Contrast and Complexity in the Low-Temperature Kinetics of CN(v = 1) with O₂ and NO: Simultaneous Kinetics and...

Bimolecular rate coefficients were determined for the reaction CN(v = 1) + NO and O₂ using continuous wave cavity ringdown spectroscopy in a uniform supersonic flow (UF-CRDS). The well-matched time scales for ringdown and reaction under pseudo-first-order conditions allow for the use of the SKaR method (simultaneous kinetics and ringdown) in which the full kinetic trace is obtained on each ringdown. The reactions offer an interesting contrast in that the CN(v = 1) + NO system is nonreactive and proceeds by complex-mediated vibrational relaxation, while the CN(v = 1) + O₂ reaction is primarily reactive. The measured rate coefficients at 70 K are (2.49 ± 0.08) × 10^-11 and (10.49 ± 0.22) × 10^-11 cm³ molecule^-1 s^-1 for the reaction with O₂ and NO, respectively. The rate for reaction with O₂ is a factor 2 lower than previously reported for v = 0 in the same temperature range, a surprising result, while that for NO is consistent with extrapolation of previous high-temperature measurements to 70 K. The latter is also discussed in light of theoretical calculations and measurements of the rate constants for the association reaction in the high-pressure limit. The measurements are complicated by the presence of a metastable population of high-J CN formed by photolysis of the precursor BrCN, and a kinetic model is developed to treat the competing relaxation and reaction. It is particularly problematic for reactions at low temperatures where the rotational relaxation and reaction have similar rates, precluding a reliable determination of the rate coefficients at 30 K. Also presented are important modifications to the data acquisition and control for the instrument that have yielded considerably enhanced stability and throughput.