IONIC LEWIS SUPERACIDS IN THE GAS-PHASE .5. COMPETING LIGAND DISPLACEMENTS IN THE ADDUCTS BETWEEN SIF3-X(OH)X-3) IONS AND (H2O)-O-18( (X = 0)

The actual nature of the ionic intermediates involved in the gas-phase reaction of SiF3+ with water has been ascertained using a Fourier-tra nsform ion cyclotron resonance (FT-ICR) instrument equipped with an ex ternal ion source. The complete kinetic pattern following attack of Si F3-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 displa cement 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 relat ively fast displacement in the tetracoordinated adducts of SiF3-x(OH)x + (x = 1-3) with H-2 O-18 prevents their FT-ICR structural characteriz ation.