Experimental and theoretical studies of gas-phase reactions of SiFx+ (x = 1-3) with ammonia: intramolecular H-atom transfer reactions with SiF3+ and F2Si(NH2)(+)

Gas-phase ion-molecule reactions between the Lewis acids SiFx+ (x = 1-3) an d ammonia have been investigated both experimentally and theoretically. Exp erimental studies were performed with a selected ion flow tube (SIFT) appar atus with helium buffer gas at 294 +/- 3 K and at 0.35 +/- 0.01 Torr. The m onofluorosilicon cation SiF+ was found to be unreactive toward ammonia, whi le SiF2+ was observed to undergo electron transfer with NH3. The trifluoros ilyl cation SiF3+ reacted consecutively with two ammonia molecules by HF el imination to produce FSi(NH2)(2)(+) which subsequently reacted with three N H3 molecules in succession to form adduct ions, with no further HF eliminat ions. Molecular orbital calculations were performed on all ionic and neutra l molecular species associated with the chemistry of SiF3+. Gradient optimi zations were performed on reactants, on transition structures, and on produ cts, both at the Hartree-Fock (HF)/3-21G level of theory, and at the densit y functional Becke-Lee-Yang-Parr (B-LYP)/6-31G(d,p) level. Harmonic frequen cy calculations were performed on all optimized structures at critical poin ts at the latter level, from which, also, zero-point vibrational energies ( ZPE) were obtained. The results of molecular orbital investigations reveale d mechanistic insight into the experimentally-observed HF elimination react ions; in particular, the occurrence of H-atom transfer on a double-minimum potential-energy hypersurface. Theory also confirmed the thermodynamic legi timacy of the observed reactions and the validity of the nonobservation of a HF elimination reaction between FSi(NH2)(2)(+) and ammonia under SIFT con ditions, a process that was reported to have occurred in a previous Fourier transform ion cyclotron resonance (FTICR) study. Molecular orbital calcula tions also have shown that the lowest-energy isomer associated with the emp irical formula of the second adduct ion, FSi(NH2)(2)(NH3)(2)(+), is one whi ch has a nearly tetrahedral geometry in the heavy atoms, and solvates an am monia 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 SiFx+ (x = 1-3) with H2O. (C) 1999 Elsevier Sc ience B.V.