Gas Kinetic Limit Research Articles

An experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction with implications for the photochemical models of Titan.

We report on a combined experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction, which is of relevance in the aromatic chemistry of the atmosphere of Titan. Experimentally, the reaction was studied (i) under single-collision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy (Ec) of 31.8 kJ mol-1 to determine the primary products, their branching fractions (BFs), and the reaction micromechanism, and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 K to 296 K. Theoretically, electronic structure calculations of the doublet C6H6N potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction mechanism. The reaction is found to proceed via barrierless addition of N(2D) to the aromatic ring of C6H6, followed by formation of several cyclic (five-, six-, and seven-membered ring) and linear isomeric C6H6N intermediates that can undergo unimolecular decomposition to bimolecular products. Statistical estimates of product BFs on the theoretical PES were carried out under the conditions of the CMB experiments and at the temperatures relevant for Titan's atmosphere. In all conditions the ring-contraction channel leading to C5H5 (cyclopentadienyl) + HCN is dominant, while minor contributions come from the channels leading to o-C6H5N (o-N-cycloheptatriene radical) + H, C4H4N (pyrrolyl) + C2H2 (acetylene), C5H5CN (cyano-cyclopentadiene) + H, and p-C6H5N + H. Rate constants (which are close to the gas kinetic limit at all temperatures, with the recommended value of 2.19 ± 0.30 × 10-10 cm3 s-1 over the 50-296 K range) and BFs have been used in a photochemical model of Titan's atmosphere to simulate the effect of the title reaction on the species abundances as a function of the altitude.

Singlet (1Δg) O2 as an efficient tropospheric oxidizing agent: the gas phase reaction with the simplest Criegee intermediate.

The reaction between CH2OO and 1Δg O2 has been investigated by means of high level quantum chemical and chemical kinetic calculations. Post-CCSD(T) corrections in terms of full triplets and partial quadratic excitations, along with core corrections have been employed to estimate the reaction energetics. The title reaction was found to be effectively barrierless with the transition state lying -22.85 kcal mol-1 below the isolated reactants. Rate coefficients under tropospheric conditions have been calculated using the master equation. The calculated rate coefficient was found to be marginally over the gas kinetic limit, implying that the reaction rate would be limited by the upper limit of bimolecular collision frequency. When compared against ˙OH and O3, 1O2 was found to compete efficiently with the two well known tropospheric oxidants.

Extraction of monomer-cluster association rate constants from water nucleation data measured at extreme supersaturations

We utilize recently reported data for water nucleation in the uniform postnozzle flow of pulsed Laval expansions to derive water monomer association rates with clusters. The nucleation experiments are carried out at flow temperatures of 87.0 K and 47.5 K and supersaturations of lnS ∼ 41 and 104, respectively. The cluster size distributions are measured at different nucleation times by mass spectrometry coupled with soft single-photon ionization at 13.8 eV. The soft ionization method ensures that the original cluster size distributions are largely preserved upon ionization. We compare our experimental data with predictions by a kinetic model using rate coefficients from a previous ab initio calculation with a master equation approach. The prediction and our experimental data differ, in particular, at the temperature of 87.0 K. Assuming cluster evaporation to be negligible, we derive association rate coefficients between monomer and clusters purely based on our experimental data. The derived dimerization rate lies 2-3 orders of magnitude below the gas kinetic collision limit and agrees with the aforementioned ab initio calculation. Other than the dimerization rate, however, the derived rate coefficients between monomer and cluster j (j ≥ 3) are on the same order of magnitude as the kinetic collision limit. A kinetic model based on these results confirms that coagulation is indeed negligible in our experiments. We further present a detailed analysis of the uncertainties in our experiments and methodology for rate derivation and specify the dependency of the derived rates on uncertainties in monomer and cluster concentrations.

Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways.

