UNIMOLECULAR DECOMPOSITION OF N-C4H9 AND ISO-C4H9 RADICALS

The kinetics of the unimolecular decomposition of the n-C4H9 radical h as been studied experimentally in a heated tubular flow reactor couple d to a photoionization mass spectrometer. Rate constants for the decom position were determined in time-resolved experiments as a function of temperature (560-620 K) and bath gas density ((3-18) x 10(16) molecul es cm(-3)) in two bath gases, He and N-2. The rate constants are in th e falloff region under the conditions of the experiments. Structures, vibrational frequencies, and barriers for internal rotations of n-buty l and iso-butyl radicals and their decomposition transition states wer e obtained by ab initio calculations using UHF/6-31G and MP2/6-31G* m ethods. The results of ab initio calculation, together with the reanal ysis 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 reacti on 1 was reproduced using master equation modeling with the energy bar rier height for decomposition obtained from optimization of the agreem ent between experimental and calculated rate constants. The values of [Delta 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 mode l. The resulting models of the reactions provide the high-pressure lim it rate constants for the decomposition reactions (k(1)(infinity)(n-C4 H9 --> C2H5 + C2H4) = 1.06 x 10(13) exp(-14005 K/T), k(2)(infinity)(is o-C4H9 --> CH3 + C3H6) = 2.14 x 10(12)T(0.65) exp(-15529 K/T) s(-1)) a nd the reverse reactions (k(-1)(infinity)(C2H5 + C2H4 --> n-C4H9) = 6. 59 x 10(-21)T(2.44) exp(-2697 K/T), k(-2)(infinity)(CH3 + C3H6 --> iso -C4H9) = 1.66 x 10(-20)T(2.57) exp(-3879 K/T) cm(3) molecule(-1) s(-1) ). Parametrization of the temperature and pressure dependence of the u nimolecular rate constants for the temperature range 298-900 K and pre ssures 0.001-10 atm in He and N-2 is provided using the modified Linde mann-Hinshelwood expression.