UNIMOLECULAR DECOMPOSITION OF T-C4H9 RADICAL

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 (712-779K) and bath gas density ((3-22) X 10(16) 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 (k(-1)(infinity)(t-C4H9 --> i-C4H8 + H) = 2.18 X 10(9)T(1.48) exp(-18120 K/T) s(-1)) and the reverse reaction (k(-1)(infinity)(H + i-C4H8 --> t-C4H9) = 1.03 X 10(-11)T(0.25) exp(-737 K/T) cm(3) 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 Delta H-f degrees(298)(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 of (Delta E)(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 N-2 is provided using the modified Lindemann-Hinshelwood expression.