Unimolecular Decomposition of n-C₄H₉ and iso-C₄H₉ Radicals

Abstract

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 〈∆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 (k 1 ∞ (n-C 4 H 9 f C 2 H 5 + C 2 H 4 ) ) 1.06 × 10 13 exp(-14005 K/T), k 2 ∞ (iso-C 4 H 9 f CH 3 + C 3 H 6 ) ) 2.14 × 10 12 T 0.65 exp(-15529 K/T) s -1 ) and the reverse reactions (k -1 ∞ (C 2 H 5 + C 2 H 4 f n-C 4 H 9 ) ) 6.59 × 10 -21 T 2.44 exp(-2697 K/T), k -2 ∞ (CH 3 + C 3 H 6 f 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.X Abstract published in AdVance ACS Abstracts, February 15, 1996.n-C 4 H 9 a C 2 H 5 + C 2 H 4 (1,-1) 5318

Multichannel decomposition and isomerization of octyl radicals

“…The range covered is from 0.1-100 bar and 500-1900 K. The departures from high pressure values are expressed as log [k 1 /k] and for the 1-octyl radical are summarized in Table 2. The step size down values in wave numbers 0.3 Â T [19] where T the temperature, is the same as that used in the previous studies [5,6]. Similar tables for the other three octyl radicals can be found in the supplementary material.…”

Computational studies on the reactions of 3‐butenyl and 3‐butenylperoxy radicals

“…Figure 5a shows the unimolecular rate coefficients for butyl radical at pressure of 10 atm (N 2 ) and highpressure limiting (HPL) condition. The C C bond fission was confirmed to be predominant over the entire temperature range, 500-2000 K. The estimated rate coefficients for two major channels, * cc and * ch, in the pressure range 1-100 atm are compared with the recent evaluation by Curran [29] based on available experimental data [30] and thermochemical estimates. The present estimate, even without any adjustment, agreed well with the evaluation at relatively low temperature and at high pressures.…”

Thermal rate constants of the pyrolysis of n-Heptane

“…exp (À2697 k/T) cm 3 molecule À1 s À1 at T = 298-1500 K [45]. Some of our calculated results with the RRKM and master equation mode described above along with selected data from experiments and the recommendations are plotted in Fig.…”