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We present an experimental and theoretical investigation of the reaction of vibrationally excited CN ( v = 1) with isomers of butadiene at low temperature. The experiments were conducted using the newly built apparatus, UF-CRDS, which couples near-infrared cw-cavity ring-down spectroscopy with a pulsed Laval flow. The well-matched hydrodynamic time and long ring-down time decays allow measurement of the kinetics of the reactions within a single trace of a ring-down decay, termed Simultaneous Kinetics and Ring-down (SKaR). The pulsed experiments were carried out using a Laval nozzle designed for the 70 K uniform flow with nitrogen as the carrier gas. The measured bimolecular rates for the reactions of CN ( v = 1) with 1,3-butadiene and 1,2-butadiene are (3.96 ± 0.28) × 10 −10 and (3.06 ± 0.35) × 10 −10 cm 3 per molecule per s, respectively. The reaction rate measured for CN ( v = 1) with the 1,3-butadiene isomer is in good agreement with the rate previously reported for the reaction with ground state CN ( v = 0) under similar conditions. We report the rate of the reaction of CN ( v = 1) with the 1,2-butadiene isomer here for the first time. The experimental results were interpreted with the aid of variable reaction-coordinate transition-state theory calculations to determine rates and branching of the addition channels based on a high-level multireference treatment of the potential energy surface. H-abstraction reaction rates were also theoretically determined. For the 1,2-butadiene system, theoretical estimates are then combined with literature values for the energy-dependent product yields from the initial adducts to predict overall temperature-dependent product branching. H loss giving 2-cyano-1,3-butadiene + H is the main product channel, exclusive of abstraction, at all energies, but methyl loss forming 1-cyano-prop-3-yne is 15% at low temperature growing to 35% at 500 K. Abstraction forming HCN and various radicals is important at 500 K and above. The astrochemical implications of these results are discussed.
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This content will become publicly available on June 19, 2025
Contrast and complexity in the low temperature kinetics of CN(v=1) with O2 and NO: Simultaneous kinetics and ringdown in a uniform supersonic flow
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 timescales 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 cm3 molecule-1 s-1, respectively, for the reaction of O2 and (10.49 ± 0.22) × 10-11 cm3 molecule-1 s-1 for reaction with NO. The rate for reaction with O2 is a factor two 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 temperature 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.
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- Award ID(s):
- 2247776
- PAR ID:
- 10515766
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- Journal of physical chemistry
- ISSN:
- 0092-7325
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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