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

Abstract

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).