Atmospheric Chemistry of (Z)-CF3CH═CHCF3: OH Radical Reaction Rate Coefficient and Global Warming Potential

Chemical Investigation on the Mechanism and Kinetics of the Atmospheric Degradation Reaction of Trichlorofluoroethene by OH⋅ and Its Subsequent Fate in the Presence of O2 /NOx

The M06-2X/6-311++G(d,p) level of theory was used to examine the degradation of Trichlorofluoroethene (TCFE) initiated by OH⋅ radicals. Additionally, the coupled-cluster single-double with triple perturbative [CCSD(T)] method was employed to refine the single-point energies using the complete basis set extrapolation approach. The results indicated that OH-addition is the dominant pathway. OH⋅ adds to both the C1 and C2 carbons, resulting in the formation of the C(OH)Cl2 -⋅CClF and ⋅CCl2 -C(OH)ClF species. The associated barrier heights were determined to be 1.11 and -0.99 kcal mol-1 , respectively. Furthermore, the energetic and thermodynamic parameters show that pathway 1 exhibits greater exothermicity and exergonicity compared to pathway 2, with differences of 8.11 and 8.21 kcal mol-1 , correspondingly. The primary pathway involves OH addition to the C2 position, with a rate constant of 6.2×10-13  cm3 molecule-1 sec-1 at 298 K. This analysis served to estimate the atmospheric lifetime, along with the photochemical ozone creation potential (POCP) and ozone depletion potential (ODP). It yielded an atmospheric lifetime of 8.49 days, an ODP of 4.8×10-4 , and a POCP value of 2.99, respectively. Radiative forcing efficiencies were also estimated at the M06-2X/6-311++G(d,p) level. Global warming potentials (GWPs) were calculated for 20, 100, and 500 years, resulting in values of 9.61, 2.61, and 0.74, respectively. TCFE is not expected to make a significant contribution to the radiative forcing of climate change. The results obtained from the time-dependent density functional theory (TDDFT) indicated that TCFE and its energized adducts are unable to photolysis under sunlight in the UV and visible spectrum. Secondary reactions involve the [TCFE-OH-O2 ]⋅ peroxy radical, leading subsequently to the [TCFE-OH-O]⋅ alkoxy radical. It was found that the alkoxy radical resulting from the peroxy radical can lead to the formation of phosgene (COCl2 ) and carbonyl chloride fluoride (CClFO), with phosgene being the primary product.

First Atmospheric Measurements and Emission Estimates of HFO-1336mzz(Z)

For the past few years, short-lived unsaturated halocarbons have been marketed as environmentally friendly replacements for long-lived halogenated greenhouse gases and ozone-depleting substances. The phase-in of unsaturated halocarbons for various applications, such as refrigeration and foam blowing, can be tracked by their emergence and increase in the atmosphere. We present the first atmospheric measurements of the hydrofluoroolefin (HFO) HFO-1336mzz(Z) ((Z)-1,1,1,4,4,4-hexafluoro-2-butene, cis-CF3CH═CHCF3), a newly used unsaturated hydrofluorocarbon. HFO-1336mzz(Z) has been detected in >90% of all measurements since 2018 during multi-month campaigns at three Swiss and one Dutch location. Since 2019, it is found in ∼30% of all measurements that run continuously at the Swiss high-altitude Jungfraujoch station. During pollution events, mole fractions of up to ∼10 ppt were observed. Based on our measurements, Swiss and Dutch emissions were estimated at 2-7 Mg yr-1 (2019-2021) and 30 Mg yr-1 (2022), respectively. Modeled spatial emission distributions only partly conform to population density in both countries. Monitoring the presence of new unsaturated halocarbons in the atmosphere is crucial since long-term effects of their degradation products are still debated. Furthermore, the production of HFOs involves climate-active substances, which may leak to the atmosphere─in the case of HFO-1336mzz(Z), for example, the ozone-depleting CFC-113a (CF3CCl3).

