Atom Radical Reactions Research Articles

Gas-phase kinetics of the N + C2N reaction at low temperature.

The rate of the gas-phase N((4)S) + C2N(X(2)Πi) reaction has been measured in a continuous supersonic flow reactor down to 54 K through the relative-rate method using the N((4)S) + OH(X(2)Π) → H((2)S) + NO(X(2)Π) reaction as a reference. The microwave discharge technique was employed to produce high concentrations of atomic nitrogen. Pulsed laser photolysis of precursor molecules Cl3C2N and H2O2 at 212 nm in situ led to C2N and OH radical formation, respectively. The rate constant is shown to be approximately independent of temperature, in contrast to previous studies of atom-radical reactions involving atomic nitrogen. While the reaction rate is faster than previously estimated, astrochemical simulations indicate that this reaction is probably only a minor source of CN radicals in dense interstellar clouds.

Crossed beam study of the atom-radical reaction of ground state carbon atoms (C(3P)) with the vinyl radical (C2H3(X2A′))

The atom-radical reaction of ground state carbon atoms (C((3)P)) with the vinyl radical (C(2)H(3)(X(2)A')) was conducted under single collision conditions at a collision energy of 32.3 ± 2.9 kJ mol(-1). The reaction dynamics were found to involve a complex forming reaction mechanism, which is initiated by the barrier-less addition of atomic carbon to the carbon-carbon-double bond of the vinyl radical forming a cyclic C(3)H(3) radical intermediate. The latter has a lifetime of at least 1.5 times its rotational period and decomposes via a tight exit transition state located about 45 kJ mol(-1) above the separated products through atomic hydrogen loss to the cyclopropenylidene isomer (c-C(3)H(2)) as detected toward cold molecular clouds and in star forming regions.

A combined crossed-beam and ab initio study of the atom–radical reaction dynamics of O(3P) + C2H5→ C2H4 + OH: analysis of nascent internal state distributions of the OH product

The oxidation reaction dynamics of ethyl radicals (C(2)H(5)) in the gas phase are investigated by applying a combination of high-resolution laser induced fluorescence spectroscopy in a crossed-beam configuration and ab initio theoretical calculations. The supersonic atomic oxygen (O((3)P)) and ethyl (C(2)H(5)) reactants are produced by photodissociation of NO(2) and supersonic flash pyrolysis of a synthesized precursor (azoethane), respectively. An exothermic channel leading to the C(2)H(5) + OH (X(2)Pi: upsilon'' = 0, 1) products is identified. The nascent rovibrational state distributions of the OH product show substantial bimodal internal excitations consisting of low- and high-N'' components with neither spin-orbit nor Lambda-doublet propensities in the ground and first excited vibrational states. The averaged vibrational population (P(upsilon'')), partitioning with respect to the low-N'' components of the upsilon'' = 0 level, shows a comparable population ratio of P(0)ratioP(1) = 1 ratio 1.06. On the basis of comparison between the population analyses using ab initio and prior statistical calculations, the title atom-radical reactive scattering processes are governed by dynamic characteristics. The reaction mechanism can be rationalized by two competing mechanisms: abstraction versus addition. The major low N''-components can be described in terms of the direct abstraction process responsible for the comparable vibrational populations, while the minor but hot rotational distribution of the high N''-components implies that some fraction of radical reactants is sampled to proceed through the short-lived addition-complex forming process.

Production of Fluorine-Containing Molecular Species in Plasma-Generated Atomic F Flows

The fluorine atomic radical reactions to form molecular fluorine and hydrogen fluoride are examined by time-of-flight mass spectroscopy (TOFMS) and kinetically modeled for various reaction orders. Fluorine radicals are generated from NF 3 or F 2 in microwave-generated and low-frequency (LF)-generated plasmas and are passed through flow tubes under various flow, pressure, and dilution conditions. In general, the concentrations of the mass spectroscopically measured products F, F 2 , and HF do not depend on the specific wall material (e.g., Teflon, stainless steel, Al, Ni, Al 2 O 3 , SiO 2 ); the only variations of [F]/[F 2 ]/[HF] ratios with wall material are found at pressures below 1 Torr. The most significant changes in these ratios are observed upon varying flow rates and pressures. Specifically, the F relative concentration decreases from ∼80% to ∼20%, and the F 2 relative concentration increases from ∼20% to ∼80%, as the pressure is varied over the range 0.5-10 Torr. In all cases, the HF concentration is found to decrease as the pressure increases. Data suggest that the composition of the tube surface material does not contribute significantly to the generation of F 2 ; however, since the wall surface carries adsorbed hydrogen sources such as H, H 2 O, H 2 , and OH, it becomes important in the generation of HF. A simple kinetic analysis of the experimental data suggests a combined two-reaction mechanism for F 2 and HF generation: (1) a pseudo-second-order volume reaction (k v ) to generate F 2 , and (2) a zero- or first-order wall reaction (k w ) to generate HF. Thus, both surface and volume reactions contribute to the overall F atom loss mechanism in the gas flow from the plasma source. The model fits our data best for a k v /k w ratio of about 75. The reaction order for the loss of F atom is found to be 1.68, while the reaction order for the formation of F 2 is found to be ∼2.

Atom-radical reaction dynamics of O(3P)+C3H5→C3H4+OH: Nascent rovibrational state distributions of product OH

The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.

Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical

The absolute rate constant and primary reaction products have been determined at T=298 K for the atom–radical reaction N(4S)+C2H5 in a discharge flow system with collision-free sampling to a mass spectrometer. The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source. The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C2H5 or C2D5 as a function of time in the presence of excess N atoms. The result is k=(1.1±0.3)×10−10 cm3 molecule−1 s−1. For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD3, D2CN and C2D4 for the N+C2D5 reaction. The product identification is based on the unambiguous combination of product m/z values, the shift of the m/z peaks observed for the N+C2D5 reaction products with respect to the N+C2H5 reaction products and the photoionization threshold measured for the major products. The observed products are consistent with the occurrence of the reaction channels D2CN+CD3(2a) and C2D4+ND(2c). Formation of C2D4 product via channel (2c) accounts for approximately 65% of the C2D5 reacted. Most, if not all, of the remaining 35% is probably accounted for by channel (2a). These rate constant and product results are compared with those for the N+CH3 reaction as well as other atom+C2H5 reactions. The role of the N+C2H5 reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered. In addition, the appearance energy for the formation of C2D+3 from C2D5 was determined from photoionization threshold measurements, AE0(C2D+3,C2D5)=239.5 kcal mol−1. From this, values are derived for the zero Kelvin heats of formation of C2D+3 (266 kcal mol−1) and C2D3 (71.6 kcal mol−1).

A laser flash photolysis, time-resolved Fourier transform infrared emission study of the reaction monochlorine + ethyl .fwdarw. hydrogen chloride(v) + ethene

The atom-radical reaction Cl + C[sub 2]H[sub 5] [yields] HCl(v) + C[sub 2]H[sub 4] is studied using laser flash photolysis, time-resolved Fourier transform infrared emission spectroscopy and broad band infrared chemiluminescence. The Cl atoms and ethyl radicals are produced from a number of different precursors using one or two lasers. The initial HCl vibrational distribution is determined to be HCl (v=1/2/3/4=0.39 [plus minus] 0.04/0.29 [plus minus] 0.03/0.22 [plus minus] 0.02/0.10 [plus minus] 0.02). The vibrational distribution is characteristic of an addition-elimination mechanism and can be reproduced using modified statistical theories of energy partitioning within the [C[sub 2]H[sub 5]Cl]z[double dagger] intermediate. The time evolution of the HCl(v=4) emission is used to estimate a rate coefficient for this reaction of (3.0 [plus minus] 1.0) x 10[sup [minus]10] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. 39 refs., 8 figs., 6 tabs.

The thermal decomposition of CH3Cl using the Cl-atom absorption method and the bimolecular rate constant for O+CH3 (1609–2002 K) with a pyrolysis photolysis-shock tube technique

Rate constants for the thermal decomposition of CH3Cl and the atom–radical reaction O+CH3 have been measured in shock tube experiments. The decomposition of CH3Cl, at three loading pressures, has been carried out between 1663–2059 K with mixtures that varied from 1.25 to 24.9 ppm in Ar. The first-order rate constant k1=1.21×1010 ×exp(−27 838 K/T) s−1 describes the experimental results to within ±29% at the one standard deviation level. The bimolecular rate experiment has then been carried out over the temperature range 1609–2002 K using mixtures of SO2 (49.9 ppm) and CH3Cl (5.14 and 8.2 ppm) in Ar. The technique first involves allowing the thermal decomposition to proceed forming CH3 radicals, and this is then followed by delayed photolysis of SO2 forming the O-atom species. This new method is called the pyrolysis photolysis-shock tube (PyPh-ST) technique. A reaction mechanism had to be used to simulate the measured O-atom profiles for the various experimental conditions, and the bimolecular rate constant was found to be temperature independent with a value of k2=1.4×10−10 cm3 molecule−1 s−1. Reactions (1) and (2) are both theoretically discussed.

A stop-scan interferometer used for time-resolved FTIR emission spectroscopy

The authors describe how a low-cost teaching interferometer has been modified for use as a time-resolved instrument to study the infrared vibrational chemiluminescence of the products from atom-radical reactions and molecular photodissociations. This technique allows the entire temporal evolution of the emission spectrum to be obtained from a single interferometric scan, with spectral and temporal resolutions of 2 cm-1 and 3 mu s respectively. Illustrative results are given for discharge-flow studies of laser initiated atom-radical reactions, where product emission from highly vibrationally excited CO2 and HF is observed, and for emission from vibrationally excited products of the infrared multiple-photon dissociation of halocarbon molecules.

