Unimolecular Decomposition of n-C4H9 and iso-C4H9 Radicals

The kinetics of the unimolecular decomposition of the n -C 4 H 9 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) × 10 16 molecules cm -3 ) in two bath gases, He and N 2 . 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 〈Δ E 〉 all = −28 cm -1 (He) and −40 cm -1 (N 2 ) 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 -C 4 H 9 → C 2 H 5 + C 2 H 4 ) = 1.06 × 10 13 exp(−14005 K/ T ), ( iso -C 4 H 9 → CH 3 + C 3 H 6 ) = 2.14 × 10 12 T 0.65 exp(−15529 K/ T ) s -1 ) and the reverse reactions ( (C 2 H 5 + C 2 H 4 → n -C 4 H 9 ) = 6.59 × 10 -21 T 2.44 exp(−2697 K/ T ), (CH 3 + C 3 H 6 → iso -C 4 H 9 ) = 1.66 × 10 -20 T 2.57 exp(−3879 K/ T ) cm 3 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 N 2 is provided using the modified Lindemann−Hinshelwood expression.