Experimental and theoretical study of the C2H3 reversible arrow H+C2H2 reaction. Tunneling and the shape of falloff curves

The kinetics of the unimolecular decomposition of the C2H3 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 (879-1058 K) and bath gas density ((6-48) x 10(16) molecules cm(-3)) in He, Ar, and N-2. The rate constants are close to the low-pressure limit under the conditions of the experiments. The potential energy surface and properties of the transition state were studied by ab initio methods. Experimental results of the current and earlier studies of both the direct and reverse reactions were analyzed and used to create a transition state model of the reaction. Falloff behavior was reproduced using master equation modeling with parameters obtained from optimization of the agreement between experimental and calculated rate constants. The effects of tunneling on the shape of falloff and the values of the low-pressure-limit rate constants were investigated. It was demonstrated that these effects exceed those for the high-pressure-limit rate constants by orders of magnitude and cannot be neglected. The resulting model of the reaction provides the high-pressure-limit rate constants for the decomposition reaction (k(1)(infinity)(C2H3-->H+C2H2) = 3.86 X 10(8)T(1.62) exp(-18650 K/T) s(-1)) and the reverse reaction (k(-1)(infinity)(H+C2H2-->C2H3) = 6.04 x 10(-14)T(1.09) exp(-1328 K/T) cm(3) molecule(-1) s(-1)). Values of [Delta E](all) = -74 (He), -115 (Ar), and -117 cm(-1) (N-2) for the average energy loss per collision were obtained using an exponential-down model, Parametrization of the temperature and pressure dependence of the unimolecular rate constant for the temperature range 200-2120 K and pressures 1-10(4) Torr in He and Nz is provided using the modified Lindemann-Hinshelwood expression.