Reactions of the CN Radical with Benzene and Toluene: Product Detection and Low-Temperature Kinetics

Abstract

Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9-4.9) × 10 -10 cm 3 molecule -1 s -1 . These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 × 10 -10 cm 3 molecule -1 s -1 at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C 6 H 5 CN) is the only product recorded with no detectable evidence for a C 6 H 5 + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC 6 H 4 CH 3 ) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the ∼40 meV photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C 6 H 5 CH 2 ) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn's moon Titan (∼100 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…Kinetics for this reaction have been measured at 105 K, 165 K and 295 K and the rate coefficients are essentially unchanging over this temperature range with values between 3.9 and 4.9 × 10 -10 molecule -1 cm 3 s -1 . 112 These results are in good agreement with computational kinetics study of Woon predicting rate coefficients of 3.35 -3.50 × 10 -10 cm 3 molecule -1 s -1 over the 100 -300K temperature range. 113 This implies that reaction efficiencies are close to unity and that the reaction pathways have no significant energetic barrier.…”

Low-Temperature Mechanisms for the Formation of Substituted Azanaphthalenes through Consecutive CN and C2H Additions to Styrene and N-Methylenebenzenamine: A Theoretical Study

“…Again, the finding that the CN addition to the aromatic ring in styrene is barrierless is in line with the previous experimental and theoretical studies of the reaction of cyano radicals with benzene 18,26,33 and toluene. 26 Once the adduct i10 forms, it may lose the H atom from the attacked ortho-carbon, yielding 2-vinylbenzonitrile. A barrier of 29.2 kcal mol −1 must be surmounted for the hydrogen loss to proceed.…”

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

“…74 Using low temperature Laval nozzle techniques to study reaction rate constants by kinetic rates, numerous other reactions have been explored down to the ≈100 K temperatures relevant to Titan, including CN with benzene and toluene. 75 Recognizing that mechanisms of elementary reactions are altered from low to high temperatures, a new end station platform is under construction to couple the Laval technique with tunable vacuum ultraviolet product detection at the Chemical Dynamics Beamline, thus allowing user groups to obtain quantitative product information and kinetic rate constant data on reactions down to approximately 100 K. With the high temperature capability of the original product detection apparatus, 17 reactions may be studied definitively over a wide range of temperatures where mechanisms will most likely change from addition-elimination to direct abstraction.…”