Experimental and Theoretical Studies of Double Minima in the Potential-Energy Surfaces for HF-Elimination Reactions of SiFx(OH)y+ with H2O (x = 1−3, y = 0−2) via Intramolecular H-Atom Transfer

Abstract

HF elimination reactions between H2O and ions of the type SiFx(OH)y+ with (x = 1−3, y = 0−2) have been observed and are shown by computation to proceed by intramolecular H-atom transfer on potential-energy surfaces characterized by double minima. The chemistry was initiated by SiF+, SiF2•+ and SiF3+ in H2O using the selected-ion flow tube (SIFT) technique at 293 ± 4 K in helium buffer gas at 0.35 ± 0.01 Torr. All three cations were observed to react with H2O by sequential HF elimination until all Si−F bonds in these cations were replaced by Si−O bonds in agreement, in the case of the chemistry initiated by SiF3+, with previous low-pressure FT-ICR measurements by Speranza et al. SiOH+ does not react further with water, but the terminal Si(OH)2•+ ion in the sequence initiated by SiF2•+ reacts further with H2O by H-atom elimination (90%) and H2O addition (10%), while Si(OH)3+, the terminal ion in the sequence initiated by SiF3+, was observed to sequentially add two water molecules under SIFT conditions. Products and rate coefficients were measured for all primary and higher-order reactions. For closed-shell species gradient structure optimizations and harmonic frequency calculations were performed on critical points at HF/3-21G and with density functional theory using B-LYP/6-31G(d,p); reactants and products for open-shell species were examined at ROHF/3-21G and at ROHF/6-31G(d,p). On all the potential-energy surfaces studied, the hydrogen fluoride elimination pathway was shown to proceed through a hydrated reactant ion and an HF-solvated product ion, each at local minima, separated by a transition structure for H-atom transfer; the energies of these three critical points, in all cases, lie below those of the reactants and of the products. The measured HF-elimination efficiency increases with increasing energy defect between the initial reactants and the transition state.