An experimental and theoretical study of the reaction of ethynyl radicals with nitrogen dioxide (HC≡C+NO2)

Abstract

Pulsed laser photolysis and quantum chemical-statistical rate study of the reaction of the ethynyl radical with water vapor Potential energy surface for the CH 3 + HBr → CH 4 + Br hydrogen abstraction reaction: Thermal and stateselected rate constants, and kinetic isotope effects A pulsed laser photolysis/chemiluminescence ͑PLP/CL͒ technique was used to determine absolute rate constants of the reaction C 2 HϩNO 2 →products over the temperature range 288 -800 K at a pressure of 5 Torr (N 2 ). The reaction has a large rate constant that decreases with increasing temperature. It may be expressed in simple Arrhenius form as k 1 (T)ϭ(7.6Ϯ1.0) ϫ10 Ϫ11 exp͓(130Ϯ50) K/T͔, although there is an indication of a downward curvature for T Ͼ700 K. A three-parameter Arrhenius fit to the data, which takes this into account gives k 1 (T) ϭ(9.7Ϯ1.5)ϫ10 Ϫ9 T Ϫ0.68 exp͓(158Ϯ65) K/T͔. Our experiments also show that the 293 K rate constant is invariant to pressure between 2 and 11 Torr (N 2 ). We have also characterized the C 2 HϩNO 2 reaction theoretically. A large portion of the potential energy surface ͑PES͒ of the ͓C 2 ,H,N,O 2 ͔ system has been investigated in its electronic ͑singlet͒ ground-state using DFT with the B3LYP/6-311ϩϩG(3d f ,2p) method and MO computations at the CCSD(T)/6-311ϩ ϩG(d,p) level of theory. Seventeen isomers and thirty-two transition structures were found to connect reactants to products following eighteen different channels. Hydroxyl cyano ketone 11 and formylisocyanate 16 were found to be the most stable intermediates, although the reaction flux through them, as a fraction of the total, is likely to be small over the temperature range studied. A part of the PES corresponds with that of the HCCOϩNO reaction ͓I. V. Tokmakov, L. V. Moskaleva, D. V. Paschenko, and M. C. Lin, J. Phys. Chem. A 167, 1066 ͑2003͔͒, and the dominant product channels for C 2 HϩNO 2 proceed via the same nitrosoketene intermediate that is formed initially in the HCCOϩNO reaction. However, unlike in the latter reaction, the fate of the much more highly excited nitrosoketene formed by C 2 HϩNO 2 is likely to be governed dynamically. We present arguments as to the likely product channels for C 2 HϩNO 2 based on both statistical and dynamical considerations. A statistical description overwhelmingly favors the product set HCCOϩNO. Dynamical considerations on the other hand favor both the HCNϩCO 2 and HCCOϩNO product sets. Formation of HCNOϩCO appears unlikely. Energetically allowed paths, leading to five other product sets, namely, HNCOϩCO, HOCNϩCO, HOCCϩNO, HONCϩCO, and HNC ϩCO 2 , have also been identified, and are discussed.

Kinetics of C2H Reactions with Hydrocarbons and Nitriles in the 104−296 K Temperature Range

“…Details of the 193-nm photolysis of C 2 H 2 to create an initial concentration of C 2 H radicals and the chemiluminescence tracer method were discussed previously. 37 Under the conditions of our experiment, the chemiluminescence signal is proportional to the C 2 H concentration. A typical decay of the chemiluminescence signal is shown in Figure 1a.…”

Product State Distributions of Vibrationally Excited CO(v) for the CH(X2Π) and CH(A2Δ) Channels of the C2H + O(3P) Reaction

“…The slight preference for energy disposal into the CO vibrations indicates that the energy does not completely randomize in the transition state, which means that the reaction does not proceed through long-lived or multiple transition states, but possibly, there is only one transition state and/or the reaction exit barrier is comparable to the exothermicity of the reaction, putting more energy into translation. The large deviation of the surprisal-predicted branching ratio for R1 and R2 from previous work [22][23][24] suggests that the CO distribution for R2 could be very different than the surprisal predictions, which might suggest a more direct mechanism.…”

“…Details of the 193 nm photolysis of C 2 H 2 to create an initial concentration of C 2 H radical and the chemiluminescence tracer method are discussed previously. 24 Under the conditions of this experiment, the chemiluminescence signal is proportional to the C 2 H concentration. A typical decay of the chemiluminescence signal is shown in Figure 6a.…”

CH(A2Δ) Formation in Hydrocarbon Combustion: The Temperature Dependence of the Rate Constant of the Reaction C2H + O2 → CH(A2Δ) + CO2

“…33 The main points are briefly repeated here. C 2 H radicals were produced in a small tubular region along the center of a heatable stainless steel reaction cell by pulsed (8 Hz, 10 ns) 193 nm photolysis of ca.…”

“…22 Indeed, this reaction is routinely employed for the time-resolved monitoring of C 2 H radicals in kinetic experiments 30-32 whereas, more recently, reaction R2b has also been employed for this purpose. 33 Comparison of these two techniques using the same experimental setup clearly shows the rate constant k 2b to be orders of magnitude greater than k 1e at room temperature. This observation was quantified by Devriendt et al 28 who determined k 2b /k 1e at 290 K to be 500 ( 70 (not including the systematic uncertainty) by comparing the initial emission intensities at 430 nm following the photolysis of C 2 H 2 /O 2 /He and C 2 H 2 /N 2 O/He mixtures of known composition at 193 nm.…”