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 [(H 3 SiO) 3 Al-(OH)] 1- , Na + [(H 3 SiO) 3 Al-(OH)] 1- , [((OH) 3 SiO) 3 Al-(OH)] 1- [(H 3 SiO) 3 Si-(OH)],[((OH) 3 SiO) 3 Si-(OH)],[((OH) 3 SiO) 2 ((OH) 3 A10)Si(OH)] 1- and Na + [(H 3 SiO ) 2 (H 3 Al0) 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 Q 3 T- (OH) species where T is a So 4+ or Al 3+ . Calculated Q 3 Si-(OH) vibrations are close to the observed value in H 2 O-bearing Si0 2 glass. The theoretical Al-(OH) stretching frequency in Na + [ (H 3 SiO) 3 A1-(OH)] 1- matches the 880 cm − 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., [H 6 SiAl 2 0 9 ] 2- + H 2 O → [H 8 SiAl 2 0 10 ] 2- ) was also calculated. Hydrolysis occurs through a pentavalent Al 3+ 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 H 2 O-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.