Kinetics, thermochemistry and atmospheric implications for the reaction of OH radicals with CH3CF = CF2 (HFO-1243yc)

Unveiling the Atmospheric Oxidation of Hexafluoroisobutylene, (CF3)2C═CH2, with Cl Atom, NO3 Radical, and O3 Molecule

The CFC alternative, 3,3,3-trifluoro-2(trifluoromethyl)-1-propene, (CF3)2C═CH2 (HFIB), plays a pivotal role across various industrial sectors owing to its unique chemical properties, versatility, and diverse applications as a refrigerant, propellant, aerosol, etc. However, its extensive presence in industrial processes raises concerns about its environmental impact. In this study, atmospheric oxidation of HFIB by reaction with Cl, NO3, and O3 is investigated theoretically to unravel the reaction mechanism, thermodynamics, and kinetics. This study employs quantum chemical methods to explore the various reaction pathways via potential energy surface diagrams. The addition reactions are the dominating reactions of HFIB with atmospheric oxidants. The thermodynamics and kinetics were analyzed, revealing exothermic addition and endothermic abstraction reactions. The rate coefficients (ko,Cl, ko,NO3, and ko,O3) computed using the M06-2X/6-311++G(d,p) level of theory are 0.71 × 10-11, 1.75 × 10-18, and 9.05 × 10-20 cm3 molecule-1 s-1, respectively, which align closely with the experimental rate. The atmospheric implication studies suggested that the reaction with major atmospheric oxidants, ·OH and Cl, primarily influences the lifetime of the species. The calculated cumulative lifetime is 11.70 days, while the radiative efficiency is 0.0265 W m-2 ppb-1. The global warming potential values for the 20-, 100-, and 500-year time horizons also compare well with the experimental findings. Furthermore, the subsequent loss processes of the product radicals were investigated in the atmosphere. Thus, this study provides a crucial aspect in assessing the environmental impact of CFC alternatives.

A Computational Perspective on the Chemical Reaction of HFO-1234zc with the OH Radical in the Gas Phase and in the Presence of Mineral Dust

The gas phase and heterogeneous reaction on mineral dust aerosols of trace gases could significantly affect the tropospheric oxidation capacity and aerosol composition of the atmosphere. In this work, the OH radical-initiated oxidation of a hydrofluoroolefin, HFO-1234zc, and subsequent reaction of favorable intermediates with other reactive species, such as O₂, HO₂, and NO_x (x = 1-2) radicals, were studied, and the role of mineral dust in the form of silicate clusters on the reaction mechanism and rate constant was studied. In the gas phase, OH radical addition to HFO-1234zc is kinetically more favorable than the H-atom abstraction reaction. The calculated reaction energy barrier and thermochemical parameters show that both the initial reactions are more feasible on silicate clusters. Thus, silicates can act as chemical sinks for trapping of hydrofluoroolefins (HFOs). It is found that both gas-phase and heterogeneous reactions are responsible for the transformation of HFOs into fluorinated compounds in the atmosphere. Further, the results show that the ozone creation potential of HFO-1234zc is low, and few of the products are harmful to aquatic organisms. This study provides new insights on the formation of toxic pollutants from the oxidation of HFO-1234zc, which may have significant implications in the troposphere.

Reaction of •OH with CHCl=CH-CHF2 and its atmospheric implication for future environmental-friendly refrigerant

