(580f) Thermal Decomposition of Fluoroethane: A Shock Tube Time of Flight Mass Spectrometry Study

The thermal decomposition of fluoroethanes is often dominated by simple molecular HF elimination to often produce stable olefins. The stability of the products and the relative ease of determining the high pressure limiting rate coefficient have led to the use of these reactions as chemical thermometers or internal standards in singe pulse shock tube experiments. Recent studies of fluorinated alkanes at elevated temperatures have led to some interesting observations regarding unimolecular fall-off and vibrational relaxation [1-3]. In particular the pressure dependency of 1,1,1-trifluoroethane dissociation cannot be accurately described by standard RRKM calculations [1,2] although that of 1,1-difluroethane can [3]. Furthermore, the values of down that are required for RRKM calculations e.g 1,1-difluoroethane (1600 cm-1), are unusually large compared to similar hydrocarbons. It seems very unlikely that a simple insertion of fluorine atoms into ethane would produce such a large change in down. These abnormalities have stimulated a series of studies with fluorinated hydrocarbons and here we report the results of experiments with monofluoroethane. The thermal unimolecular dissociation of monofluoroethane was investigated (T=1200?1560 K, P=400-1500 Torr) behind reflected shock waves, in a recently developed shock tube coupled with a time of flight mass spectrometer. Strong falloff from the high pressure limiting rate constant [4] was observed in the pressure range studied. The experimental results have been simulated with RRKM calculations and are found to be in reasonable agreement with the theoretical prediction.

1. J. H. Kiefer, C. Katopodis, S. Santhanam, N. K. Srinivasan and R. S. Tranter, J. Phys. Chem. A, 2004, 108, 2443. 2. B. R. Giri and R. S. Tranter, J. Phys. Chem. A, 2007, 111, 1585. 3. H. Xu, J. H. Kiefer, Raghu Sivaramakrishnan, R. S. Tranter and B. R. Giri, Phys. Chem. Chem. Phys., 2007, DOI: 10.1039/b703124f. 4. K. Okada, E. Tschuikow-Roux and P. J. Evans, J. Phys. Chem. 1980, 84, 467.

This work was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract No. W-31-109-Eng-38