Quantifying the Competition between Intersystem Crossing and Spin-Conserved Pathways in the Thermal Reaction of V + NO.

The kinetics of V + NO and VO + NO are studied using a selected-ion flow tube from 300-600 K at pressures of 0.25-0.70 Torr helium. V + NO yields VO ( = 4.9 ± 1.0 (T/300 K) × 10 cm s) in both ground and excited states. The secondary reaction VO + NO → VO + N proceeds near the collision rate at >10 cm s, whereas thermalized VO + NO studied as a primary reaction proceeds more than 100× more slowly ( = 4.2 ± 1.0 (T/300 K) × 10 cm s). The results are best explained by contributions of competing pathways in V + NO: a spin crossing to the lower energy VO in the exit well and a spin-conserved reaction yielding an electronically excited VO. 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 10 s, and yields derived bond and transition state energies.