Reaction of ammonia with SO3 as a potential source of sulfamic acid in the troposphere has been investigated by means of electronic structure and chemical kinetic calculations. Besides, the hydrolysis reaction, which is known to be a major atmospheric decay channel of SO3, has also been investigated. The catalytic effects of ammonia and water on both the reactions have been studied. Rate coefficients for all the studied reaction channels were calculated using the transition state theory employing pre-equilibrium approximation. Calculated rate coefficients for a number of catalyzed hydrolysis and ammonolysis processes were found to be much higher (by ∼105 to ∼109 times) than the gas kinetic limit at ambient temperature. With decrease in temperature because of negative temperature dependence of rate coefficients, that difference became even larger (up to ∼1016 times). Therefore, in order to remove the discrepancies, rate coefficients for all the studied reaction channels were calculated by means of the master equation. The results showed marked improvements, with only one channel showing a slightly higher rate coefficient above the gas kinetic limit. The rate coefficients for catalyzed channels obtained from the master equation also showed negative temperature dependence, albeit to a much smaller extent. The uncatalyzed ammonolysis reaction, similar to the corresponding hydrolysis, was found to be too slow to have any practical atmospheric implication. For both reactions, ammonia-catalyzed pathways have higher rate coefficients than water-catalyzed ones. Between hydrolysis and ammonolysis, the latter showed a higher rate coefficient. In spite of that, ammonolysis is expected to have negligible contribution in the tropospheric loss process of SO3 because of large difference in concentration values between water and ammonia in the troposphere.

Water nucleation at extreme supersaturation.

We report water cluster formation in the uniform postnozzle flow of a Laval nozzle at low temperatures of 87.0 and 47.5 K and high supersaturations of lnS ∼ 41 and 104, respectively. Cluster size distributions were measured after soft single-photon ionization at 13.8 eV with mass spectrometry. Critical cluster sizes were determined from cluster size distributions recorded as a function of increasing supersaturation, resulting in critical sizes of 6-15 and 1, respectively. Comparison with previous data for propane and toluene reveals a systematic trend in the nucleation behavior, i.e., a change from a steplike increase to a gradual increase of the maximum cluster size with increasing supersaturation. Experimental nucleation rates of 5 · 1015 cm-3 s-1 and 2 · 1015 cm-3 s-1 for lnS ∼ 41 and 104, respectively, were retrieved from cluster size distributions recorded as a function of nucleation time. These lie 2-3 orders of magnitude below the gas kinetic collision limit assuming unit sticking probability, but they agree very well with a recent prediction by a master equation model based on ab initio transition state theory. The experimental observations are consistent with barrierless growth at 47.5 K, but they hint at a more complex nucleation behavior for the measurement at 87.0 K.

Bimolecular Reactions of Vibrationally Excited Molecules. Roaming Atom Mechanism at Low Kinetic Energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.

A CROSSED MOLECULAR BEAM, LOW-TEMPERATURE KINETICS, AND THEORETICAL INVESTIGATION OF THE REACTION OF THE CYANO RADICAL (CN) WITH 1,3-BUTADIENE (C4H6). A ROUTE TO COMPLEX NITROGEN-BEARING MOLECULES IN LOW-TEMPERATURE EXTRATERRESTRIAL ENVIRONMENTS

