Advanced Kinetic Studies of CF2(3B1) Reactions via Laser Photodissociation

1. METHODOLOGY • LITERATURE •  Information about the kinetics of CF2(3B1) is scarce. There are no results at elevated temperatures apart from the reactions of CF2(3B1) with O2 and C2F4[1] ; data have been derived from fitting to a complex model[2] or have been obtained as relative results. •  In these previous measurements the following reaction was always used as a source of CF2(3B1): • C2F4 + O(3P)  CF2(3B1) + CF2O (1a) • with k1 = 7.1 x 10-13 cm3 molecule-1 s-1 at room temperature • -There is however considerable discussion concerning the yield of CF2(3B1), estimates ranging between 1 and 80 % [3,4] • -The above mentioned formation reaction is relatively slow compared to the subsequent CF2(3B1) removal reactions, making the kinetic analysis of these reactions difficult. • NEW METHOD •  Formation of CF2(3B1), by photodissociation of C2F4 by an excimer laser pulse at 193 nm. Subsequent detection of the CF2(3B1)  CF2(1A1) luminescence[5]. Pseudo first order conditions are always maintained by ensuring that the concentration of the reaction partner is in high excess over the CF2(3B1) concentration. •  The time profiles of the luminescence can be described by a tri-exponential function, indicating that the formation process proceeds via two intermediate species. The conversion of these intermediates is fast enough to allow a straightforward absolute determination of the rate constants of subsequent gas phase reactions of CF2(3B1) with reactants (such as O2, NO and hydrocarbons) for [CF2(3B1)]- decays at larger t. For the first time temperature-dependent rate coefficients have been measured for various CF2(3B1) reactions. The temperature dependences of the CF2(3B1) reactions, for T = 288 K - 550 K, follow a simple Arrhenius form described by (in cm3 s-1 molecule-1 ): CF2(3B1)+C3H8:k(T) = (6.5 ± 2.2) × 10-13 exp[-(252 ± 130)K/T] CF2(3B1)+C3H6:k(T) = (7.8 ± 2.3) × 10-13 exp[+(500 ± 100)K/T] CF2(3B1)+iso-C4H8:k(T) = (1.4 ± 0.2) × 10-12 exp[+(762 ± 36)K/T] CF2(3B1)+C3H4:k(T) = (2.1 ± 0.5) × 10-12 exp[+(790 ± 83)K/T] The Arrhenius expression for k(CF2(3B1) + C3H8) is consistent with an H-atom abstraction reaction, while the rate constants for the reactions with alkenes, having a negative temperature dependence thus indicating a negligible energy barrier, are consistent with an addition-to-the-double-bond process. Theoretical calculations concerning the product distributions of the above reactions are currently undertaken. • CALCULATIONS • The reaction of CF2(3B1) with C3H6: The various stationary points along the reaction path were optimized using UMP2/6-311++G(d, p) followed by single-point energy calculations using G2M(UCC, MP2). The rate constants were calculated using conventional TST theory in which tunneling corrections were made for the abstraction channel using the un-symmetry Eckart potential. •  Plot of the temperature dependence of k(CF2(3B1) + C3H6) and its various reaction path components (abstraction and addition). The overall calculated rate constant is slightly higher than the experimental one, probably due to the overestimation of the addition rate. However the slight curvature at higher temperatures, as hinted by the experimental results, is reproduced in the calculations. The fact that the curvature seems to manifests itself at lower temperatures for the experimental values can again be attributed to an overestimation of the addition rate in the calculations. Absolute rate measurements and product distribution study of the reactions of electronically excited CF2(3B1) with several hydrocarbons Bart Dils, Raviraj M. Kulkarni, Thanh L. Nguyen, Shaun A. Carl and Jozef Peeters University of Leuven, Department of Chemistry, Belgium E-mail : Raviraj.Kulkarni@chem.kuleuven.ac.be EXPERIMENTAL SETUP Flow tubes Heating coils Excimer Laser Filter PM 193 nm RESULTS A Plot of the obtained pseudo first order rate constants k’ as a function of [X] yields a straight line with a slope that corresponds to the rate constant of the CF2(3B1) + X reaction at a given temperature, whereas the intercept corresponds with the rate of various CF2(3B1) removal processes as diffusion and reaction with C2F4 ,all constant over the series of experiments. Rate constants were obtained over a temperature range between 288 and 550K  Plot of the temperature dependence of k(CF2(3B1) + C3H8), k(CF2(3B1) + C3H6), k(CF2(3B1) + iso-C4H8) and k(CF2(3B1) + C3H4 (Allene)) and corresponding Arrhenius fits. • Published papers on the subject: • B. Dils, S.A. Carl, J. Peeters, Absolute rate coefficient of the reaction of CF2(ã3B1) with O2 between 288 K and 600 K, Phys. Chem. Chem. Phys., 5, 2376-2380, 2003 • B. Dils, R.M.I. Elsamra, J. Peeters, S.A. Carl, Absolute rate coefficients of the reactions of CF2(ã3B1) with NO and H2 between 287 and 600K, Phys. Chem. Chem. Phys., 5, 5405-5408, 2003 • B. Dils, R.M. Kulkarni, J. Peeters, S.A. Carl, Absolute rate coefficients of the reactions of CF2(ã3B1) with C3H8, C3H6, iso-C4H8 and C3H4 between 295 and 550 K, Phys. Chem. Chem. Phys., accepted for publication (2004) 1. Koda S., J. Phys. Chem., Vol. 83, No. 16, 2065-2073, 1979 2. Dodonov A.F., Zelenov V.V., Kukui A.S., Sov. J. 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