New method of determining rate constants of elementary reactions of atoms and radicals

Summary A new method is proposed for the determination of absolute values of the rate constants of elementary reactions of atoms and radicals with molecules in the gas phase.In conclusion the authors express their thanks to N. N. Semenov and V. N. Kondrat'ev for valuable advice.     

A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons--pathways to hydrogen deficient hydrocarbon radicals in combustion flames

Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.     

Laser flash photolysis studies of elementary reactions of significance in combustion

Recent measurements of radical-radical and radical-atom rate coefficients and of the heats of formation of ethyl and t-butyl radicals are discussed, in the context of the provision of rate data for elementary reactions of importance in combustion and pyrolysis.     

Association reactions of atoms and radicals

The pressure and temperature dependences of association reactions involving atoms and/or radicals is discussed and illustrated by reference to the reactions CH₃ + CH₃ → C₂H₆, CH₃ + O₂ → CH₃O₂, CH₃ + H → CH₄, and H + C₂H₄ → C₂H₅. Recent experimental measurements of the rate coefficients, k([M], T) are described, particular attention being paid to experiments designed to measure the rate coefficient over wide ranges of pressure and temperature. Methods of fitting the experimental data, to obtain estimates of the limiting rate coefficients, k⁰ and k^∞, and to permit extrapolation to regions beyond the experimental range, are discussed. These methods include the Troe factorization technique, a combination of master equation and variational RRKM theory, and recent calculations by Wagner and Wardlaw using the technique developed by Wardlaw and Marcus to describe loose transition states.     

Interactions between theory and experiment in the investigation of elementary reactions of importance in combustion

Elementary reactions are a central component of models of combustion processes. Rate constants and channel yields are needed for those models. Both experimental and theoretical methods used to determine such rate data are discussed in this tutorial review, which is of interest to reaction kinetics and combustion engineering communities. Applications to combustion present particular problems because the conditions required can be well outside the ranges of temperature and pressure accessible to experiment, and the rate data can show a complex dependence on conditions. Under these conditions, the interplay between theory and experiment becomes important.     

The kinetics and mechanisms of elementary NF2 reactions with O and N atoms

A combined EPR–LMR spectrometer with a fast-flow system has been used to investigate the kinetics and mechanisms of NF₂ reactions with O and N atoms at 298 K. The overall rate constants of these reactions are: k₀ = (2.8 ± 0.4) × 10^?11 cm³/s and k_N = (5.7 ± 0.8) × 10^?11 cm³/s. The stoichiometry of the reactions with respect to O, N, NF₂, F, and NO has been determined. The statistical theory of bimolecular reactions has been used for interpretation of the results obtained.     

Distribution and transfer of atoms by elementary reactions

Elementary reactions are considered to result from transitions of the initial species to the final atomic groups. Matrices of distribution and transfer of atoms (MDA and MTA, respectively) and their adequate bigraphs are introduced. Several types of mechanisms specified by different MDA's, which can be determined by labelling methods, are possible for one elementary reaction.

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Crossed beam studies of radical-radical reactions: O(3P) + C3H5 (allyl)

The dynamics of the radical-radical reaction O((3)P) + C(3)H(5) has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 73.0 kJ mol(-1); the reaction mechanism of the H-displacement channel has been elucidated, while experimental evidence of the occurrence of one or more C-C bond-breaking channels at this collision energy has been obtained.     

Formation of surface CN from the coupling of C and N atoms on Pt(111)

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption have been used to study the coupling of C and N atoms on Pt(111) to form surface CN. This reaction underlies the important synthesis of HCN from methane and ammonia over platinum catalysts. Since CH4 and NH3 do not thermally dissociate on Pt(111) under ultrahigh vacuum conditions, we used CH3I and electron bombardment of NH3 to generate reactive surface species. Surface CN is formed at a temperature of 500 K from surface Nads and Cads atoms. The presence of surface CN is detected by HCN desorption and through the reaction of hydrogen with CNads to form a surface >CNH2 (aminocarbyne) species, which has a characteristic RAIR spectrum.     

