Quantifying the Competition between Intersystem Crossing and Spin-Conserved Pathways in the Thermal Reaction of V⁺ +...

The kinetics of V⁺ + N₂O and VO⁺ + N₂O are studied using a selected-ion flow tube from 300-600 K at pressures of 0.25-0.70 Torr helium. V⁺ + N₂O yields VO⁺ (k = 4.9 ± 1.0 (T/300 K)^-0.3±0.2 × 10^-10 cm³ s^-1) in both ground and excited states. The secondary reaction VO⁺ + N₂O → VO₂⁺ + N₂ proceeds near the collision rate at >10^-10 cm³ s^-1, whereas thermalized VO⁺ + N₂O studied as a primary reaction proceeds more than 100× more slowly (k = 4.2 ± 1.0 (T/300 K)^-1.4±0.2 × 10^-12 cm³ s^-1). The results are best explained by contributions of competing pathways in V⁺ + N₂O: a spin crossing to the lower energy ³VO⁺ in the exit well and a spin-conserved reaction yielding an electronically excited ⁵VO⁺. The intersystem crossing occurs in 35 ± 20% and 37 ± 15% of reactive interactions at 300 and 600 K, respectively. Statistical modeling of relevant reaction coordinates supports the lack of a temperature dependence, indicates an intersystem crossing rate constant of 10¹¹ s^-1, and yields derived bond and transition state energies.