Unimolecular Decomposition of t-C4H9 Radical

Article abstract of DOI:10.1021/j100071a018

The kinetics of the unimolecular decomposition of the t-C4H9 radical has been studied.The reaction was isolated for quantitative study 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 (712 - 779 K) and bath gas density ((3-22) x 10¹⁶ molecules cm^-3) in He.The rate constants are in the falloff region under the conditions of the experiments.A transition-state model was created to obtain values of the microcanonical rate constants, k(E), needed to solve the master equation.The transition-state model provides the high-pressure limit rate constants for the decomposition reaction (k₁ <*>(t-C4H9 -> i-C4H8 + H) = 2.18 x 10⁹T^1.48exp(-18120 K/T) s^-1) and the reverse reaction (k-1^<*>(H = i-C4H8 -> t-C4H9) = 1.03 x 10^-11T^0.25exp(-737 K/T) cm³ molecule^-1 s^-1).The results of earlier studies of reactions of decomposition of t-C4H9 and the reverse reaction which reported the value of the enthalpy of formation of t-C4H9 were reanalyzed, and the resultant value of ΔH_f⁰₂₉₈(t-C4H9) is in agreement with the most current measurements.The falloff behavior was analyzed using a master equation approach.The hindered internal rotations of methyl groups in the involved molecules were treated classically.Simple formulas for the density-of-states and sum-of-states functions of a one-dimensional hindered rotor based on the inverse Laplace transform of the classical partition function are reported.The average value <ΔE>_down = 230 cm^-1 for the energy loss probability was obtained using the exponential-down model.Parametrization of the temperature and pressure dependence of the unimolecular rate constant for the temperature range 298 - 1500 K and pressures 0.001 - 10 atm in He and N2 is provided using the modified Lindemann-Hinshelwood expression.