In order to understand the atmospheric implication of the chlorinated hydrofluoroolefin (HFO), the geometrical structures and the IR absorption cross sections of the stereoisomers 1-chloro-3,3-difluoropropene were studied using the B3LYP/6-31G(3df) and M06-2X/6-31G(3df) methods in the gas phase. The cis-trans isomerization was assessed using the M06-2X/6-311++G(3df,p)//6-31+G(3df,p) method. The latter method was also employed for thermochemistry and the rate coefficients of the reactions of •OH with the cis- and trans-isomers in the temperature ranging from 200 to 400 K. The computational method CCSD/cc-pVTZ//M06-2X/6-31+G(3df,p) was used to benchmark the rate coefficients. It turns out that, the trans-isomer is more stable than cis-isomer and the trans- to cis-isomerization is thermodynamically unfavorable. The rate coefficient follows the Gaussian law with respect to the inverse of temperature. At the global temperature of stratosphere, the calculated rate coefficients served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP). This yielded lifetimes of 4.31 and 7.31 days and POCPs of 3.80 and 2.23 for the cis- and trans-isomer, respectively. The radiative forcing efficiencies gave 0.0082 and 0.0152 W m−2 ppb−1 for the cis- and trans-isomer, respectively. The global warming potential approached zero for both stereoisomers at 20, 100, and 500 years time horizons.

Theoretical investigation of the atmospheric implication for the reaction of •OH radical with CF2C(CH3)-CX3, X = H, F

The atmospheric implication of the hydrofluoroolefines (HFO) CF₂C(CH₃)-CX₃, X=(H,F), through the reactions with ^•OH radical were assessed using the M06-2X/6-311++G(df,p)//6-31+G(df,p) method. The rate coefficient was calculated over the temperature range 200-500 K, and was accurately expressed in non-Arrhenius form exp(a+bT^-1+cT^-2) cm³molecule^-1s^-1, where a, b, and c are real constants. This served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP), which yielded lifetimes of 0.39 and 3.53 days and POCPs of 51.00 and 3.57 for X = H and F, respectively. The radiative forcing efficiencies (RFEs) were also estimated at G96LYP/6-311G(df,p) along with the global warming potentials (GWPs). The results showed negligible impact towards global warming for the HFOs.

FTIR product study of the Cl-initiated oxidation products of CFC replacements: (E/Z)-1,2,3,3,3-pentafluoropropene and hexafluoroisobutylene

A product study of the reactions of (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) and hexafluoroisobutylene ((CF3)2C[double bond, length as m-dash]CH2) initiated by Cl atoms were developed at 298 ± 2 K and atmospheric pressure. The experiments were carried out in a 1080 L quartz-glass environmental chamber coupled via in situ FTIR spectroscopy to monitor the reactants and products. The main products observed and their yields were as follows: CF3C(O)F (106 ± 9)% with HC(O)F (100 ± 8)% as a co-product for (E/Z)-CF3CF[double bond, length as m-dash]CHF, and CF3C(O)CF3 (94 ± 5)% with HC(O)Cl (90 ± 7)% as a co-product for (CF3)2C[double bond, length as m-dash]CH2. Atmospheric implications of the end-product degradation are assessed in terms of their impact on ecosystems to help environmental policymakers consider HFOs as acceptable replacements.

Insight of UV-vis spectra and atmospheric implication for the reaction of ˙OH radical towards glyphosate herbicide and its hydrates

The rate constant of the reactions of ˙OH radicals with glyphosate (GPS) and its hydrates (GPS(H₂O)_n=1–3) were evaluated using the dual method M06-2X/6-311++G(df,p)//6-31+G(df,p) over the temperature range of 200–400 K. The results served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP). The calculations yielded an atmospheric lifetime of 2.34 hours and a POCP of 24.7 for GPS. Upon addition of water molecules, there is an increase of lifetime and decrease of POCP for water monomer and water dimer. The POCP for water trimer is slightly above the gaseous GPS. However, the POCPs of GPS and its hydrates are comparable to that of alkanes. The GPS and its hydrates were found to be a potential reservoir of CO₂. The acidification potential (AP) of GPS was found to be 0.189 and decreases upon addition of water molecules. This shows negligible contribution to rain acidification as the AP is less than that of SO₂. The UV-vis spectra were attained using the M06-L/6-311++G(3df,3pd) method and cover the range 160–260 nm which fits well with experiment.

The rate constant of the reactions of ˙OH radical with glyphosate (GPS) and its hydrates (GPS(H₂O)_n=1–3) were evaluated using the dual method M06-2X/6-311++G(df,p)//6-31+G(df,p) over the temperature range of 200–400 K.