Ab initio molecular orbital study of the effects of basis set size on the calculated structure and acidity of hydroxyl groups in framework molecular sieves

The structures, force constants, and relative acidities of a series of molecules that mimic the geometries of terminal and bridging hydroxyl groups in various substituted zeolites and clays are calculated by ab initio molecular orbital methods. The molecules are structural analogs of disiloxane H₃T-O-TH₃, and the protonated form H₃T-OH-TH₃, where T = Si, Al, B, and P. Also included are H₃SiO^- and H₃SiOH that mimic terminal hydroxyl groups that occur at the zeolite surface and defect sites. Results are presented for restricted Hartree-Fock (RHF) calculations at levels of theory that range from the minimal RHF/STO-3G to double- and triple-zeta basis sets with multiple polarization functions. The study shows that the structures of the molecules converge toward limiting values at the higher levels of theory. The theoretical trend in acidity of the terminal and bridging hydroxyls, as determined by the proton affinity, the length of the O-H bond, the charge on the hydroxyl proton, and the O-H stretching force constants and frequencies, is presented and is in agreement with experimental determinations. The calculated proton affinity is also shown to depend on the basis set size; however, the acidity trend is well reproduced at all levels of theory except RHF/STO-3G. Force parameters suitable for use in classical mechanical simulations are presented for O-H and T-O bond stretches and T-O-T and T-OH-T angle bends that are key structural features for zeolites and clays. The coupled motion of the T-O bond and T-O-T angles is calculated and compared to proposed experimental relationships. The preferential sites for substitution of Si by Al or B in the zeolite framework are discussed in terms of changes in the related T-O bond lengths and T-O-T bond angles.