An Experimental and Theoretical Investigation of the C(1D) + N2 → C(3P) + N2 Quenching Reaction at Low Temperature

State-resolved modeling of electronic excitation in weakly ionized oxygen mixtures

Electronic excitation and ionization in oxygen-argon mixtures are analyzed using a three-temperature electronic state-resolved model and evaluated using recent experimental data from reflected shock experiments. A detailed description of the model formulation and parameter selection is provided. Excellent agreement is obtained between model predictions and experimental measurements of O_{2} number density during dissociation in mixtures of 2%-5% O_{2} dilute in argon. Next, electron number density measurements are leveraged to infer a rate constant for the heavy particle impact excitation of argon, facilitating improved modeling of net ionization and a clearer understanding of the electronic excitation kinetics of oxygen. The electronic state-resolved model is then assessed using measured data for three electronic states of atomic oxygen. The model successfully reproduces the multistage behavior observed in the measured time histories and yields new insights into the multistage behavior that revises previous interpretations. For several experiments, the modeling choices involved in the calculation of escape factors significantly influence the predicted time histories. A global sensitivity analysis considering nearly 300 parameters is then conducted to identify which model parameters most sensitively influence the predicted excited state populations. Excitation of the measured states from the metastable levels and collisional excitation between the three measured states are important across all conditions. The excited state populations demonstrate complex sensitivities involving a large number of collisional and radiative processes, highlighting the importance of adopting a detailed modeling approach when interpreting excited state measurements.

Quantum and Semiclassical Dynamics of Nonadiabatic Electronic Excitation of C(3P) to C1D) by Hyperthermal Collisions with N2

The dynamics and kinetics of nonadiabatic excitation of C(3P) to C(1D) induced by hyperthermal collisions with N2 molecules are investigated using a quantum mechanical and two semiclassical nonadiabatic methods. The full-dimensional interaction potential energy surfaces and spin-orbit coupling, which facilitates the spin-forbidden process, are represented by a recently constructed diabatic potential energy matrix. The multistate quantum dynamics for selected partial waves found small transition probabilities due to the weak spin-orbit coupling. The spin-flip transition is the most favored near the threshold due to effective curve crossing. Strong oscillations are also found in the probabilities, which are attributable to resonances supported by the deep well in the singlet-state potential. Vibrational state-specified rate coefficients are reported from J-shifted quantum dynamics calculations, and they follow the Arrhenius form. Vibrational excitation in the N2 collision partner is found to increase the excitation rate at low temperatures, but the trend is reversed at high temperatures. The two semiclassical methods qualitatively reproduce the quantum rate coefficients.

Hyperthermal Dynamics and Kinetics of the C(3P) + N2(X1Σg+) → CN(X2Σ+) + N(4S) Reaction

The hyperthermal dynamics and kinetics of the title reaction, which plays an important role in hypersonic chemistry for atmospheric entry vehicles, are investigated using quasi-classical trajectory methods on a recently developed ground electronic state potential energy surface. The dynamics calculations indicated that the reaction follows a complex-forming mechanism, despite its large endoergicity. The calculated differential cross section is forward-backward symmetric, consistent with a long-lived reaction intermediate supported by the NCN potential well. The lifetime of the reaction complex is sufficiently long that the vibrational distribution of the CN product can be predicted by the phase space theory. The calculated vibrational state specific and thermal rate coefficients follow the Arrhenius behavior, and the agreement with existing low-temperature experimental thermal rate coefficients is satisfactory. Extrapolations to high temperatures relevant to hypersonic conditions are provided.

Reaction N(2D) + CH2CCH2 (Allene): An Experimental and Theoretical Investigation and Implications for the Photochemical Models of Titan

