Reactions Of CN Radicals Research Articles

Laser measurements of the quenching and reaction of CN radicals with BrCN and CICN

The laser-induced fluorescence (LIF) technique was used to measure the rate at which CN radicals ( v = 0,1,2) are removed in collisions with BrCN and CICN. The rate constants are (9 ± 1) × 10 −14 cm 3 molecule −1 s −1 for BrCN and (2.1 ± 0.3) × 10 −13 cm 3 molecule −1 for CICN. They are independent of the CN vibrational energy content. It is therefore concluded that chemical reaction dominates the removal and that the upper limit for vibrational quenching of both species is (1 – 2) × 10 −14 cm 3 molecule −1 s −1.

The reaction of CN radicals with cyanoacetylene

The decay of CN radicals has been measured as a function of cyanoacetylene concentration by laser-induced fluorescence. The room-temperature rate constant obtained is (1.70 ± 0.08 ) × 10 −11 cm 3 molecule −1 s −1. This is about ten times slower than those of CN with unsaturated hydrocarbons, suggesting that there may be a small activation energy of about 1.5 kcal mol −1. On this basis, the rate constant at 150 K (an average temperature in Titan's stratosphere) would be 1.4 × 10 −12 cm 3 molecule −1 s −1. The reaction is interpreted as CN addition to an unsaturated bond, followed by H atom elimination. The product of this reaction may be C 4N 2 (which is formed in Titan's stratosphere).

High temperature rate coefficient for the reaction CN+O2→NCO+O using different CN-sources

The reaction of CN radicals with oxygen has been studied behind reflected shock waves using two different thermal sources of CN radicals, viz. C2N2 and BrCN. Mixtures of C2N2/O2 and BrCN/O2 highly diluted in argon were heated by reflected shock waves and the concentrations of N and O atoms were monitored in the post shock reaction zone by using the Atomic Resonance Absorption Spectroscopy (ARAS). From boty, C2N2/O2 and BrCN/O2 experiments, a mean rate coefficient for the reaction: CN+O2→k2NCO+OK2=7.0×1012cm3mol−1s−1

Measurements of absolute rate coefficients for the reactions of CN radicals with H2O, H2, and CO2 in the temperature range 295 K≤T≤1027 K

Absolute rate coefficients were measured at different temperatures by means of laser photolysis-laser induced fluorescence technique for the reactions: CN+H2O→HCN+OH (1) CN+H2→HCN+H (2) CN+CO2→OCN+CO. (3) The temperature dependent rate coefficients for reactions (1) and (2) could be fit by Arrhenius expressions and are: k1(518−1027 K)=(8.0±0.8)×1012 exp [−(31.2±0.6) kJ/RT] cm3 mol−1 s−1k2(295−1000 K)=(2.1±0.2)×1013 exp [−(19.7±0.5) kJ/RT] cm3 mol−1 s−1. From thermodynamical properties of Rxn. (1) we evaluated the temperature dependent rate coefficient for the reverse reaction: k−1=(7.7±1)×1012 exp [−(34.6±1) kJ/RT] cm3 mol−1 s−1. In the CN+CO2 system we did not observe CN 1st order rate constants attributable to chemical reaction at temperatures up to 1000 K. We estimated an upper limit for k3 of 2.5×109 cm3 mol−1 s−1. The experimental results are compared with literature data. In the case of Rxn. (1) and (2) the agreement with previous experimental and theoretical studies is good, whereas our results for k3 are strongly different from former measurements.

Laser induced fluorescence studies of CN reactions with alkanes, alkenes and substituted aliphatic species

Gas phase bimoleuclar rate constants at 294 ± 3 K (ambient temperature) for the reaction of CN radicals in the ν″ = 0 and 1 levels with a number of aliphatic organic compounds were measured using laser-induced fluorescence (LIF). Most of the rate constants are very large. The rate constants for CN(ν″ = 0) reactions with the saturated hydrocarbons increases from 5.6 × 10 −13 cm 3 s −1 molecule −1 for CH 4 to 2.6 × 10 −10 cm 3 s −1 molecule −1 for C 7H 16. Except for CH 4 the vibrational energy of the CN radical has little or no effect on the observed rate constants, indicating that any barriers occur early in the reaction coordinate. Theoretical model calculations using a long-range attractive potential suggest that many of these reactions are driven by long-range forces. These long-range forces may enhance the probability for the reaction by increasing the interaction time between the two molecules.

A shock tube study of CN radical reactions with H 2 and NO verified by H, N and O atom measurements

The reactions of CN radical with H2 and NO have been studied behind reflected shock waves respectively in the temperature ranges 2050–2590 K and 2480–3160 K at pressures of about 2 bar using Atomic Resonance Absorption Spectroscopy (ARAS). C2N2/H2 mixtures highly diluted in argon were shock-heated and H atom concentrations were monitored in the post-shock reaction zone. By comparing the measured H atom profiles with computer simulations, the rate coefficients of the reactions. CN+H2→HCN+H(R2)C2N2+H→HCN+CN(R3)k2k3=3.5×exp⁡(−2400K/T)k2=3×10−10exp⁡(−4000K/T)cm3s−1 were determined. In shock-heated C2N2/NO mixtures highly diluted in argon, N and O atom concentrations were measured and compared with computed values based on a kinetic mechanism. Both atom concentrations were sensitive to the rate of the reaction: CN+NO→NCO+N(R6)k6=1.6×10−10exp⁡(−21200K/T)cm3s−1 The resulting rate coefficient k6 determined from N atom profiles is found to be slightly higher than the value obtained from the O atom profiles. The given rate coefficient is the mean value which is very close to the one reported by Colket [7].

Reaction of CN radicals with CH4 and O2

A flash-photolysis laser-induced fluorescence technique has been used to study CN radical kinetics at 300 K. A radiative lifetime of 66.70 ± 4.60 ns has been measured for the B2Σ+ state of this radical. Rate constants for the reaction of the CN radical with CH4 and O2 have been measured to be (1.14 ± 0.06)× 10–12 and (18.2 ± 1.2)×10–12 cm3 molecule–1s–1, respectively.

THE STUDY OF THE REACTION RATE AND MECHANISM OF CN(<I>V</I>″=0,1)RADICAL WITH HYDROCARBONS BY USING LASER INDUCED FLUORESCENCE METHOD

The rate constants of eleven hydrocarbons and fluorocarbons with CN (V=0, 1) at 300K have been measured by using Laser Induced Fluorescence(LIF) method. For the saturated hydrocarbons, the rate constants are chagned from (5.6 +/- 0.3) x 10(-13) for CH4 to (2.3 + 0.2) x 10(-10)cm(3) molecu(-1),8(-1) for C7H16. The rate constants of the reaction of CN with alkenes and alkynes are close to the gas kinetic rate in spite of the structure of the molecules. The rate constants and mechanism of CN with the saturated hydrocarbons, H-2 and CH3OH can be explained satisfactorily by the long distance attractive potential. The reaction of CN with alkenes and alkynes can only proceed through the addition into pi bond. The influence of vibrational energy on the reaction rate was not found in the reactions of CN radical with hydrocarbon compounds.

Temperature dependence of the CN radical reactions with C 2H 2 and C 2H 4

The temperature dependence of the reaction rates for CN radicals with C 2H 4 and C 2H 4 and C 2H 2 has been measured from room temperature to 700 K. The two laser photoionization/LIF-probe technique was used by photolyzing ICN at 266 nm and monitoring CN depletion via B ↔ X LIF at 388 nm. A resistively heated slow-flow gas reactor was employed at 50 Torr total pressure for the temperature dependence study. Both reactions were found to have rate constants that decreased with temperature, fitting k C 2H 4 = (4.72±0.25)×10 −11exp[(509±20)/ T] and k C 2H 2 = (3.49±10 −11exp[(571±23)/ T] cm 3 molecule −1 s −1, indicating that both reactions occur by addition—elimination mechanism. No pressure dependence was observed within experimental errors.

Kinetics of CN radical reactions with selected molecules at room temperature

The two-laser photoinitiation/probe technique has been used to obtain room-temperature second-order rate constants for the reaction of CN radicals with compounds key to atmospheric and combustion chemistry. CN was generated by 266 nm photolysis of ICN. Laser-induced flourescence probing via both CN(A ← X) and CN(B ← X) has been utilized. Values reported are: k(H 2)=(4.9±0.4)×10 −14, k(CH 4)=(1.1±0.1)×10 −12, k(C 2H 6)= (2.9±0.1)×10 −11, k(C 2H 2)=(2.3±0.1) ×10 −10, k(C 2H 4)=(2.7±0.1)×10 −10, k(C 3H 6)=(2.3±0.3)×10 −10, and k(O 2)=(2.5±0.2)×10 −11 cm 3 molecule −1 s −1. A comparison with available literature data is made and mechanism are discussed.