We present a joint crossed molecular beam and kinetics investigation combined with electronic structure and statistical calculations on the reaction of the ground-state cyano radical, CN(X 2Σ+), with the 1,3-butadiene molecule, H2CCHCHCH2(X 1 A g), and its partially deuterated counterparts, H2CCDCDCH2(X 1 A g) and D2CCHCHCD2(X 1 A g). The crossed beam studies indicate that the reaction proceeds via a long-lived C5H6N complex, yielding C5H5N isomer(s) plus atomic hydrogen under single collision conditions as the nascent product(s). Experiments with the partially deuterated 1,3-butadienes indicate that the atomic hydrogen loss originates from one of the terminal carbon atoms of 1,3-butadiene. A combination of the experimental data with electronic structure calculations suggests that the thermodynamically less favorable 1-cyano-1,3-butadiene isomer represents the dominant reaction product; possible minor contributions of less than a few percent from the aromatic pyridine molecule might be feasible. Low-temperature kinetics studies demonstrate that the overall reaction is very fast from room temperature down to 23 K with rate coefficients close to the gas kinetic limit. This finding, combined with theoretical calculations, indicates that the reaction proceeds on an entrance barrier-less potential energy surface (PES). This combined experimental and theoretical approach represents an important step toward a systematic understanding of the formation of complex, nitrogen-bearing molecules--here on the C5H6N PES--in low-temperature extraterrestrial environments. These results are compared to the reaction dynamics of D1-ethynyl radicals (C2D; X 2Σ+) with 1,3-butadiene accessing the isoelectronic C6H7 surface as tackled earlier in our laboratories.

Comment on “Kinetics of the reactions of Cl atoms with 2-buten-1-ol, 2-methyl-2-propen-1-ol, and 3-methyl-2-buten-1-ol as a function of temperature” by Rodriguez et al. (J. Atmos. Chem. (2008) 59:187–197)

The Arrhenius expressions and the data plotted in Figure 2 of Rodriguez et al. 2008 give rate coefficients of approximately 2 × 10-8 cm3 molecule-1 s-1 at 255 K. Such values are approximately two orders of magnitude larger than expected from simple collision theory (Finlayson-Pitts and Pitts 1986). The rate coefficients reported at sub-ambient temperatures are substantially greater than the gas kinetic limit and are not physically plausible. The rate coefficients reported by Rodriguez et al. imply a long range attraction between the reactants which is not reasonable for reaction of neutral species such as chlorine atoms and unsaturated alcohols. We also note that the pre-exponential A factors (10-23-10-20) and activation energies (−15 kcal mol-1) are not physically plausible. We conclude that there are large systematic errors in the study by Rodriguez et al. (Atmos Chem 59:187–197, 2008).

Rate Coefficients for the Reactions of C2(a3Πu) and C2(X1Σg+) with Various Hydrocarbons (CH4, C2H2, C2H4, C2H6, and C3H8): A Gas-Phase Experimental Study over the Temperature Range 24−300 K

The kinetics of reactions of C2(a(3)Pi(u)) and C2(X(1)Sigma(g)(+)) with various hydrocarbons (CH4, C2H2, C2H4, C2H6, and C3H8) have been studied in a uniform supersonic flow expansion over the temperature range 24-300 K. Rate coefficients have been obtained by using the pulsed laser photolysis-laser induced fluorescence technique, where both radicals were produced at the same time but detected separately. The reactivity of the triplet state was found to be significantly lower than that of the singlet ground state for all reactants over the whole temperature range of the study. Whereas C2(X(1)Sigma(g)(+)) reacts with a rate coefficient close to the gas kinetic limit with all hydrocarbons studied apart from CH4, C2(a(3)Pi(u)) appears to be more sensitive to the molecular and electronic structure of the reactant partners. The latter reacts at least one order of magnitude faster with unsaturated hydrocarbons than with alkanes, and the rate coefficients increase very significantly with the size of the alkane. Results are briefly discussed in terms of their potential astrophysical impact.

A Kinetic and Spectroscopic Study of the CH3I−Cl and ICH2I−Cl Adducts

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).

