Experimental and theoretical studies of gas-phase reactions of SiF x+ ( x = 1–3) with ammonia: intramolecular H-atom transfer reactions with SiF 3+ and F 2Si(NH 2)

Abstract

Gas-phase ion-molecule reactions between the Lewis acids SiF x + ( x = 1–3) and ammonia have been investigated both experimentally and theoretically. Experimental studies were performed with a selected ion flow tube (SIFT) apparatus with helium buffer gas at 294 ± 3 K and at 0.35 ± 0.01 Torr. The monofluorosilicon cation SiF + was found to be unreactive toward ammonia, while SiF 2 ·+ was observed to undergo electron transfer with NH 3. The trifluorosilyl cation SiF 3 + reacted consecutively with two ammonia molecules by HF elimination to produce FSi(NH 2) 2 + which subsequently reacted with three NH 3 molecules in succession to form adduct ions, with no further HF eliminations. Molecular orbital calculations were performed on all ionic and neutral molecular species associated with the chemistry of SiF 3 +. Gradient optimizations were performed on reactants, on transition structures, and on products, both at the Hartree-Fock (HF)/3-21G level of theory, and at the density functional Becke-Lee-Yang-Parr (B-LYP)/6-31 G( d, p) level. Harmonic frequency calculations were performed on all optimized structures at critical points at the latter level, from which, also, zero-point vibrational energies (ZPE) were obtained. The results of molecular orbital investigations revealed mechanistic insight into the experimentally-observed HF elimination reactions; in particular, the occurrence of H-atom transfer on a double-minimum potential-energy hypersurface. Theory also confirmed the thermodynamic legitimacy of the observed reactions and the validity of the nonobservation of a HF elimination reaction between FSi(NH 2) 2 + and ammonia under SIFT conditions, a process that was reported to have occurred in a previous Fourier transform ion cyclotron resonance (FTICR) study. Molecular orbital calculations also have shown that the lowest-energy isomer associated with the empirical formula of the second adduct ion, FSi(NH 2) 2(NH 3) 2 +, is one which has a nearly tetrahedral geometry in the heavy atoms, and solvates an ammonia molecule by hydrogen bonding. Comparisons also were made between the results of the present study and those of an earlier SIFT investigation of the reactions of Lewis acids SiF x + (x = 1–3) with H 2O.