Rate Coefficients for the Gas-Phase Reaction of ( E)- and ( Z)-CF3CF═CFCF3 with the OH Radical and Cl-Atom

The rate coefficients, k, for the gas-phase reaction of the OH radical and Cl-atom with ( E)- and ( Z)-CF₃CF═CFCF₃ were measured using a relative rate technique over a range of temperature (240-375 K) and bath gas pressure (50-630 Torr, He). The obtained rate coefficients were found to be independent of pressure under these conditions. The obtained rate coefficients for the reaction of Cl-atom with ( E)- and ( Z)-CF₃CF═CFCF₃ at 296 K were k₁(296 K) = (7.23 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1 and k₂(296 K) = (6.70 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1, respectively, with the temperature dependence described by the Arrhenius expressions: k₁( T) = (3.47 ± 0.35) × 10^-12 exp[(210 ± 25)/ T] cm³ molecule^-1 s^-1 and k₂( T) = (3.37 ± 0.35) × 10^-12 exp[(199 ± 25)/ T] cm³ molecule^-1 s^-1. The rate coefficients for the OH radical reaction with ( E)- and ( Z)-CF₃CF═CFCF₃ were found to be k₃(296-375 K) = (4.34 ± 0.45) × 10^-13 cm³ molecule^-1 s^-1 and k₄(296-375 K) = (3.30 ± 0.35) × 10^-13 cm³ molecule^-1 s^-1, respectively. The quoted rate coefficient uncertainties are 2σ (95% confidence level) and include estimated systematic errors. The rate coefficients for the reaction of OH with a mixture of the two stereoisomers were determined using a pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique for comparison with previous kinetic measurements using stereoisomer mixtures. The effective rate coefficient for the 0.7/0.3 ( E)/( Z) stereoisomer sample was found to be nearly independent of temperature over the range 222-375 K with a value of (4.47 ± 0.36) × 10^-13 cm³ molecule^-1 s^-1. The atmospheric lifetimes for ( E)- and ( Z)-CF₃CF═CFCF₃ due to OH-reactive loss are estimated to be 25 and 35 days, respectively. The lifetime-corrected radiative efficiencies (W m^-2 ppb^-1) and 100 year time horizon global warming potentials derived in this work are 0.05 and 1.2 for ( E)-CF₃CF═CFCF₃ and 0.13 and 4.1 for ( Z)-CF₃CF═CFCF₃. The photochemical ozone creation potentials for ( E)- and ( Z)-CF₃CF═CFCF₃ are estimated to be 2.5 and 2.1, respectively.

Rate Constants for the Reactions of OH Radicals with the ( E)/( Z) Isomers of CFCl=CFCl and ( E)-CHF=CHF

The rate constants for the OH radical reactions with halogenated ethenes were investigated experimentally and computationally. The rate constants for the reactions of OH radicals with ( E)-CFCl=CFCl ( k1), ( Z)-CFCl=CFCl ( k2), and ( E)-CHF=CHF ( k3) were measured using flash and laser photolysis methods. The temporal profile of the OH radical was monitored by a laser-induced fluorescence technique. Kinetic measurements were carried out over the temperature range of 250-430 K. Arrhenius rate constants were determined to be k1 = (1.67 ± 0.06) × 10-12·exp[(140 ± 10) K/ T], k2 = (1.75 ± 0.04) × 10-12·exp[(140 ± 10) K/ T], and k3 = (3.99 ± 0.15) × 10-12·exp[(260 ± 10) K/ T] cm3 molecule-1 s-1. The quoted uncertainties are 95% confidence levels and do not include systematic errors. Infrared absorption spectra were measured at room temperature. The atmospheric lifetimes and the global warming potentials of ( E)-CFCl=CFCl, ( Z)-CFCl=CFCl, and ( E)-CHF=CHF were estimated to be 4.3, 4.2, and 1.2 days and 0.035, 0.036, and 0.0056, respectively. The ozone depletion potentials of ( E)-CFCl=CFCl and ( Z)-CFCl=CFCl were determined to be 0.00011 and 0.00010, respectively. The photochemical ozone creation potentials of the halogenated ethenes were less than 1/4 that of ethene. In addition, the ( E)/( Z) differences in the energy and IR spectra of the CFCl=CFCl and CHF=CHF molecules were computationally examined. The reactivities of these halogenated ethenes toward OH radicals were investigated through the combination of DFT and ab initio computations. The rate constants calculated for the OH radical reactions of these halogenated ethenes showed reasonable agreement with the experimentally determined values. Our computational results for the CFCl=CFCl and CHF=CHF ( E)/( Z) isomeric pairs indicated that the rate constants toward OH radicals are larger for the higher-energy geometrical isomers than for the lower-energy counterparts.

