Molecular Structures, Acid−Base Properties, and Formation of Group 6 Transition Metal Hydroxides

Abstract

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) were used to study the group 6 metal (M = Cr, Mo, W) hydroxides: MO3−m(OH)2m (m = 1−3), M2O6−m(OH)2m (m = 1−5), M3O9−m(OH)2m (m = 1, 2), and M4O11(OH)2. The calculations were done up to the complete basis set (CBS) limit for the CCSD(T) method. Molecular structures of many low-energy conformers/isomers were located. Bronsted acidities in the gas phase and pKa values in aqueous solution were predicted for MO3−m(OH)2m (m = 1−3) and MnO3n−1(OH)2 (n = 2−4). In addition, Bronsted basicities and Lewis acidities (fluoride affinities) were predicted for MO3−m(OH)2m (m = 1−3) as well as the metal oxide clusters MnO3n (n = 1−3). The metal hydroxides were predicted to be strong Bronsted acids and weak to modest Bronsted bases and Lewis acids. The pKa values can have values as negative as −31. Potential energy surfaces for the hydrolysis of the MnO3n (n = 1−4) clusters were calculated. Heats of formation of the metal hydroxides were predicted from t...