Thermal decomposition of H3O− produced in reaction of OH− with H2CO

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The ion–molecule reaction OH−+H2CO→H3O−+CO has been studied at 300 K with isotopic labeling of reactants. The H3O− product is only observed in small abundance because the ion dissociates into OH−+H2 upon multiple collisions in a helium buffer gas. Without isotopic labeling, the pseudo-first-order kinetics plots for the reactions of OH− with H2CO and OD−+D2CO were found to be curved as a result of the regeneration of OH− or OD− reactant. A scavenger technique was used to remove the H3O− (or D3O−) produced prior to dissociation, to reveal the true first-order attenuation of OH− (or OD−) in reaction with H2CO (or D2CO). The rate constant for the OH−+H2CO reaction is 7.6×10−10 cm3 s−1, and for OD−+D2CO is 5.7×10−10 cm3 s−1. For the isotopically mixed cases OH−+D2CO and OD−+H2CO, the rate constants are equal to 1.3×10−9 cm3 s−1, about twice as large as those for the reactions involving only a single hydrogen isotope, indicating that isotopic exchange is an important process. The rate constants for the thermal dissociation of H3O− and D3O− in helium were found to be 1.6×10−12 and 1.1×10−12 cm3 s−1, respectively, within a factor of 2. The results are discussed in terms of other thermal dissociation reactions of ions.

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