Gas-phase condensation reactions of SixOyHz- oxyanions with H2O

Water was reacted with gas-phase oxyanions having the general composition S ixOyHz- that were formed and isolated in an ion trap-secondary ion mass spe ctrometer (IT-SIMS). The radical SiO2.- reacted slowly with H2O to abstract HO., forming SiO3H-, at a rate of 8 x 10(-13) cm(3) molecule(-1) s(-1), co rresponding to an efficiency of about 0.03% compared with the theoretical c ollision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3- at a rate that was approximately 0.4-0.7% efficient. SiO4H3- did not undergo f urther reaction with water. The multiple reaction pathways by which radical SiO3.- reacted with H2O were kinetically modeled using a stochastic approa ch. SiO3.- reacted with water by three parallel reaction pathways: (1) abst raction of a radical H-. to form SiO3H-, which then reacted with a second H 2O to form SiO4H3-; (2) abstraction of a radical OH. to form SiO4H-, which further reacted by consecutive H-. abstractions to form SiO4H2.- and then S iO4H3-; and (3) condensation with H2O to form SiO4H2.-, which subsequently abstracted a radical H-. from a second H2O to form SiO4H3-. In all of these reactions, the rate constants were determined to be very slow, as determin ed by both direct measurement and stochastic modeling. For comparison, the even electron ion Si2O5H- was also investigated: it underwent condensation with H2O to form Si2O6H3-, with a rate constant corresponding to 50% effici ency. The reactions were also modeled using ab initio calculations at the U B3LYP/6-311 + G(2d,p) level. Addition of H2O to SiO3.-, SiO3H-, and Si2O5H- was calculated to be approximately 42, 45, and 55 kcal mol(-1) exothermic, respectively, and encountered low activation barriers. Modeling Of SiO2.- and SiO3.- reactions with H2O failed to produce radical abstraction reactio n pathways observed in the IT-SIMS, possibly indicating that alternative re action mechanisms are operative.