Kinetic study of the OH + ethylene reaction using frequency‐modulated laser absorption spectroscopy

Theoretical investigation of the OH-initiated atmospheric degradation mechanism of CX2CHX (X = H, F, Cl) by advanced quantum chemical and transition state theory methods

Halogenated olefins are anthropogenic compounds with many industrial applications but at the same time raising many environmental and health concerns. Gas-phase electrophilic addition of the OH radical to the olefinic CC bond represents the primary sink for these chemicals in the atmosphere, with the degree and type of halogenation playing a significant role in their overall reactivity. In this work, we present a theoretical investigation of the reaction mechanisms and kinetics for the reactions between the OH radical and CH2CH2 (ethylene, ETH), CF2CHF (trifluoroethylene, TFE) and CCl2CHCl (trichloroethylene, TCE), simulated by state-of-the-art protocols and methods, with the aim of providing a detailed interpretation of the available experimental results, as well as new data of relevance to tropospheric chemistry. Specifically, potential energy surfaces (PESs) are obtained using the jun-Cheap (jChS) composite scheme, whereas temperature and pressure dependent rate coefficients and product distributions in the 100-600 K temperature range are calculated within the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) framework. The rates for barrierless channels are obtained from variable reaction coordinate-variational transition state theory (VRC-VTST) combined with the two transition state model. While the reactions with ETH and TFE proceed mainly via the formation of addition adducts at P = 1 atm and T = 298 K, the dominant channel for TCE is the Cl-elimination reaction. Global rate constants for the two halogenated olefins, TFE and TCE, are found to be pressure-independent, contrary to the case of ETH. The computed rate constants, as well as their temperature and pressure dependence, are in remarkable agreement with the available experimental data, and they are used to derive atmospheric lifetimes (τ) for both TFE and TCE as a function of altitude (h) in the atmosphere, by taking into account variations in the rate coefficients (k (T, P)) and [OH] concentration.

OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study

The reaction between the ground-state hydroxyl radical, OH(²Π), and ethylene, C₂H₄, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH₂CHOH (vinyl alcohol) + H, CH₃CHO (acetaldehyde) + H, and H₂CO (formaldehyde) + CH₃ product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH₂CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H₂CO + CH₃ channel for ∼58%, while the CH₃CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Dissociative Photodetachment Dynamics of the OH-(C2H4) Anion Complex

Photoelectron-photofragment coincidence (PPC) measurements on OH^-(C₂H₄) anions at a photon energy of 3.20 eV revealed stable and dissociative photodetachment product channels, OH-C₂H₄ + e^- and OH + C₂H₄ + e^-, respectively. The main product channel observed was dissociation to the reactants (>67%), OH + C₂H₄ (v = 0, 1, 2) + e^-, where vibrational excitation in the C-H stretching modes of the C₂H₄ photofragments corresponds to a minor channel. The low kinetic energy release (KER) of the dissociating fragments is consistent with weak repulsion between the OH + C₂H₄ reactants near the transition state as well as the partitioning of energy into rotation of the dissociation products. An impulsive model was used to account for rotational energy partitioning in the dissociative photodetachment (DPD) process and showed good agreement with the experimental results. The low KER of the dissociating fragments and the similarities in the photoelectron spectra between stable and dissociative events support a mechanism involving the van der Waals complex formed upon photodetachment of OH^-(C₂H₄) as an intermediate in the dominant OH + C₂H₄ + e^- dissociative channel.

Direct Trace Fitting of Experimental Data Using the Master Equation: Testing Theory and Experiments on the OH + C2H4 Reaction

Laser flash photolysis coupled with laser-induced fluorescence observation of OH has been used to observe the equilibration of OH + C₂H₄ ↔ HOC₂H₄ over the temperature range 563-723 K and pressures of bath gas (N₂) from 58 to 250 Torr. The time-resolved OH traces have been directly and globally fitted with a master equation in order to extract Δ_RH₀⁰, the binding energy of the HOC₂H₄ adduct, with respect to reagents. The global approach allows the role that OH abstraction plays at higher temperatures to be identified. The resultant value ofΔ_RH₀⁰, 111.8 kJ mol^-1, is determined to be better than 2 kJ mol^-1 and is in agreement with our ab initio calculations (carried out at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level), 111.4 kJ mol^-1, and other state of the art calculations. Parameters for the abstraction channel are also in good agreement with previous experimental studies. To effect this analysis, the MESMER master equation code was extended to directly incorporate secondary chemistry: diffusional loss from the observation region and reaction with the photolytic precursor. These extensions, which, among other things, resolve issues related to the merging of chemically significant and internal energy relaxation eigenvalues, are presented.

Single-tone mid-infrared frequency modulation spectroscopy for sensitive detection of transient species

A single-tone mid-infrared frequency modulation (MIR-FM) spectrometer consisting of a cw-OPO-based laser system, a 500 MHz LiTaO ₃ electro-optical modulator (EOM), and a high-bandwidth GaAs mid-infrared detector has been developed. In order to assess the instrument's sensitivity and time resolution, FM spectra of selected CH ₄ transitions around 3070 cm ^-1 were measured and the reaction Cl + CH ₄ following the 193 nm excimer laser photolysis of oxalyl chloride was investigated by recording concentration-time profiles of HCl at 2925.90 cm ^-1 in a low-pressure slow-flow reactor. Furthermore, OH radicals were generated by UV photolysis of H ₂O ₂ and its transients were recorded at 3447.27 cm ^-1. The minimal detectable absorption of the spectrometer was determined to be A _min=4⋅10^-4 (Δ f _BW=1 MHz, ν~=3447 cm ^-1) by using the Allan approach. Mainly due to thermal noise contributions of the easy-to-saturate photodetector, the detection limit is about a factor of 4 above the shot-noise limit. To the best of our knowledge, this work reports the first implementation of a single-tone MIR-FM spectrometer based on an external EOM modulation scheme and its use for the detection of transient molecular species.