Two-photon laser-induced fluorescence studies of HS radicals,DS radicals,and I atoms

A two-photon laser-induced excitation and fluorescence technique has been used to study the A ²Σ⁺-X ²Π transition of HS and DS radicals and various high-lying ⁴P^o, ²D^o, and ⁴D^o states of the I atom. The two-photon excitation cross sections and detection sensitivity are discussed.     

Crossed molecular beam studies on the reaction dynamics of O(1D)+N2O

The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(1D) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The 46N2O product of the isotope exchange channel, 18O+44N2O-->16O+46N2O, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.     

Alkenoxy radicals in gas-phase reactions of alkenes with oxygen atoms or ozone

Observations in the O₃ + trans-2-butene reaction system and in the O + trans-2-butene + O₂ reaction system suggest the intermediacy of alkenoxy radicals. A mechanism is proposed for the production of C_n and C_m (m <n) alkenoxy radicals by the reaction of C_nH_2n alkenes with oxygen atoms or with ozone.     

Ultracold collisions and reactions of vibrationally excited OH radicals with oxygen atoms

We report a quantum dynamics study of O + OH (v = 1, j = 0) collisions on its ground electronic state, employing two different potential energy surfaces: the DIMKP surface by Kendrick and Pack, and the XXZLG surface by Xu et al. A time-independent quantum mechanical method based on hyperspherical coordinates has been adopted for the dynamics calculations. Energy-dependent probabilities and rate coefficients are computed for the elastic, inelastic, and reactive channels over the collision energy range E(coll) = 10(-10)-0.35 eV, for J = 0 total angular momentum. Initial state-selected reaction rate coefficients are also calculated from the J = 0 reaction probabilities by applying a J-shifting approximation, for temperatures in the range T = 10(-6)-700 K. Our results show that the dynamics of the collisional process and its outcome are strongly influenced by long-range forces, and chemical reactivity is found to be sensitive to the choice of the potential energy surface. For O + OH (v = 1, j = 0) collisions at low temperatures, vibrational relaxation of OH competes with reactive scattering. Since long-range interactions can facilitate vibrational relaxation processes, we find that the DIMKP potential (which explicitly includes van der Waals dispersion terms) favours vibrational relaxation over chemical reaction at low temperatures. On the DIMKP potential in the ultracold regime, the reaction rate coefficient for O + OH (v = 1, j = 0) is found to be a factor of thirteen lower than that for O + OH (v = 0, j = 0). This significantly high reactivity of OH (v = 0, j = 0), compared to that of OH (v = 1, j = 0), is attributed to enhancement caused by the presence of a HO(2) quasibound state (scattering resonance) with energy near the O + OH (v = 0, j = 0) dissociation threshold. In contrast, the XXZLG potential does not contain explicit van der Waals terms, being just an extrapolation by a nearly constant function at large O-OH distances. Therefore, long-range potential couplings are absent in calculations using the XXZLG surface, which does not induce vibrational relaxation as efficiently as the DIMKP potential. The XXZLG potential leads to a slightly higher reactivity (a factor of 1.4 higher) for O + OH (v = 1, j = 0) compared to that for O + OH (v = 0, j = 0) at ultracold temperatures. Overall, both potential surfaces yield comparable values of reaction rate coefficients at low temperatures for the O + OH (v = 1, j = 0) reaction.     

On the combination reactions of hydrogen atoms with resonance-stabilized hydrocarbon radicals

Procedures for accurately predicting the kinetics of H atom associations with resonance stabilized hydrocarbon radicals are described and applied to a series of reactions. The approach is based on direct CASPT2/cc-pvdz evaluations of the orientation dependent interaction energies within variable reaction coordinate transition state theory. One-dimensional corrections to the interaction energies are estimated from a CASPT2/aug-cc-pvdz minimum energy path (MEP) on the specific reaction of interest and a CASPT2/aug-cc-pvtz MEP for the H + CH3 reaction. A dynamical correction factor of 0.9 is also applied. For the H + propargyl, allyl, cyclopentadienyl, and benzyl reactions, where the experimental values appear to be quite well determined, theory and experiment agree to within their error bars. Predictions are also made for the combinations with triplet propargylene, CH2CCCH, CH3CCCH2, CH2CHCCH2, CH3CHCCH, cyclic-C4H5, CH2CCCCH, and CHCCHCCH.     

