Ionic Lewis superacids in the gas phase. Part 5. Competing ligand displacements in the adducts between SiF3-x(OH)x+ (x = 0-3) ions and H218O.

The actual nature of the ionic intermediates involved in the gas-phase reaction of SiF3+ with water has been ascertained using a Fourier-transform ion cyclotron resonance (FT-ICR) instrument equipped with an external ion source. The complete kinetic pattern following attack of SiF3-x(OH)x+ (x = 0-3) on H-2 O-18 has been investigated, revealing the occurrence of competing H-2 O-18-to-HF and H-2 O-18-to-H-2 O-16 displacement processes, when x = 1 or 2. The factors governing the relative extent of such competing reactions and the overall reactivity of SiF3-x(OH)x+ (x = 0-3) toward H-2 O-18 are discussed. Occurrence of a relatively fast displacement in the tetracoordinated adducts of SiF3-x(OH)x+ (x = 1-3) with H-2 O-18 prevents their FT-ICR structural characterization.