EXPERIMENTAL AND THEORETICAL-STUDY OF THE SEC-C4H9-REVERSIBLE-ARROW-CH3+C3H6 REACTION

The kinetics of the unimolecular decomposition of the sec-C4H9 radical has been studied experimentally in a heated tubular flow reactor coup led to a photoionization mass spectrometer. Rate constants for the dec omposition were determined in time-resolved experiments as a function of temperature (598-680 K) and bath gas density ((3-18) x 10(16) molec ules cm(-3)) in three bath gases: He, Ar, and Nz. The rate constants a re in the falloff region under the conditions of the experiments. The results of earlier studies of the reverse reaction were reanalyzed and used to create a transition state model of the reaction. This transit ion state model was used to obtain values of the microcanonical rate c onstants, k(E). Falloff behavior was reproduced using master equation modeling with the energy barrier height for decomposition (necessary t o calculate k(E)) obtained from optimization of the agreement between experimental and calculated rate constants. The resulting model of the reaction provides the high-pressure limit rate constants for the deco mposition reaction (k(1)(infinity)(sec-C4H9 --> C3H6 + CH3) = 2.73 x 1 0(10)T(1.11) exp(- 15712 K/T) s(-1)) and the reverse reaction (k(-1)(i nfinity)(CH3 + C3H6 --> sec-C4H9)= 2.13 x 10(-19)T(2.28) exp(- 3319 K/ T) cm(3) molecule(-1) s(-1)). Average values of [Delta E](down) = 363 (He), 447 (Ar), and 506 cm(-1) (N-2) for the average energy loss per d eactivating collision were obtained using an exponential-down model. P arametrization of the temperature and pressure dependence of the unimo lecular rate constant for the temperature range 298-1500 K and pressur es 0.001-10 atm in He, Ar, and N-2 is provided using the modified Lind emann-Hinshelwood expression.