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

The reactions of atomic carbon in its first excited ¹D state with both CH₄ and C₂H₆ have been investigated using a continuous supersonic flow reactor over the 50–296 K temperature range. C­(¹D) atoms were generated in situ by the pulsed laser photolysis of CBr₄ at 266 nm. To follow the reaction kinetics, product H atoms were detected by vacuum ultraviolet laser-induced fluorescence at 121.567 nm. Absolute H-atom yields for both reactions were determined by comparison with the H-atom signal generated by the reference C­(¹D) + H₂ reaction. Although the rate constant for the C­(¹D) + CH₄ reaction is in excellent agreement with earlier work at room temperature, this process displays a surprising reactivity increase below 100 K. In contrast, the reactivity of the C­(¹D) + C₂H₆ system decreases as the temperature falls, obeying a capture-type rate law. The H-atom product yields of the C­(¹D) + CH₄ reaction agree with the results of earlier crossed-beam experiments at higher collision energy. Although no previous data is available on the product channels of the C­(¹D) + C₂H₆ reaction, comparison with earlier work involving the same singlet C₃H₆ potential energy surface allows us to draw conclusions from the measured H-atom yields

Unusual Low-Temperature Reactivity of Water: The CH + H₂O Reaction as a Source of Interstellar Formaldehyde?

Water is an important reservoir species for oxygen in interstellar space and plays a key role in the physics of star formation through cooling by far-infrared emission. While water vapor is present at high abundances in the outflows of protostars, its contribution to the chemical evolution of these regions is a minor one due to its limited low temperature reactivity in the gas-phase. Here, we performed kinetic experiments on the barrierless CH + H₂O reaction in a supersonic flow reactor down to 50 K. The measured rate increases rapidly below room temperature, confirming and extending the predictions of previous statistical calculations. The open product channels for this reaction suggest that this process could be an important gas-phase route for formaldehyde formation in protostellar envelopes

Ring-Polymer Molecular Dynamics for the Prediction of Low-Temperature Rates: An Investigation of the C(¹D) + H₂ Reaction

Quantum mechanical calculations are important tools for predicting the rates of elementary reactions, particularly for those involving hydrogen and at low temperatures where quantum effects become increasingly important. These approaches are computationally expensive, however, particularly when applied to complex polyatomic systems or processes characterized by deep potential wells. While several approximate techniques exist, many of these have issues with reliability. The ring-polymer molecular dynamics method was recently proposed as an accurate and efficient alternative. Here, we test this technique at low temperatures (300–50 K) by analyzing the behavior of the barrierless C­(¹D) + H₂ reaction over the two lowest singlet potential energy surfaces. To validate the theory, rate coefficients were measured using a supersonic flow reactor down to 50 K. The experimental and theoretical rates are in excellent agreement, supporting the future application of this method for determining the kinetics and dynamics of a wide range of low-temperature 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(³P) + 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

Gas-Phase Kinetics of the Hydroxyl Radical Reaction with Allene: Absolute Rate Measurements at Low Temperature, Product Determinations, and Calculations

The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ <i>T</i>/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10^–11 cm³ molecule^–1 s^–1. Product formation from the reaction of OH and OD radicals with allene (C₃H₄) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C₃H₄OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH₃ or CH₂D + H₂CCO (ketene). Moreover, the rate constant for the OD + C₃H₄ reaction is also found to be 1.4 times faster than the rate constant for the OH + C₃H₄ reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations