2231
J. Phys. Chem. 1986, 90, 2231-2236 of k6 in Figure 9 indicates that in the regime of flame temperatures the values for the rate coefficient of (6) are up to a factor 5 larger than the extrapolated values from the low-temperature experiments of Michael et al.27 With a simple TST calculation we tried to support at least the tendency of the observed non-Arrhenius behavior (see Appendix B). The main purpose of the present measurements was to obtain high-temperature rate coefficients of methylene reactions which are of importance in combustion systems. The extension of this work to reactions of methylene with atomic and molecular oxygen will be presented shortly.
Acknowledgment. W e are very grateful to Dr. F. Zabel, Gesamthochschule Bielefeld, West Germany, who made available to us the original measurements from his thesis (ref 15b). We also thank Dr. F. Bachmaier, Dr. G. Kuth, and Mr. M. Kapernaum for preparing and analyzing the ketene samples. The financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Appendix A
Rate C a l c ~ l a t i o for n ~ ~the Unimolecular Decomposition of Ketene. The same nomenclature has been used as in ref 30: A = CHZCO, M = Ar; KAM = 20.48; uAM N 4.07; P A M N 193 K; A0 = 9.4 cm-', Bo = 0.32 cm-' (from ref 18); & / ( l = -( AE),,,,,/(F(Eo)RT);Eo = 86 kcal/mol for singlet Morse poE 1.2 to tential. Eo = 74 kcal/mol and (ZB*Z~')'/*/(ZB~Z~~)'/Z 1.5. I* and p refer to moments of inertia at potential maximum and ground state, respectively. Appendix
B
WT) =
(~BT/~)Q(TST)/(QHQCH,CO) ~xP(-Eo/~BT)
kBand h have the usual meaning. With the calculated partition functions Q s and a slight potential barrier Eo of 4 kcal/mol at the entrance channel for H H 2 C C 0 , we obtained k ( T ) for several temperatures. Our results could be expressed by an empirical correlation curve
+
k(T) = 4.54 x i09T'~28 exp(-159O/T)
+
TST Calculation for H CHzCO.Since the reaction of H atoms with ketene is relatively strongly exothermic
H
temperature range of 300-500 K. Since no data on possible TST structures based upon potential energy surface calculations are known, we had to estimate the relevant parameters. Such TST calculations are of limited value; however, they may serve at least to make the observed non-Arrhenius behavior plausible. The measurements of Michael and co-workersz7 and our own hightemperature data can be satisfactorily correlated by the simple TST expression for k( T ) as given below. Details of the TST Complex. Structure: OCC linear; OC: 1.2 A; CC: 1.6 A; CH3 group: one H atom, 2.1 A; the two others, 1.08 A. Frequencies (in cm-I): CO, 1950; CH, group: CH, 2950 (2); C
