Unimolecular Decomposition of n-C4H9 and iso-C4H9 Radicals

Abstract

The kinetics of the unimolecular decomposition of the n-C4H9 radical has been studied experimentally in a heated tubular flow reactor coupled to a photoionization mass spectrometer. Rate constants for the decomposition were determined in time-resolved experiments as a function of temperature (560−620 K) and bath gas density ((3−18) × 1016 molecules cm-3) in two bath gases, He and N2. The rate constants are in the falloff region under the conditions of the experiments. Structures, vibrational frequencies, and barriers for internal rotations of n-butyl and iso-butyl radicals and their decomposition transition states were obtained by ab initio calculations using UHF/6-31G* and MP2/6-31G* methods. The results of ab initio calculation, together with the reanalysis of earlier studies of the reverse reactions, were used to create transition-state models of the reactions of unimolecular decomposition of n-butyl (1) and iso-butyl (2) radicals. Falloff behavior of reaction 1 was reproduced using master equation modeling with the energy barrier height for decomposition obtained from optimization of the agreement between experimental and calculated rate constants. The values of 〈ΔE〉all = −28 cm-1 (He) and −40 cm-1 (N2) for the average energy loss per collision were obtained using an exponential-down model. The resulting models of the reactions provide the high-pressure limit rate constants for the decomposition reactions ( (n-C4H9 → C2H5 + C2H4) = 1.06 × 1013 exp(−14005 K/T), (iso-C4H9 → CH3 + C3H6) = 2.14 × 1012T0.65 exp(−15529 K/T) s-1) and the reverse reactions ( (C2H5 + C2H4 → n-C4H9) = 6.59 × 10-21T2.44 exp(−2697 K/T), (CH3 + C3H6 → iso-C4H9) = 1.66 × 10-20T2.57 exp(−3879 K/T) cm3 molecule-1 s-1). Parametrization of the temperature and pressure dependence of the unimolecular rate constants for the temperature range 298−900 K and pressures 0.001−10 atm in He and N2 is provided using the modified Lindemann−Hinshelwood expression.