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Issue 31, 2007
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Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

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Abstract

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500–2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late observations. We thus include a laser schlieren study (1717–2332 K, 75 < P < 482 torr in 10 and 4% vinyl fluoride in Kr) of this dissociation. This latter work also includes a set of experiments using shock-tube time-of-flight mass spectrometry (4% vinyl fluoride in neon, 1500–1980 K, 500 < P < 1300 torr). These time-of-flight experiments confirm the theoretical expectation that the only reaction in vinyl fluoride is HF elimination. The dissociation experiments are augmented by laser schlieren measurements of vibrational relaxation (1–20% C2H3F in Kr, 415–1975 K, 5 < P < 50 torr, and 2 and 5% C2H4F2 in Kr, 700–1350 K, 6 < P < 22 torr). These experiments exhibit very rapid relaxation, and incubation delays should be negligible in dissociation. An RRKM model of dissociation in 1,1-difluoroethane based on a G3B3 calculation of barrier and other properties fits the experiments but requires a very large 〈ΔEdown of 1600 cm−1, similar to that found in a previous examination of 1,1,1-trifluoroethane. Dissociation of vinyl fluoride is complicated by the presence of two parallel HF eliminations, both three-center and four-center. Structure calculations find nearly equal barriers for these, and TST calculations show almost identical k. An RRKM fit to the observed falloff again requires an unusually large 〈ΔEdown and the experiments actually support a slightly reduced barrier. These large energy-transfer parameters now seem routine in these large fluorinated species. It is perhaps a surprising result for which there is as yet no explanation.

Graphical abstract: Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

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Publication details

The article was received on 01 Mar 2007, accepted on 16 Apr 2007 and first published on 24 May 2007


Article type: Paper
DOI: 10.1039/B703124F
Phys. Chem. Chem. Phys., 2007,9, 4164-4176

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    Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

    H. Xu, J. H. Kiefer, R. Sivaramakrishnan, B. R. Giri and R. S. Tranter, Phys. Chem. Chem. Phys., 2007, 9, 4164
    DOI: 10.1039/B703124F

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