Time-resolved pulsed FTIR emission studies of atom-radical reactions: Product chemiluminescence from the O( 3P)+CF 2(X̃ 1A 1) reaction

A time-resolved interferometer has been used to study the product vibrational infrared chemiluminescence from atom-radical reactions. The entire temporal evolution of the emission spectrum is obtained from a single interferometric scan. Results are given for the O( 3P) + CF 2(X̃ 1A 1) reaction, which exhibits strong emission from highly vibrationally excited CO 2, formed by reaction of O atoms with FCO, the major primary product of the title reaction.

Energy partitioning in atom–radical reactions: The reaction of F atoms with NH2

An extension of the low-pressure infrared chemiluminescence technique has allowed the measurement of energy partitioning in the atom/radical reactions: F+NH2→HF+NH, F+ND2→DF+ND. A complete numerical model of the experiment is described in detail including its parametrization. This model allows the unambiguous determination of the primary energy distribution of the above reactions. These reactions give inverted product energy distributions, in contrast to the isoelectronic F+OH→HF+O reaction. The inverted primary energy distribution for F+NH2/ND2 indicates a direct abstraction mechanism. Ab initio quantum chemical computations on some features of the relevant potential energy surfaces support this direct abstraction route. An energetically accessible transition state, having approximately zero barrier, is found on the triplet surface which directly correlates reagents and products. The geometry of this triplet transition state is also suggestive of strong HF vibrational excitation. Abstraction on the triplet surface provides an alternative pathway to reaction on the lowest singlet surface, which contains a deep potential energy well corresponding to NH2F.

Variational transition state theory calculations for an atom–radical reaction with no saddle point: O+OH

We have extended the general polyatomic canonical variational theory formalism of Isaacson and one of the authors to improved canonical and microcanonical variational theory. We have calculated the rate constants for the reaction in the title over the temperature range 200–2500 K using all three variational theories and the Melius–Blint ab initio potential energy surface. The results are compared to canonical variational calculations based on the reaction-path interpolation scheme of Quack and Troe, to the trajectory calculations of Miller, and to experiment. We find that the microcanonical variational transition states have a strong energy dependence and the generalized free energy of activation curves have two maxima. Quantization effects appear to be important at the lower temperatures, and recrossing effects may be important at higher temperatures.

Energy partitioning in some atom/radical reactions found in atmospheric pollution systems

A modification of the arrested relaxation infrared chemiluminescence technique has been used to study the energy disposal in two atom–radical reactions. Vinyl and formyl radicals were generated chemically and their subsequent reactions with O atoms (in the case of vinyl) and F atoms (in the case of formyl) were followed. Product CO emission was observed in the system O/C2H3. Emission from both CO and HF was seen in the F/HCO system. The O/C2H3 reaction gives rise to substantially higher excitation in the CO product than does the F/HCO case. In each case, bound intermediates may be formed; the differences in energy diposal are discussed in terms of the possible influence of these intermediates.

Evidence for a biphotonic process in the mercury-photosensitized decomposition of ethylene oxide in the gas phase

The quantum yields of CO, H 2 and CH 4 at incident resonance radiation intensities of I 0 = 38.6 × 10 15 and 1.23 × 10 15 quanta s −1, used in the equations Φ = kI n−1 0, lead to values of n which are greater than unity. Considering the nature of the possible radical—radical and atom—radical reactions involved, this clearly indicates the presence of a biphotonic step or steps leading to the formation of these products in the mercury-photosensitized decomposition of ethylene oxide. The pressure dependence of the product yields at the higher intensity is thought to be the result of the deactivation of the intermediate (probably a triplet ethylene oxide molecule) before the second excitation can occur.

Absolute Rates of the Reactions H+C2H4 and H+C2H5

The hydrogen atom–ethylene system was studied at 298°K employing the methods of resonance fluorescence and absorption by hydrogen atoms of Lyman α radiation at 1216 Å. The contribution of hydrogen atom–radical reactions was evaluated under varying experimental conditions, and the rate of disappearance of H atoms in ethylene was measured under conditions where stoichiometric corrections became significant. Measurements in the literature of reaction rates for H+C2H4 at low total pressure are now in good agreement; however the limiting high-pressure absolute rate constants thus far reported differ depending on the assignment of stoichiometric factors. Our results indicate that stoichiometric factors obtained under low-pressure conditions may not be applicable to high pressure. Furthermore, extrapolations based on plots of inverse rate constant vs inverse pressure may be in error due to significant curvature in such plots. Our high-pressure limiting rate constant for H+C2H4, extrapolated from data at pressures higher than those used by other workers, is free of stoichiometric corrections and is in agreement with our earlier measurements. Absolute rate constants obtained in this work are H+C2H4→k1 = 13.6 ± 1.9 × 10−13 (cm3molecule−1·sec−1) (extrapolated to infinite He pressure), H+C2H5→k4 = 6.0 ± 2.0 × 10−11 (cm3molecule−1·sec−1) (50 torr He). The latter rate has been estimated from the variation of the rate of disappearance of H atoms as a function of initial H atom concentration at 50 torr total pressure.