Kinetic and Product Study of the Reactions of C(1D) with CH4 and C2H6 at Low Temperature

Kinetic Study of the Gas-Phase O(1D) + CH3OH and O(1D) + CH3CN Reactions: Low-Temperature Rate Constants and Atomic Hydrogen Product Yields

Atomic oxygen in its first excited singlet state, O(¹D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(¹D) with methanol, CH₃OH, and acetonitrile, CH₃CN, over the 50-296 K temperature range. A continuous supersonic flow reactor is used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser-induced fluorescence to generate and monitor O(¹D) atoms, respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions are also performed, through laser-induced fluorescence measurements of H(²S) atom formation. On the kinetic side, the rate constants for these reactions are seen to be large (>2 × 10^-10 cm³ s^-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(¹D) + CH₃OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical works. For the O(¹D) + CH₃CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels are performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(¹D) + CH₃CN reaction.

An Experimental and Theoretical Investigation of the Gas-Phase C(3P) + N2O Reaction. Low Temperature Rate Constants and Astrochemical Implications

The reaction between atomic carbon in its ground electronic state, C(³P), and nitrous oxide, N₂O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(³P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the ³A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (k_C(³P)+N2O (296 K) = (3.4 ± 0.3) × 10^-11 cm³ s^-1), reaching a value of k_C(³P)+N2O (50 K) = (7.9 ± 0.8) × 10^-11 cm³ s^-1 at 50 K. As current astrochemical models do not include the C + N₂O reaction, we tested the influence of this process on interstellar N₂O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N₂O abundances decrease significantly at intermediate times (10³ - 10⁵ years) when gas-phase C(³P) abundances are high.

Experimental and theoretical studies of the gas-phase reactions of O(1D) with H2O and D2O at low temperature

Here we report the results of an experimental and theoretical study of the gas-phase reactions between O(¹D) and H₂O and O(¹D) and D₂O at room temperature and below. On the experimental side, the kinetics of these reactions have been investigated over the 50-127 K range using a continuous flow Laval nozzle apparatus, coupled with pulsed laser photolysis and pulsed laser induced fluorescence for the production and detection of O(¹D) atoms respectively. Experiments were also performed at 296 K in the absence of a Laval nozzle. On the theoretical side, the existing full-dimensional ground X ¹A potential energy surface for the H₂O₂ system involved in this process has been reinvestigated and enhanced to provide a better description of the barrierless H-atom abstraction pathway. Based on this enhanced potential energy surface, quasiclassical trajectory calculations and ring polymer molecular dynamics simulations have been performed to obtain low temperature rate constants. The measured and calculated rate constants display similar behaviour above 100 K, showing little or no variation as a function of temperature. Below 100 K, the experimental rate constants increase dramatically, in contrast to the essentially temperature independent theoretical values. The possible origins of the divergence between experiment and theory at low temperatures are discussed.

A kinetic study of the N(2D) + C2H4 reaction at low temperature

Electronically excited nitrogen atoms N(²D) are important species in the photochemistry of N₂ based planetary atmospheres such as Titan. Despite this, few N(²D) reactions have been studied over the appropriate low temperature range. During the present work, rate constants were measured for the N(²D) + ethene (C₂H₄) reaction using a supersonic flow reactor at temperatures between 50 K and 296 K. Here, a chemical reaction was used to generate N(²D) atoms, which were detected directly by laser induced fluorescence in the vacuum ultraviolet wavelength region. The measured rate constants displayed very little variation as a function of temperature, with substantially larger values than those obtained in previous work. Indeed, considering an average temperature of 170 K for the atmosphere of Titan leads to a rate constant that is almost seven times larger than the currently recommended value. In parallel, electronic structure calculations were performed to provide insight into the reactive process. While earlier theoretical work at a lower level predicted the presence of a barrier for the N(²D) + C₂H₄ reaction, the present calculations demonstrate that two of the five doublet potential energy surfaces correlating with reagents are likely to be attractive, presenting no barriers for the perpendicular approach of the N atom to the C[double bond, length as m-dash]C bond of ethene. The measured rate constants and new product channels taken from recent dynamical investigations of this process are included in a 1D coupled ion-neutral model of Titan's atmosphere. These simulations indicate that the modeled abundances of numerous nitrogen bearing compounds are noticeably affected by these changes.

Experimental and Theoretical Study of the O(1D) + HD Reaction

This work addresses the kinetics and dynamics of the gas-phase reaction between O(¹D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(¹D) production and pulsed-laser-induced fluorescence for O(¹D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction. In parallel, theoretical rate constants and branching ratios were calculated by using three different techniques; mean potential phase space theory (MPPST), the statistical quantum mechanical method (SQM), and ring polymer molecular dynamics (RPMD). Although the agreement between experimental and theoretical rate constants is reasonably good, with differences not exceeding 30% over the entire temperature range, the theoretical branching ratios derived by the MPPST and SQM methods are as much as 50% larger than the experimental ones. These results are presented in the context of earlier work, while the possible origins of the discrepancies between experiment and theory are discussed.