Reaction of Carbon Atoms, C (2p2,3P), with Hydrogen Sulfide, H2S (XA1): Overall Rate Constant and Product Channels

The multichannel C + H2S reaction was studied, at room temperature, in a low-pressure fast-flow reactor. Carbon atoms were obtained from the reaction of CBr4 with potassium atoms. The overall rate constant was found to be (2.1 ± 0.5) × 10-10 cm3 molecule-1 s-1, close to the gas kinetic limit. Absolute product branching ratio was estimated over the channels yielding H atoms probed by resonance fluorescence in the vacuum ultraviolet: H + HCS or HSC (75 ± 25)%. Ab initio studies of the different stationary points relevant to this reaction have been performed at the CCSD(T)/cc-pVTZ//QCISD/cc-pVDZ level. In agreement with the measured rate constant, the insertion transition structure leading to triplet HSCH is found lower in energy than the reactants. The reaction is found to proceed via H2SC (3A‘ ‘) intermediate that then rearrange to HSCH. On the basis of RRKM calculations, this last species leads mainly to HCS + H products.

Selected ion flow tube studies of the reactions of the isomers C 2H 5OH 2+ and (CH 3) 2OH + with a series of molecules

The reactivities of the isomeric forms C 2H 5OH 2 + and (CH 3) 2OH + of C 2H 7O + with a series of molecules (C 2H 5OCHO, (CH 3) 2CHOH, CH 3CO 2H, CH 3C 6H 5, CH 3CHO, C 3H 4 (allene), CH 3OH, c-C 3H 6, H 2O and C 2H 6) having proton affinities ranging from 143.6 to 193.1 kcal mol −1 have been studied in a selected ion flow tube at 300 K. The reactivities of these species are generally consistent with the reaction energetics. When exothermic, proton transfer is a significant product channel for both isomers and the reactions proceed at close to the gas kinetic limit. When proton transfer is significantly endothermic, association is the only reaction mechanism with varying rate coefficients, but with the C 2H 5OH 2 + reaction being ≈ 50% more efficient than that for (CH 3) 2OH +. In cases with intermediate energetics, there are appreciable differences between the products and in the reactivities of these two isomers by as much as two orders of magnitude. Association is competitive with proton transfer in many cases and, for C 2H 5OH 2 +, channels are observed which can be viewed as initial association followed by ejection of H 2O. Such a mechanism is compatible with recent potential surface calculations which show that C 2H 5OH 2 + is closely coupled to the high energy proton bound dimer H 2O…H +…C 2H 4.

Resonant vibration–vibration energy transfer between highly vibrationally excited O2(X 3Σ−g,v=15–26) and CO2, N2O, N2, and O3

Vibrational-state-specific total-removal relaxation rate constants, kv(M), for O2(X 3Σ−g, v=15 to 26) by M=CO2, N2O, and N2 have been obtained using the stimulated emission pumping (SEP) method in a pump–dump and probe configuration. Relaxation by O3 was studied using the chemical activation method, where the reaction: O(3P)+O3→O2(v)+O2, was employed to produce highly vibrationally excited O2 in an excess of ozone. Efficient (1%–2% of the gas kinetic limit) near-resonant 2–1 and/or 1–1 vibration-to-vibration (V–V) energy exchange was observed whenever the energy resonant condition was fulfilled and the transition in the quench partner would have been an allowed infrared transition in the isolated molecule. For M=CO2 and N2O, the temperature dependence of the 2–1 near-resonant energy transfer rate constants was found to be inverted. In contrast, the temperature dependence of the V–R, T relaxation rate constants for M=O2 was normal. For M=N2, a weak but positive temperature dependence was found. By extrapolating the temperature dependence to mesospheric temperatures (200 K) the effect of highly vibrationally excited O2 on the thermal budget can be discussed. The rate constant for the reaction of O(3P)+O3 was determined for an elevated collision energy of ∼10 kcal/mol and was found to be 5000 times larger than the room temperature rate constant.

