EXPERIMENTAL AND THEORETICAL-STUDY OF THE C2H3-REVERSIBLE-ARROW-H- TUNNELING AND THE SHAPE OF FALLOFF CURVES(C2H2 REACTION )

The kinetics of the unimolecular decomposition of the C2H3 radical has been studied. The reaction was isolated for quantitative study in a h eated tubular flow reactor coupled to a photoionization mass spectrome ter. Rate constants for the decomposition were determined in time-reso lved experiments as a function of temperature (879-1058 K) and bath ga s density ((6-48) x 10(16) molecules cm(-3)) in He, Ar, and N-2. The r ate constants are close to the low-pressure limit under the conditions of the experiments. The potential energy surface and properties of th e transition state were studied by ab initio methods. Experimental res ults of the current and earlier studies of both the direct and reverse reactions were analyzed and used to create a transition state model o f the reaction. Falloff behavior was reproduced using master equation modeling with parameters obtained from optimization of the agreement b etween experimental and calculated rate constants. The effects of tunn eling on the shape of falloff and the values of the low-pressure-limit rate constants were investigated. It was demonstrated that these effe cts exceed those for the high-pressure-limit rate constants by orders of magnitude and cannot be neglected. The resulting model of the react ion provides the high-pressure-limit rate constants for the decomposit ion 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 exp onential-down model, Parametrization of the temperature and pressure d ependence 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 t he modified Lindemann-Hinshelwood expression.