Kinetics of atmospheric reactions of 4-chloro-1-butene

Chloroalkenes are among the important anthropogenic organic compounds emitted in the atmosphere as a result of their wide use in synthetic processes in industry. Despite their well-known adverse effects on human health and air quality, the chemistry of these chloroalkenes remains poorly explored. In this work, reactions of 4-chloro-1-butene (CBE), a representative example of chloroalkenes, with O₃, OH, NO₃, and Cl are investigated in a 100-L Teflon reaction chamber equipped with gas chromatography-flame ionization detector (GC-FID). The absolute rate method was used for the reaction with O₃ while the relative rate method was used for reactions with OH, NO₃, and Cl. The following rate constants were obtained at room temperature (298 ± 2) K and atmospheric pressure: (3.96 ± 0.43) × 10^-18, (2.63 ± 0.96) × 10^-11, (4.48 ± 1.23) × 10^-15, and (2.35 ± 0.90) × 10^-10 cm³ molecule^-1 s^-1, for reactions with O₃, OH, NO₃, and Cl, respectively. Atmospheric lifetimes of CBE calculated from rate constants of the different reactions obtained in this work showed that reaction with OH is the main loss process for CBE, while in coastal areas and in the marine boundary layer, the CBE loss by Cl reaction becomes important. Estimation of the value of the photochemical ozone creation potential (POCP) indicated that CBE has a large ozone formation potential. The present work underlines the need for further studies on the atmospheric chemistry of chlorinated VOCs.

Rate Coefficient Measurements and Theoretical Analysis of the OH + ( E)-CF3CH═CHCF3 Reaction

Rate coefficients, k, for the gas-phase reaction of the OH radical with ( E)-CF₃CH═CHCF₃ (( E)-1,1,1,4,4,4-hexafluoro-2-butene, HFO-1336mzz(E)) were measured over a range of temperatures (211-374 K) and bath gas pressures (20-300 Torr; He, N₂) using a pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique. k₁( T) was independent of pressure over this range of conditions with k₁(296 K) = (1.31 ± 0.15) × 10^-13 cm³ molecule^-1 s^-1 and k₁( T) = (6.94 ± 0.80) × 10^-13exp[-(496 ± 10)/ T] cm³ molecule^-1 s^-1, where the uncertainties are 2σ, and the pre-exponential term includes estimated systematic error. Rate coefficients for the OD reaction were also determined over a range of temperatures (262-374 K) at 100 Torr (He). The OD rate coefficients were ∼15% greater than the OH values and showed similar temperature dependent behavior with k₂( T) = (7.52 ± 0.44) × 10^-13exp[-(476 ± 20)/ T] and k₂(296 K) = (1.53 ± 0.15) × 10^-13 cm³ molecule^-1 s^-1. The rate coefficients for reaction 1 were also measured using a relative rate technique between 296 and 375 K with k₁(296 K) measured to be (1.22 ± 0.1) × 10^-13 cm³ molecule^-1 s^-1, in agreement with the PLP-LIF results. In addition, the 296 K rate coefficient for the O₃ + ( E)-CF₃CH═CHCF₃ reaction was determined to be <5.2 × 10^-22 cm³ molecule^-1 s^-1. A theoretical computational analysis is presented to interpret the observed positive temperature dependence for the addition reaction and the significant decrease in OH reactivity compared to the ( Z)-CF₃CH═CHCF₃ stereoisomer reaction. The estimated atmospheric lifetime of ( E)-CF₃CH═CHCF₃, due to loss by reaction with OH, is estimated to be ∼90 days, while the actual lifetime will depend on the location and season of its emission. Infrared absorption spectra of ( E)-CF₃CH═CHCF₃ were measured and used to estimate the 100 year time horizon global warming potentials (GWP) of 32 (atmospherically well-mixed) and 14 (lifetime-adjusted).

