Absolute Rate Constants of Cr2 (X 1Σg+) with O2, NH3, and C2H4 in the Gas Phase at Room Temperature

Abstract

The gas-phase reactivities of ground-state chromium dimers (Cr2 (X 1Σg+)) for three small molecules (O2, NH3, and C2H4) were investigated in a mass-flow controlled cell. The transient concentration of Cr2 (X 1Σg+, v = 0) produced by pulsed 266-nm laser photolysis of [Cr(CO)6] was monitored by a time-resolved laser-induced fluorescence (LIF) at 459.67 nm (A 1Σu+, v′′ = 0 ← X 1Σg+, v′ = 0). The pseudo-first-order decay rates of Cr2 (X 1Σg+) were observed to increase linearly with increasing O2 and NH3 pressure, giving pseudo-second-order rate constants of (2.3 ± 0.3)×10-11 cm3 molecule-1 s-1 for Cr2 + O2 and (4.3 ± 0.8)×10-13 cm3 molecule-1 s-1 for Cr2 + NH3 under 6.0-Torr total pressure with Ar buffer gas at room temperature. On the other hand, C2H4 was apparently found not to react with Cr2 (X 1Σg+) under our experimental conditions. In the Cr2 + O2 reaction, the pseudo-second-order rate constants were found to depend on the total pressure, and the limiting low-pressure rate constant was estimated to be (5.9 ± 2.5)×10-28 cm6 molecule-2 s-1.

12 Chromium, molybdenum and tungsten

“…It was determined, however, that the molybdenum species are kinetically capable of intramolecular oxidative addition of a C-H bond of the ortho substituents on the aryl rings to yield cyclometalated derivatives, but that these are less stable than the aryloxy-hydride derivatives. 30 35 A protocol using Mo(CO) 6 as an in situ condensed source of CO has been described and tested for some palladiumcatalysed carbonylation reactions of aryl halides. 36 Laser pyrolysis of W(CO) 6 in the gas-phase at moderate temperatures has led to the unsaturated species, W(CO) n (n < 6), which were shown to be effective oxygen atom abstractors from a range of organic substrates.…”