Ion-molecule reactions of gas-phase chromium oxyanions: Cr xO yH z− + O 2

Abstract

Chromium oxyanions, Cr xO yH z −, were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O 2. Only CrO 2 − of the Cr 1O yH z − envelope was observed to react with oxygen, producing primarily CrO 3 −. The rate constant for the reaction of CrO 2 − with O 2 was ∼38% of the Langevin collision constant at 310 K. CrO 3 −, CrO 4 −, and CrO 4H − were unreactive with O 2 in the ion trap. In contrast, Cr 2O 4 − was observed to react with O 2 producing CrO 3 − + CrO 3 via oxidative degradation at a rate that was ∼15% efficient. The presence of background water facilitated the reaction of Cr 2O 4 − + H 2O to form Cr 2O 5H 2 −; the hydrated product ion Cr 2O 5H 2 − reacted with O 2 to form Cr 2O 6 − (with concurrent elimination of H 2O) at a rate that was 6% efficient. Cr 2O 5 − also reacted with O 2 to form Cr 2O 7 − (4% efficient) and Cr 2O 6 − + O (2% efficient); these reactions proceeded in parallel. By comparison, Cr 2O 6 − was unreactive with O 2, and in fact, no further O 2 addition could be observed for any of the Cr 2O 6H z − anions. Generalizing, Cr xO yH z − species that have low coordinate, low oxidation state metal centers are susceptible to O 2 oxidation. However, when the metal coordination is >3, or when the formal oxidation state is ≥5, reactivity stops.