We report on a combined experimental and theoretical investigation of the N(²D) + CH₂CCH₂ (allene) reaction of relevance in the atmospheric chemistry of Titan. Experimentally, the reaction was investigated (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 (E _c) of 33 kJ/mol to determine the primary products and the reaction micromechanism and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 to 296 K. Theoretically, electronic structure calculations of the doublet C₃H₄N 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 one of the two equivalent carbon-carbon double bonds of CH₂CCH₂, followed by the formation of several cyclic and linear isomeric C₃H₄N intermediates that can undergo unimolecular decomposition to bimolecular products with elimination of H, CH₃, HCN, HNC, and CN. The kinetic experiments confirm the barrierless nature of the reaction through the measurement of rate constants close to the gas-kinetic rate at all temperatures. Statistical estimates of product branching fractions (BFs) on the theoretical PES were carried out under the conditions of the CMB experiments at room temperature and at temperatures (94 and 175 K) relevant for Titan. Up to 14 competing product channels were statistically predicted with the main ones at E _c = 33 kJ/mol being formation of cyclic-CH₂C(N)CH + H (BF = 87.0%) followed by CHCCHNH + H (BF = 10.5%) and CH₂CCNH + H (BF = 1.4%) the other 11 possible channels being negligible (BFs ranging from 0 to 0.5%). BFs under the other conditions are essentially unchanged. Experimental dynamical information could only be obtained on the overall H-displacement channel, while other possible channels could not be confirmed within the sensitivity of the method. This is also in line with theoretical predictions as the other possible channels are predicted to be negligible, including the HCN/HNC + C₂H₃ (vinyl) channels (overall BF < 1%). The dynamics and product distributions are dramatically different with respect to those observed in the isomeric reaction N(²D) + CH₃CCH (propyne), where at a similar E _c the main product channels are CH₂NH (methanimine) + C₂H (BF = 41%), c-C(N)CH + CH₃ (BF = 32%), and CH₂CHCN (vinyl cyanide) + H (BF = 12%). Rate coefficients (the recommended value is 1.7 (±0.2) × 10^-10 cm³ s^-1 over the 50-300 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 abundance (including any new products formed) as a function of the altitude.

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.

Electronic Structure and Bonding of the Fastidious Species CN2 and CP2: A First-Principles Study

The transient molecular species CN₂ (CNN, NCN, c(yclic)-CN₂) and CP₂ (CPP, PCP, c(yclic)-CP₂), along with the isoelectronic to CNN and isovalent to CPP, CCO, have been studied theoretically through the ab initio methodologies multireference configuration interaction (MRCI) and RCCSD(T) coupled with augmented correlation-consistent quintuple and sextuple basis sets. For the CNN, NCN, and c-CN₂ molecules, the examined states are [X̃³Σ^-, ã¹Δ, b̃¹Σ⁺, óΠ, and c̃¹Π], X̃³Σ_g^-, and X̃¹A₁, respectively. The analogous phosphorous system CPP has been studied theoretically for the first time. Our results show that the symmetries ³Σ^-, ¹Δ, and ¹Σ⁺ are not stationary states; thence, the ground state of CPP is of ³Π symmetry and of similar electronic structure to that of the Ã³Π state of CNN. For most of the symmetries studied, we have constructed fully optimized potential energy profiles or "cuts" through the corresponding surfaces at the MRCI level of theory in an effort to follow the (valence) electronic distributions from the "selected" adiabatic species to equilibrium. Our numerical results are in excellent agreement with existing experimental data and previous, although limited, high-level ab initio calculations. Finally, it should be said that some of our findings like dissociation energies, permanent electric dipole moments, and bonding considerations are addressed for the first time.

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.

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.

First-principles dynamics of collisional intersystem crossing: resonance enhanced quenching of C(1D) by N2

Intersystem crossing is a common and important nonadiabatic process in molecular systems. Revealed with a first-principles investigation, the quenching of C(¹D) by N₂ is efficient due to multiple passages via long-lived collisional resonances, despite relatively small spin–orbit couplings.

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.

An experimental and theoretical investigation of the C(1D) + D2 reaction

Rate constants derived from ring polymer molecular dynamics calculations confirm the validity of this method for studying low-temperature complex-forming reactions

Quantum Tunneling Enhancement of the C + H2O and C + D2O Reactions at Low Temperature

Recent studies of neutral gas-phase reactions characterized by barriers show that certain complex forming processes involving light atoms are enhanced by quantum mechanical tunneling at low temperature. Here, we performed kinetic experiments on the activated C((3)P) + H2O reaction, observing a surprising reactivity increase below 100 K, an effect that is only partially reproduced when water is replaced by its deuterated analogue. Product measurements of H- and D-atom formation allowed us to quantify the contribution of complex stabilization to the total rate while confirming the lower tunneling efficiency of deuterium. This result, which is validated through statistical calculations of the intermediate complexes and transition states has important consequences for simulated interstellar water abundances and suggests that tunneling mechanisms could be ubiquitous in cold dense clouds.