Vibrationally quantum-state-specific dynamics of the reactions of CN radicals with organic molecules in solution

The dynamics of reactions of CN radicals with cyclohexane, d(12)-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ～500 cm(-1) bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v(3)) and H-C-N (D-C-N) bending (v(2)) modes, and for DCN products there is also evidence of vibrational excitation of the v(1) mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ～130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm(-1) and 2065 cm(-1) (in CHCl(3)) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC.     

Kinetics of CN reactions with N2O and CO2

The rate constants for the reaction of CN with N₂O and CO₂ have been measured by the laser dissociation/laser-induced fluorescence (two-laser pump-probe) technique at temperatures between 300 and 740 K. The rate of CN + N₂O was measurable above 500 K, with a least-squares averaged rate constant, k = 10^−11.8±0.4 exp(−3560 ± 181/T) cm³/s. The rate of CN + CO₂, however, was not measurable even at the highest temperature reached in the present work, 743 K, with [CO₂] ⩽ 1.9 × 10¹⁸ molecules/cm³. In order to rationalize the observed kinetics, quantum mechanical calculations based on the BAC-MP4 method were performed. The results of these calculations reveal that the CN + N₂O reaction takes place via a stable adduct NCNNO with a small barrier of 1.1 kcal/mol. The adduct, which is more stable than the reactants by 13 kcal/mol, decomposes into the NCN + NO products with an activation energy of 20.0 kcal/mol. This latter process is thus the rate-controlling step in the CN + N₂O reaction. The CN + CO₂ reaction, on the other hand, occurs with a large barrier of 27.4 kcal/mol, producing an unstable adduct NCOCO which fragments into NCO + CO with a small barrier of 4.5 kcal/mol. The large overall activation energy for this process explains the negligibly low reactivity of the CN radical toward CO₂ below 1000 K. Least-squares analyses of the computed rate constants for these two CN reactions, which fit well with experimental data, give rise to for the temperature range 300–3000 K.     

Kinetics of the reactions of pinonaldehyde with OH radicals and with Cl atoms

The rate constant for the reaction of OH radicals with pinonaldehyde has been measured at 293 ± 6 K using the relative rate method in the laboratory in Wuppertal (Germany) using photolytic sources for the production of OH radicals and in the EUPHORE smog chamber facility in Valencia (Spain) using the in situ ozonolysis of 2,3‐dimethyl‐2‐butene as a dark source of OH radicals. In all the experiments pinonaldehyde and the reference compounds were monitored by FTIR spectroscopy, and in addition in the EUPHORE smog chamber the decay of pinonaldehyde was monitored by the HPLC/DNPH method and the reference compound by GC/FID. The results from all the different series of experiments were in good agreement and lead to an average value of k(pinonaldehyde + OH) = (4.0 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. This result lead to steady‐state estimates of atmospheric pinonaldehyde concentrations in the ppbV range (1 ppbV ≈ 2.5 × 10¹⁰ molecule cm⁻³ at 298 K and 1 atm) in regions with substantial α‐pinene emission. It also implies that atmospheric sinks of pinonaldehyde by reaction with OH radicals could be half as important as its photolysis. The rate constant of the reaction of pinonaldehyde with Cl atoms has been measured for the first time. Relative rate measurements lead to a value of k(pinonaldehyde + Cl) = (2.4 ± 1.4) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. In contrast to previous studies which suggested enhanced kinetic reactivity for pinonaldehyde compared to other aldehydes, the results from both the OH‐ and Cl‐initiated oxidation of pinonaldehyde in the present work are in line with predictions using structure‐activity relationships. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 291–301, 1999