Theoretical Study of Barrierless Chemical Reactions Involving Nearly Elastic Rebound: The Case of S(1D) + X2, X = H, D

For some values of the total angular momentum consistent with reaction, the title processes involve nonreactive trajectories proceeding through a single rebound mechanism during which the internal motion of the reagent diatom is nearly unperturbed. When such paths are in a significant amount, the classical reaction probability is found to be markedly lower than the quantum mechanical one. This finding was recently attributed to an unusual quantum effect called diffraction-mediated trapping, and a semiclassical correction was proposed in order to take into account this effect in the classical trajectory method. In the present work, we apply the resulting approach to the calculation of opacity functions as well as total and state-resolved integral cross sections (ICSs) and compare the values obtained with exact quantum ones, most of which are new. As the title reactions proceed through a deep insertion well, mean potential statistical calculations are also presented. Seven values of the collision energy, ranging from 30 to 1127 K, are considered. Two remarkable facts stand out: (i) The corrected classical treatment strongly improves the accuracy of the opacity function as compared to the usual classical treatment. When the entrance transition state is tight, however, those trajectories crossing it with a bending vibrational energy below the zero point energy must be discarded. (ii) The quantum opacity function, particularly its cutoff, is finely reproduced by the statistical approach. Consequently, the total ICS is also very well described by the two previous approximate methods. These, however, do not predict state-resolved ICSs with the same accuracy, proving thereby that (i) one or several genuine quantum effects involved in the dynamics are missed by the corrected classical treatment and (ii) the dynamics are not fully statistical.

Low-Temperature Rate Constants and Product-Branching Ratios for the C(1D) + H2O Reaction

The gas-phase reaction between atomic carbon in its first electronically excited ¹D state and water has been studied over the 50-296 K temperature range using a supersonic flow apparatus. C(¹D) atoms were produced by pulsed ultraviolet multiphoton dissociation of carbon tetrabromide; a process that also generates ground-state atomic carbon C(³P). The reaction was followed by detecting product H-atoms by pulsed vacuum ultraviolet laser-induced fluorescence. Two types of experiment were performed. First, temperature-dependent rate constants were derived by recording H-atom formation curves at various gas-phase water concentrations at each temperature. Secondly, temperature-dependent H-atom yields were extracted by comparing the H-atom fluorescence intensities generated by the target C(¹D) + H₂O reaction with those of a reference reaction. The second-order rate constants are large and increase to low temperature, whereas the measured H-atom yields are close to the theoretical maximum value of 2 above 100 K. At 50 K, neither rate constants nor H-atom yields could be derived because of H-atom formation by quantum tunneling in the activated C(³P) + H₂O reaction. The present results are discussed in the context of earlier work on the C(¹D)/C(³P) + H₂O reactions.

Conical intersection-regulated intermediates in bimolecular reactions: Insights from C(1D) + HD dynamics

The importance of conical intersections (CIs) in electronically nonadiabatic processes is well known, but their influence on adiabatic dynamics has been underestimated. Here, through combined experimental and theoretical studies, we show that CIs induce a barrier and regulate conversion from a precursor metastable intermediate (CI-R) to a deep well one. This results in bond-selective activation, influencing the adiabatic dynamics markedly in the C(1D) + HD reaction. Theory is validated by experiment; quantum dynamics calculations on highly accurate ab initio potential energy surfaces yield rate coefficients and product branching ratios in excellent agreement with the experiment. Quasi-classical trajectory calculations reveal that the CI-R intermediate leads to unusual reaction mechanisms (designated as C─H activation complex conversion and cyclic complex), which are responsible for large branching ratios. We also reveal that CI-R intermediates exist in other reactive systems, and the dynamical effects uncovered here may have general significance.

A low temperature investigation of the N(2D) + CH4, C2H6 and C3H8 reactions

Low temperature rate constants for the N(²D) + C₂H₆, C₃H₈ reactions are shown to be much smaller than previously thought.

Rate constants for the N(2D) + C2H2 reaction over the 50–296 K temperature range

The reactions of metastable atomic nitrogen N(²D) are important processes in the gas-phase chemistry of several planetary atmospheres.

A combined theoretical and experimental investigation of the kinetics and dynamics of the O(1D) + D2 reaction at low temperature

The O(¹D) + H₂ reaction is a prototype for simple atom-diatom insertion type mechanisms considered to involve deep potential wells. While exact quantum mechanical methods can be applied to describe the dynamics, such calculations are challenging given the numerous bound quantum states involved. Consequently, efforts have been made to develop alternative theoretical strategies to portray accurately the reactive process. Here we report an experimental and theoretical investigation of the O(¹D) + D₂ reaction over the 50-296 K range. The calculations employ three conceptually different approaches - mean potential phase space theory, the statistical quantum mechanical method and ring polymer molecular dynamics. The calculated rate constants are in excellent agreement over the entire temperature range, exhibiting only weak temperature dependence. The agreement between experiment and theory is also very good, with discrepancies smaller than 26%. Taken together, the present and previous theoretical results validate the hypothesis that long-lived complex formation dominates the reaction dynamics at low temperature.