Abstract |
In this MSc work, the rate coefficients for the reactions of Cl atoms and OH radicals with
trifluoroethylene, (HFO-1123) (1), and the (E)- and (Z)-isomers of 1,2-difluoroethylene (HFO1132(E)/(Z)) (2), (3) and (5), (6), were measured, in the gas phase, as a function of
temperature, 223 – 363 K and pressure, 30 – 700 Torr, employing the Thermostated
PhotoChemical Reaction technique (TPCR/FTIR) and relative rate methods (RR). The endoxidation products for the atmospheric degradation of the three HFO were also identified
using infrared spectroscopy and the product-yields were measured, where possible. HFO are
the most recently proposed hydrofluorocarbons (HFC) alternatives and the first generation
of substitutes that contain unsaturated bonds in their molecule, so as to ensure their high
chemical reactivity. The experimental results were combined with quantum-mechanical
molecular calculations in an attempt to further investigate the impact of the degree and
positioning of fluorination on hydrofluoroethylenes (the simplest HFO series, C2-HFO)
reactivity. The physical interpretation of the observed behaviour was also explored.
The kinetic measurements revealed that the rate coefficients, k(T,P), of the studied
reactions, exhibit a negative temperature dependence (Ea < 0), while the corresponding ones
for the Cl atom reactions appeared to be also pressure dependent. The latter observations
are consistent with electrophilic association mechanism that proceeds via a rovibrationally
excited intermediate adduct. Cl kinetics experimental results were very well represented by
Troe expression that models complex reactions proceeding via an intermediate adduct
formation. From Troe’s expression, the fall-off parameters, at the zero- and infinite-pressure
limits were obtained, using the NASA/JPL data evaluation panel recommendation for the
collision broadening factor, Fc = 0.6. As far as the kinetics of OH radicals with the two
isomers are concerned, the reaction rate coefficients are well represented with Arrhenius
expressions.
Regarding the end-oxidation products, the following ones were identified for each reaction:
monofluoroformaldehyde (HC(O)F) and fluoroformaldehyde (FC(O)F), in reaction (1) and
monofluoroformaldehyde (HC(O)F) in reactions (2)-(6).
To further investigate the reactions mechanisms, key thermodynamic and kinetic
parameters were calculated using Gaussian16 program suite, aiming to evaluate the
intermediate adducts relative stability and the potent interconversion at CCSD(T)/AUG-ccpVDz//B3LYP/6-311++G(2df,2p) level of theory. Finally, the experimental results were used
to obtain critical environmental metrics, namely the Radiative Efficiency (RE) and the Global
Warming Potential (GWP), in order to assess the impact of the HFO series on air quality and
climate.
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