Temperature-Dependent Rate Constants for the Reactions of Gas-Phase Lanthanides with O2

Article abstract of DOI:10.1021/jp991647c

The reactivity of the gas-phase lanthanide atoms Ln (Ln = La-Yb with the exception of Pm) with O₂ is reported. Lanthanide atoms were produced by the photodissociation of [Ln(TMHD)₃] and detected by laser-induced fluorescence. For all the lanthanides studied with the exception of Yb, the reaction mechanism is bimolecular abstraction of an oxygen atom. The bimolecular rate constants (in molecule^-1 cm³ s^-1) are described in Arrhenius form by k[Ce(¹G₄)] = (3.0 ± 0.4) × 10^-10 exp(-3.4 ± 1.3 kJ mol^-1/RT); Pr(⁴I_9/2), (3.1 ± 0.7) × 10^-10 exp(-5.3 ± 1.5 kJ mol^-1/RT); Nd(⁵I₄), (3.6 ± 0.3) × 10^-10 exp(-6.2 ± 0.4 kJ mol^-1/RT); Sm(⁷F₀), (2.4 ± 0.4) × 10^-10 exp(-6.2 ± 1.5 kJ mol^-1/RT); Eu(⁸S_7/2), (1.7 ± 0.3) × 10^-10 exp(-9.6 ± 0.7 kJ mol^-1/RT); Gd(⁹D₂), (2.7 ± 0.3) × 10^-10 exp(-5.2 ± 0.8 kJ mol^-1/RT); Tb(⁶H_15/2), (3.5 ± 0.6) × 10^-10 exp(-7.2 ± 0.8 kJ mol^-1/RT); Dy(⁵I₈), (2.8 ± 0.6) × 10^-10 exp(-9.1 ± 0.9 kJ mol^-1/RT); Ho(⁴I_15/2), (2.4 ± 0.4) × 10^-10 exp(-9.4 ± 0.8 kJ mol^-1/RT); Er(³H₆), (3.0 ± 0.8) × 10^-10 exp(-10.6 ± 1.1 kJ mol^-1/RT); Tm(²F_7/2), (2.9 ± 0.2) × 10^-10 exp(-11.1 ± 0.4 kJ mol^-1/RT), where the uncertainties represent ±2σ. The reaction barriers are found to correlate to the energy required to promote an electron out of the 6s subshell. The reaction of Yb(¹S₀) with O₂ reacts through a termolecular mechanism. The limiting low-pressure third-order rate constants are described in Arrhenius form by k₀[Yb(¹S₀)] = (2.0 ± 1.3) × 10^-28 exp(-9.5 ± 2.8 kJ mol^-1/RT) molecule^-2 cm⁶ s^-1.