less than 25 cm-1 in all cases consistent with the small H-D frequency shifts observed in silica (Mysen and Virgo, 1986) and Na-aluminosilicate glasses (McMillan et al., 1993).Hydrolysis of a three-membered aluminosilicate ring into a trimeric chain (i.e., [H6SiAl2O9]2- + H2O -> [H8SiAl2O10]2-) was also calculated. Hydrolysis occurs through a pentavalent Al3+ transition state complex. Calculated energies of the reactants and product in the above reaction predict that the chain configuration is in a marginally lower potential energy state (-36 kJ/mol) than the H2O-ring dimer. An activation energy of +171 kJ/mol is predicted between the two species for hydrolysis and +207 kJ/mol for the reverse reaction.