(580d) Shock Tube Study of Dissociation and Relaxation in 1,1-Difluoroethane and Vinyl Fluoride

The decomposition of the fluorinated ethanes is often
dominated by simple molecular HF elimination. As such this reaction, with just
one channel and often stable products, is ideal for investigation by
non-specific methods, i.e., those that do not provide an actual product
analysis, such as the shock tube laser-schlieren (ST/LS) technique in shock
waves. Previously we used this to explore such HF elimination in
1,1,1-trifluoroethane[1] at very high temperatures (1600-2400K), and found an anomalous falloff suggesting a possible non-RRKM process. A ?failure' of statistical theory in a simple thermal reaction such as the HF elimination that occurs in 1,1,1-trifluoroethane is certainly surprising and unprecedented, although the possibility of such an occurrence, perhaps through slow IVR, has been well-recognized and extensively discussed since the theory's inception.[2-4] In fact, this result has now led to some theoretical studies that seriously question the interpretation[5, 6] of the original ST/LS results. In any case, it seems that further study of this and related reactions is strongly indicated. One possible experimental approach to further understanding might then be through an investigation of HF elimination from other partially fluorinated ethanes. This is the approach used here.

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, 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
<ΔE>_down of 1600cm^-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 near equal barriers
for these, and TST calculations show almost identical k_∞. An
RRKM fit to the observed falloff again requires an unusually large <ΔE>_down. These surprisingly large
energy-transferparameters now
seem routine in these large fluorinated species.

References:

1.         Kiefer, J.H., et al., A Shock-Tube, Laser-Schlieren
Study of the Dissociation of 1,1,1-Trifluoroethane: An Intrinsic Non-RRKM
Process. Journal of Physical Chemistry A, 2004. 108(13): p.
2443-2450.

2.         Bunker, D.L., Theory of elementary gas
reaction rates. The International encyclopedia of physical chemistry and
chemical physics. Topic 19: Gas kinetics, ed. A.F. Trotman-Dickenson. Vol. 1.
1966, Oxford, New York: Pergamon Press Ltd.

3.         Forst, W., Theory of unimolecular reactions.
Physical chemistry, a series of monographs. Vol. 30. 1973, New York: Academic
Press.

4.         Bunker, D.L. and W.L. Hase, On non-RRKM unimolecular
kinetics: Molecules in general, and CH₃NC in particular. The
Journal of Chemical Physics, 1973 59(9): p. 4621-4632

5.         Barker, J.R., et al., CF3CH3 -> HF +
CF2CH2: A Non-RRKM Reaction? Journal of Physical Chemistry A, 2006. 110(9):
p. 2944-2954.

6.         Stimac, P.J. and J.R. Barker, Intramolecular Vibrational
Energy Redistribution Involving the Torsion in CF3CH3: A Molecular Dynamics
Study. Journal of Physical Chemistry A, 2006. 110(21): p. 6851-6859.