Ion−Molecule Reactions of Gas-Phase Chromium Oxyanions: CrxOyHz- + H2O

Abstract

Chromium oxyanions having the general formula Cr x O v H z - play a key role in many industrial, environmental, and analytical processes, which motivated investigations of their intrinsic reactivity. Reactions with water areperhaps the most significant, and were studied by generating Cr x O y H z - in the gas phase using a quadrupole ion trap secondary ion mass spectrometer. Of the ions in the Cr 1 O y H z envelope (y = 2, 3, 4; z = 0, 1), only CrO 2 - was observed to react with H 2 O, producing the hydrated CrO 3 H 2 - at a slow rate (∼0.07% of the ion-molecule collision constant at 310 K). CrO 3 -, CrO 4 -, and CrO 4 H - were unreactive. In contrast, Cr 2 O 4 -, Cr 2 O 5 -, and Cr 2 O 5 H 2 - displayed a considerable tendency to react with H 2 O. Cr 2 O 4 - underwent sequential reactions with H 2 O, initially producing Cr 2 O 5 H 2 - at a rate that was ∼7% efficient. Cr 2 O 5 H 2 - then reacted with a second H 2 O by addition to form Cr 2 O 6 H 4 - (1.8% efficient) and by OH abstraction to form Cr 2 O 6 H 3 -(0.6% efficient). The reactions of Cr 2 O 5 - were similar to those of Cr 2 O 5 H 2 -: Cr 2 O 5 - underwent addition to form Cr 2 O 6 H 2 - (3% efficient) and OH abstraction to form Cr 2 O 6 H - (<1% efficient). By comparison, Cr 2 O 6 -was unreactive with H 2 O, and in fact, no further H 2 O addition could be observed for any of the Cr 2 O 6 H z -anions. Hartree-Fock ab initio calculations showed that reactive Cr x O y H z - species underwent nucleophilic attack by the incoming H 2 O molecules, which produced an initially formed adduct in which the water O was bound to a Cr center. The experimental and computational studies suggested that Cr 2 O y H z - species that have bi- or tricoordinated Cr centers are susceptible to attack by H 2 O; however, when the metal becomes tetracoordinate, reactivity stops. For the Cr 2 O y H z - anions the lowest energy structures all contained rhombic Cr 2 O 2 rings with pendant O atoms and/or OH groups. The initially formed [Cr 2 O y - + H 2 O] adducts underwent H rearrangement to a gem O atom to produce stable dihydroxy structures. The calculations indicated that rearrangement did not occur in the [Cr 2 O 5 H 2 - + H 2 O] adduct, possibly because the rearranged product could not accommodate the negative charge.