Philippe Caubet

Revealing atom-radical reactivity at low temperature through the N + OH reaction.

Rates have been measured for a chemical transformation of interstellar interest in which both reagents are unstable. More than 100 reactions between stable molecules and free radicals have been shown to remain rapid at low temperatures. In contrast, reactions between two unstable radicals have received much less attention due to the added complexity of producing and measuring excess radical concentrations. We performed kinetic experiments on the barrierless N(4S) + OH(2Π) → H(2S) + NO(2Π) reaction in a supersonic flow (Laval nozzle) reactor. We used a microwave-discharge method to generate atomic nitrogen and a relative-rate method to follow the reaction kinetics. The measured rates agreed well with the results of exact and approximate quantum mechanical calculations. These results also provide insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar clouds.

Reaction kinetics to low temperatures. Dicarbon + acetylene, methylacetylene, allene and propene from 77 ≤ T ≤ 296 K

The temperature dependence of the reactions of the dicarbon molecule in its ground singlet (X1Sigma(g)+) and first excited triplet (a 3Pi(u)) states with acetylene, methylacetylene, allene and propene has been studied using a recently constructed continuous supersonic flow reactor. Four Laval nozzles have been designed to access specified temperatures over the range of 77 < or = T < or = 220 K and measurements have been performed at 296 K under subsonic flow conditions. C2 was produced in its two lowest electronic states via the in situ multiphoton dissociation of C2Br4 at 266 nm. The time dependent losses of C2 in these two states in the presence of an excess of co-reagent species were simultaneously followed by laser-induced fluorescence in the Mulliken and Swan bands for the detection of singlet and triplet state C2, respectively. The rate coefficients were measured to be very fast, with values larger than 10(-10) cm3 molecule(-1) s(-1) and up to 5 x 10(-10) cm3 molecule(-1) s(-1). The reactions of 1C2 are seen to be essentially temperature independent from 77 < or = T < or = 296 K whereas the rate coefficients for the 3C2 reactions are seen to increase until they are equivalent to the 1C2 values at 77 K.

Gas‐Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory

Reactions of the hydroxyl radical with propene and 1-butene are studied experimentally in the gas phase in a continuous supersonic flow reactor over the range 50≤T/K≤224. OH radicals are produced by pulsed laser photolysis of H(2)O(2) at 266 nm in the supersonic flow and followed by laser-induced fluorescence in the (1, 0) A(2)Σ(+)←X(2)Π(3/2) band at about 282 nm. These reactions are found to exhibit negative temperature dependences over the entire temperature range investigated, varying between (3.1-19.2) and (4.2-28.6)×10(-11) cm(3) molecule(-1) s(-1) for the reactions of OH with propene and 1-butene, respectively. Quantum chemical calculations of the potential energy surfaces are used as the basis for energy- and rotationally resolved Rice-Ramsperger-Kassel-Marcus calculations to determine the rate constants over a range of temperatures and pressures. The negative temperature dependences of the rate constants are explained by competition between complex redissociation and passage to the adducts by using a model with two transition states. The results are compared and contrasted with earlier studies and discussed in terms of their potential relevance to the atmosphere of Saturn.

A low temperature investigation of the N(4S degrees) + NO reaction.

The kinetics of the N((4)S degrees) + NO(X(2)Pi) reaction have been studied in a continuous supersonic flow reactor over the range 48 K <or= T <or= 211 K. Nitrogen atoms were produced by microwave discharge upstream of the Laval nozzle and were probed in the vacuum ultraviolet by resonance fluorescence. The reaction has been found to exhibit a small negative temperature dependence, with a rate coefficient decreasing from (5.8 +/- 0.3) x 10(-11) cm(3) molecule(-1) s(-1) at 48 K to (3.3 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1) at 211 K with the statistical uncertainties cited at the level of a single standard deviation from the mean.

The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol

Rate constants for the C(3P) + CH3OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K ≤ T ≤ 296 K. C(3P) was created by the in situ pulsed laser photolysis of CBr4, a multiphoton process which also produced some C(1D), allowing us to investigate simultaneously the low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H(2S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2 × 10−11 cm3 molecule−1 s−1 at 296 K to 20.0 × 10−11 cm3 molecule−1 s−1 at 50 K. CH3 and HCO are found to be the major products under our experimental conditions. As this reaction is not considered in current astrochemical networks, its influence on interstellar methanol abundances is tested using a dense interstellar cloud model.

CN(A2Πi→X2Σ+) chemiluminescence from the N+C2N, N+CCl, and N+C2 reactions under low-pressure fast-flow conditions

Abstract The reaction of potassium atoms with C 2 NCl 3 , CCl 4 , and C 2 Cl 4 have been used to produce C 2 N, CCl, and C 2 radicals. The addition of nitrogen atoms to the products of these reactions yields CN( A 2 Π i → X 2 Σ + ) chemiluminescence, detected from vibrational levels v A =2–16. The vibrational distribution of CN(A) are, respectively, for different reactions, similar to a statistical decrease from v A =2 to v A =14 for N+C 2 N, inverted with a maximum for v A =8 for N+CCl, and limited to v A =2 level for N+C 2 .

Vibrational distribution in CN(X2Σ+) from the N + C2 → CN + C reaction

Abstract C 2 radicals were prepared from the overall reaction 4K+C 2 Cl 4 →4KCl+C 2 and mixed with atomic nitrogen in a low-pressure fast-flow reactor. CN(X 2 Σ + ) produced by the exoergic reaction N+C 2 →CN+C was probed by laser-induced fluorescence. Vibrational distributions at different distances from the reactant mixing zone were determined. Fitting of the evolution of the vibrational distribution, using a kinetic scheme which included the collisional transfer between X 2 Σ + and A 2 π i vibrational levels and CN removal by the N+C 2 →C+N 2 reaction, allowed to determine the nascent vibrational distribution given by the N+C 2 reaction. This distribution was found to be non-statistical and is compared to that obtained from the C+NO→CN+O reaction in a previous crossed-beam study.

Faraday communications. A pulsed crossed supersonic molecular beam study of the reaction C(3P)+ OCS(X1Σ+)→ CS(a 3Πr, X1Σ+)+ CO(X1Σ+)

The C + OCS → CS + CO reaction has been investigated in a pulsed crossed supersonic molecular beam experiment at a centre-of-mass kinetic energy of 0.32 eV. CS radicals, produced in both the a 3Πr and X 1Σ+ states, have been detected by laser-induced fluorescence (LIF). A newly observed CS transition, A 1Π–a 3Πr, is also reported. Comparison of relative LIF intensities following CS(a 3Πr) and CS(X 1Σ+) excitation, shows that the reaction pathway producing CS(a 3Πr)+ CO(X 1Σ+) is highly efficient.