Molybdenum chloride catalysts for Z-selective olefin metathesis reactions

The development of catalyst-controlled stereoselective olefin metathesis processes has been a pivotal recent advance in chemistry. The incorporation of appropriate ligands within complexes based on molybdenum, tungsten and ruthenium has led to reactivity and selectivity levels that were previously inaccessible. Here we show that molybdenum monoaryloxide chloride complexes furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the commercially available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene. Furthermore, otherwise inefficient and non-stereoselective transformations with Z-1,2-dichloroethene and 1,2-dibromoethene can be effected with substantially improved efficiency and Z selectivity. The use of such molybdenum monoaryloxide chloride complexes enables the synthesis of representative biologically active molecules and trifluoromethyl analogues of medicinally relevant compounds. The origins of the activity and selectivity levels observed, which contradict previously proposed principles, are elucidated with the aid of density functional theory calculations.

Atmospheric chemistry of Z- and E-CF3CHCHCF3

The first determination of the atmospheric chemistry of E-CF₃CHCHCF₃ and of the Cl and O₃ initiated atmospheric chemistry of Z-CF₃CHCHCF₃.

Atmospheric chemistry of short-chain haloolefins: photochemical ozone creation potentials (POCPs), global warming potentials (GWPs), and ozone depletion potentials (ODPs)

Short-chain haloolefins are being introduced as replacements for saturated halocarbons. The unifying chemical feature of haloolefins is the presence of a CC double bond which causes the atmospheric lifetimes to be significantly shorter than for the analogous saturated compounds. We discuss the atmospheric lifetimes, photochemical ozone creation potentials (POCPs), global warming potentials (GWPs), and ozone depletion potentials (ODPs) of haloolefins. The commercially relevant short-chain haloolefins CF3CFCH2 (1234yf), trans-CF3CHCHF (1234ze(Z)), CF3CFCF2 (1216), cis-CF3CHCHCl (1233zd(Z)), and trans-CF3CHCHCl (1233zd(E)) have short atmospheric lifetimes (days to weeks), negligible POCPs, negligible GWPs, and ODPs which do not differ materially from zero. In the concentrations expected in the environment their atmospheric degradation products will have a negligible impact on ecosystems. CF3CFCH2 (1234yf), trans-CF3CHCHF (1234ze(Z)), CF3CFCF2 (1216), cis-CF3CHCHCl (1233zd(Z)), and trans-CF3CHCHCl (1233zd(E)) are environmentally acceptable.

Methyl-perfluoroheptene-ethers (CH3OC7F13): measured OH radical reaction rate coefficients for several isomers and enantiomers and their atmospheric lifetimes and global warming potentials

Mixtures of methyl-perfluoroheptene-ethers (CH3OC7F13, MPHEs) are currently in use as replacements for perfluorinated alkanes (PFCs) and poly-ether heat transfer fluids, which are persistent greenhouse gases with lifetimes >1000 years. At present, the atmospheric processing and environmental impact from the use of MPHEs is unknown. In this work, rate coefficients at 296 K for the gas-phase reaction of the OH radical with six key isomers (including stereoisomers and enantiomers) of MPHEs used commercially were measured using a relative rate method. Rate coefficients for the six MPHE isomers ranged from ∼ 0.1 to 2.9 × 10(-12) cm(3) molecule(-1) s(-1) with a strong stereoisomer and -OCH3 group position dependence; the (E)-stereoisomers with the -OCH3 group in an α- position relative to the double bond had the greatest reactivity. Rate coefficients measured for the d3-MPHE isomer analogues showed decreased reactivity consistent with a minor contribution of H atom abstraction from the -OCH3 group to the overall reactivity. Estimated atmospheric lifetimes for the MPHE isomers range from days to months. Atmospheric lifetimes, radiative efficiencies, and global warming potentials for these short-lived MPHE isomers were estimated based on the measured OH rate coefficients along with measured and theoretically calculated MPHE infrared absorption spectra. Our results highlight the importance of quantifying the atmospheric impact of individual components in an isomeric mixture.

Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

Global climate change is a major concern as it leads to an increase in the average temperature of the earth&rsquo;s atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.