Temperature-dependent structure-activity relationship of OH + haloalkene rate coefficients under atmospheric conditions and supporting measurements
Résumé
Hydrofluoroolefins (HFOs) and related haloalkenes are a family of compounds, primarily man-made, with many industrial applications. Gas phase electrophilic addition of OH to the olefinic bond represents the primary sink for these chemicals in the atmosphere. The degree and type of halogenation strongly affect their chemical reactivity, leading to differing reactivities with the OH radical that have presented a challenge for structure-activity relationships (SARs). Here, we investigate and extend the SARs to estimate temperature-dependent OH reaction rate coefficients, k(T), at tropospheric temperatures.
We considered two techniques: the group additivity approach of Atkinson and co-workers and the recent method of Tokuhashi and co-workers; we found the latter to make superior predictions for halogenated olefins. We extended Tokuhashi et al.’s SAR to include more olefins and to predict temperature dependencies.
We compared SAR predictions against new absolute k(T) for two HFOs (3,3,3 trifluoropropene and 1,1,3,3 tetrafluoropropene) measured using the pulsed laser photolysis–laser-induced fluorescence technique from 212 to 373K. The Arrhenius expressions were determined as kOH+CF3CH=CH2(T) = (8.86±0.82)×10 13exp[(159±26)/T] and kOH+CF2HCH=CF2(T) = (7.46±0.34)×10 13exp[(365±12)/T]. The measured k(T) was predicted accurately between 200 and 400K using the modified Tokuhashi approach. Our new measurements of the OH reaction rate coefficient with 3,3,3 trifluoropropene were in excellent agreement with literature determinations. We represent the first reported k(T) for 1,1,3,3 tetrafluoropropene.
Given that new HFOs enter the market frequently, such estimation techniques may be a helpful screening tool for assessing their environmental impact before they are examined further and reach the mass–production phase.
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