TRANSITION-METAL ATOM REACTION-KINETICS IN THE GAS-PHASE - ASSOCIATION AND OXIDATION REACTIONS OF S-7(3) CHROMIUM ATOMS

The kinetics of gas-phase reactions of ground-state (7S3) chromium atoms with O2, NO, N2O, and CCl4 have been examined over the buffer gas pressure range of 1-700 Torr and between 298 and 348 K. Ground-state (7S3) Cr atoms were produced in a static pressure cell by laser multiphoton dissociation (MPD) of Cr(CO)6 at 559 nm and probed by laser-induced fluorescence at 425.43 nm (z7P°4 → a7S3) or 357.87 nm (y7P°4 → 37S3). Relaxation of excited states of Cr produced in the MPD process was monitored and found to be complete within 2 μs. Pseudo-second-order rate constants for the reaction of Cr with O2 exhibited strong buffer gas pressure dependence, which is interpreted as arising from formation of CrO2 in a termolecular association reaction. The pressure dependence was modeled by using Tree's simplified falloff expression, yielding values of k0 = (6.5 ± 1.1) × 10-29 (Ar, 298 K), (5.6 ± 0.9) × 10-29 (Ar, 323 K), and (7.3 ± 1.6) × 10-29 cm6 molecule-2 s-1 (N2, 298 K), k∞ = (1.25 ± 0.06) × 10-10 cm3 molecule-1 s-1 (Ar, 298 K), and Fc = 0.67 ± 0.08 (Ar, 298 K), 0.64 ± 0.06 (Ar, 323 K), and 0.8 ± 0.1 (N2, 298 K). Similar pressure dependence of the pseudo-second-order rate constant was observed for the reaction Cr + NO + Ar in the pressure range 10-600 Torr of Ar at 298 K. Values of k0 = (1.2 ± 0.2) × 10-30 cm6 molecule-2 s-1 and k∞ = (3.2 ± 0.8) × 10-11 cm3 molecule-1 s-1 were found for an assumed value of Fc = 0.7. Strong collision, low-pressure recombination rate constants have been calculated for the reactions of Cr with O2 and NO, using the simplified form of RRKM theory described by Troe. Comparisons are made with experimental results. The reactions of Cr with N2O and CCl4 exhibited second-order kinetic behavior, with bimolecular rate constants (cm3 molecule-1 s-1) (1.0 ± 0.1) × 10-14 (N2O, 298 K), (3.3 ± 0.1) × 10-14 (N2O, 348 K), and (1.94 ± 0.04) × 10-10 (CCl4, 298 K) in Ar buffer gas. The temperature dependence of the Cr + N2O reaction yielded values of Ea ≃ 5 kcal mol-1 and A ≃ 4 × 10-11 cm3 molecule-1 s-1, assuming simple Arrhenius behavior. Upper limits for second-order rate constants are given for the removal of Cr atoms by CO, CO2, CF3Cl, C2H4, CH4, H2, D2, n-hexane, benzene, OCS, H2S, dimethyl ether, and methanol, for which no measurable reactivity was observed at 298 K. The influence of electronic configuration and spin multiplicity on the reactivity of Cr atoms is discussed.