ChemInform Abstract: CALCULATION OF ACTIVATION ENERGIES FOR HYDROGEN‐ATOM ABSTRACTIONS BY RADICALS CONTAINING CARBON TRIPLE BONDS

Activation energies are calculated by the bond-energy-bond-order (BEBO) and the bond-strength-bond-length (BSBL) methods for the reactions of C2H radicals with H2, CH4, and C2H6 and for the reactions of CN radicals with H2 and CH4. The BSBL technique accurately predicts the activation energies for these reactions while the BEBO method yields energies averaging 9 kcal higher than those observed. A possible reason for the disagreement is considered.

Absolute rate coefficients for the reaction of H-atoms with N 2O and some reactions of CN radicals

The reaction of H atoms with N 2 O was studied in an isothermal flow reactor (718°K≤ T ≤1111°K) with quantitative ESR detection of atoms and radicals. The measured stoichiometry of [H] (reacted)/[OH] (formed) substantiated the reaction H+N 2 O→N 2 +OH (1) The temperature dependence of the rate coefficient of reaction (1) is described by k 1 =(2.2±0.7)·10 14 exp(− E A /RT ) cm 3 /mol sec E A =(72.4±2.9) kJ/mol Reactions of CN radicals were investigated directly in an isothermal flow reactor (275°K≤ T ≤398°K). The decay of CN radicals, produced by C 2 N 2 + hv (λ≥165 nm)→2CN in the presence of a high excess of reactants was followed by kinetic absorption spectroscopy. The rate coefficients of the reactions CN+H 2 →HCN+H CN+O 2 →NCO+O were found to be k 12 =(6±2)·10 13 exp(− E A /RT ) cm 3 /mol sec E A =(22.2±2.5) kJ/mol and k 16 =(3.2±1)·10 13 exp(− E A /RT ) cm 3 /mol sec E A =(4.2±1.4) kJ/mol For the reaction CN+O→CO+N no temperature dependence of k 17 (298°K)=(1.2±0.4)·10 13 cm 3 /mol sec between 275° and 387°K could be observed.

Reactive Species Produced by Laser Irradiation. The Reaction of CN Radicals with Hydrocarbons

Abstract The reactive species, CN radicals, were produced by the irradiation of the focused laser beam on the metal cyanide target. The reaction of CN radicals with low-molecular-weight hydrocarbons gave fragmented hydrocarbons and CN (and NC) compounds, while that with hydrogen gave hydrogen cyanide. From the experimental results, it seems that the products may be formed mainly by the reaction of the reactive species with fragment hydrocarbon radicals which are produced by the collision of the high-temperature reactive species (CN*) with the parent hydrocarbons.

Reactions of cyanogen radicals. Part 3.—Arrhenius parameters for reactions with alkanes

Using the technique of gas phase pulse radiolysis, the Arrhenius parameters for the reactions of CN radicals with methane and ethane have been determined. With methane there is an activation energy of 8.36± 0.5 kJ mol–1 whereas for the reaction with ethane the value is 0(< 800) J mol–1. Also reported are the rate constants at 300 K for reaction with CD4 and propane. The effects of vibrational excitation of the CN radicals on the apparent rate constants is discussed.

Reactions of cyanogen radicals. Part 2.—Reactions with (CN)2 and O2

The gas phase reactions of CN radicals with (CN)2 and O2 have been studied as a function of temperature and vibrational excitation of the CN. The first reaction has Eact≃ 13.1 kJ mol–1 and only inefficient vibration → translational transfer occurred with the excited CN levels. The reaction with O2 has no activation energy, but the rate constants increase with CN vibrational level. NCO radicals were produced more rapidly than CN(X2∑; 0, 0 band) radicals were removed, indicating that vibrational excitation may contribute to the overall kinetics of this system.

Reactions of cyanogen radicals with hydrocarbons

The gas-phase radiolysis of C2N2+ argon mixtures has been used as a means of following CN radical reactions with a number of hydrocarbons. Rate constants for the reaction with ethylene, propylene, 1,3-butadiene and benzene approach the collisional limit and reflect the extremely electrophilic nature of CN radicals. Rate constants for hydrogen abstraction from methane and ethane have also been determined and enable comparisons to be made with halogen atom reactions.

The Reactions of CN Radicals in the Gas Phase

The Reactions of CN Radicals in the Gas Phase