Kinetics and Products of the Reaction SO3 + NH3 + N2

The kinetics of the gas phase reaction SO3 + NH3 + M were measured in N2 at 295 K over the pressure range 10−400 Torr with laminar flow reactors coupled to a chemical ionization mass spectrometer. The pressure dependence of the second-order rate coefficient is fit well by the generalized Troe formalism giving k0 = (3.9 ± 0.8) × 10-30 cm6 molecule-2 s-1 and k∞ = (4.7 ± 1.3) × 10-11 cm3 molecule-1 s-1. The association product of the SO3 + NH3 reaction and secondary acid cluster products were detected by chemical ionization mass spectrometry. Models of the temporal behavior of SO3, sulfamic acid, and sulfamic acid dimer show that sulfamic acid dimerizes at near the hard sphere gas kinetic limit (k > 5 × 10-11 cm3 molecule-1 s-1, 20 Torr N2, 295 K) and the dimer is relatively stable with respect to decomposition to monomers (ΔG°298 ≤ −9 kcal mol-1).

Temperature dependence of the reactions of CH radicals with NO, NH 3 and N 2O in the range 200–1300 K

Bimolecular rate constants for the reactions of CH(X 2Π) radicals with nitric oxide, ammonia and nitrous oxide were measured under pseudo-first-order conditions at 4 Torr total pressure, as a function of temperature in the range 300–1300 K for the CH+NO and CH + NH 3 reaction and in the range 200–1250 K for the CH + N 2O reaction. CH radicals were generated by excimer laser photolysis of CHClBr 2/Ar mixtures and were detected by laser-induced flourescence. The rate constant of the reaction CH+NO was found to be independent of temperature. The measured rate constants of the reactions CH + N 2O and CH + NH 3 exhibit negative temperature dependences which are described by the following Arrhenius equations (with E A in units of kJ/mol): k CH + N 2O ( T) = (3.09 ± 0.07) × 10 −11 exp[(2.14 ± 0.08)/ RT] cm 3 s −1, k CH + NH 3 ( T) = (7.23 ± 0.17) × 10 −11 exp[(2.64 ± 0.10)/ RT] cm 3 s −1 and k CH + NO ( T) = (1.87 ± 0.06) × 10 −10 exp[(0.00 ± 0.15)/ RT] cm 3 s −1. All reactions were found to be very fast, with rate constants close to the gas kinetic limit. The negative temperature dependences for the reactions CH + N 2O and CH + NH 3 are in accord with an addition/decomposition mechanism and are similar to those observed for the reactions of CH radicals with saturated and unsaturated hydrocarbons.

Charge transfer reactions OF Kr 2+ and Ne 2+ions with several molecular gases at 300 k

Measurements are reported of the rate coefficients and production distributions for the reactions of Kr 2+ and Ne 2+ions with H 2, N 2,O 2, CO 1, C0 2 and CH 4 at 300 K. The data are obtained with a selected ion flow tube (SIFT). The reaction rates are generally fast, proceeding at or near the gas kinetic limit. In the Kr 2+ reactions the only mechanism observed is single charge transfer, whereas for the more energetic Ne2+ ions parallel single and double charge transfer product channels are evident, the double charge transfer channels usually being dominant. The general features of the data are discussed in relation to those previously obtained For the corresponding reactions of Ar 2+ and Xe 2+ ions.

Studies of the binary reactions of H3O+⋅(H2O)0,1,2 ions and their deuterated analogues with D2O, H2O, and NH3

The rate coefficients and product ion distributions for the binary reactions of H3O+⋅(H2O)0,1,2 and D3O+⋅(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus. The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy into the SIFT. All the reactions proceeded at or near the gas kinetic limit. In the D2O and H2O thermoneutral isotopic exchange reactions, the distribution of H and D amongst the product ions and neutrals was seen to be purely statistical. This implies that these reactions proceed via the formation of an intermediate long-lived association ion in which total randomization of the H and D atoms takes place prior to unimolecular decomposition. No appreciable isotopic exchange occurred in the exothermic NH3 reactions which apparently proceeded via the simpler mechanisms of D+ (or H+) or D3O+ (or H3O+) transfer. The differing mechanisms for the H2O and D2O reactions compared to the NH3 reactions are rationalized in terms of the thermicities of the reactions and the lifetimes of the respective intermediate ions.