T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U.S . Patent Office 0 Copyright, 1975, by the American Chemical Society
SEPTEMBER 25, 1975
VOLUME 79, NUMBER 20
Chemically Activated 14CH3CF3from Cross Combination of 14CH3with CF3. An Introductory Experimental Study R. R. Pettljohn, G. W. Mulch, and J. W. Root’ Department of Chemistry, University of California,Davis, California 956 16 (Received July 25, 1974; Revised Manuscript Received May 28, 1975) Publication costs assisted by the U.S. Air Force Office of Scientific Research
The cophotolysis of acetone-l,3-14C-hexafluoroacetonemixtures has been used in a study of the unimolecular behavior of chemically activated 14CH3CF3. Conclusive evidence was obtained for the spurious loss of the unimolecular decomposition product 14CHz=CFz through secondary free-radical addition reactions, and a kinetic procedure was developed for controlling this nuisance problem. Experiments carried out in several bath gases suggested that these l4CH2=CF2 losses could be rendered negligible through the addition to the reaction mechanism of a rapid hydrogen abstraction or olefin addition channel involving CF3 radicals.
Introduction During the past decade CH3CF3 has been utilized in a variety of experimental studies in unimolecular The discovery that chemically activated fluorinated ethanes decompose through molecular HF elimination was made by Whittle and coworker^:^ CH3
+ CF3
-+
(CH3CF3)t
(1)
in which the superscript dagger denotes excess internal energy. Provided that the excitation level is sufficient, the activated molecule can undergo unirnolecular reaction (D) vs. collisional stabilization (S): D
(CH3CF3)t---* CHz-CFz (CH3CF3)”+ M
S -+
+ HF
(2)
+M
(3)
CH3CF3
Here the excitation energy has been shown to fall below the reaction threshold in a single “strong” collision with M, a molecule of the bath gas. This “unit deactivation” behavior is often rather well approximated by large efficient colliders in the high-pressure part of the decomposition vs. stabilization competition. As the pressure is reduced deviations from unit deactivation become more apparent and are due to a cascading energy transfer ~ e q u e n c e . For ~ . ~ ineffi-
cient colliders many step deactivation is observed at all pressures. The pioneering research on the theoretical description of unimolecular hydrogen halide elimination has been carried out at the laboratories of Setser’.la In addition to the early work with chloro and bromo alkanes several fluorinated ethanes have also been investigated, and the threshold energy for reaction 2 has been determined as 68 f 2 kcal m01-l.~ Other pyrolysis6 and chemical activation experim e n t ~have ~ been carried out on CH3CF3, and the related Arrhenius parameters and decomposition thermochemistry have been thoroughly characteri~ed.5~J3.~ Berry and Pimentel and Polanyi et al. employed chemical laser techniques to determine the internal energy content of HF produced via reaction 1.l0 Excited vibrational states up to the fourth are populated by reaction 2, but large levels of rotational excitation are apparently not involved. Another series of non-Boltzmann experiments using CH3CF3 has been reported by Root and coworkers, who investigated the unimolecular behavior of species formed in high-energy atomic substitution reactions:lI
-
+ (CH3CF2’’F)t + F
3H* -I-C H ~ C F B (CHz3HCF3)t H lSF*
+ CH3CF:
