Cycloaddition Reactions of CF with Unsaturated Hydrocarbons. Correlation of Activation Energies and Frequency Factors with the Hydrocarbon Ionization Energies

Abstract

Absolute rate coefficients of the elementary reactions of CF(X2Π,v=0) with a series of unsaturated hydrocarbons have been measured for the first time over an extended temperature range. The rate coefficients of the CF reactions with ethene, propene, isobutene and 2,3-dimethyl-2-butene, as well as with allene, 1,3-butadiene, isoprene, and propyne, have been determined at temperatures between 294 and 455 K and at pressures in the 2−10 Torr range, using pulse laser photolysis−laser-induced fluorescence (PLP-LIF) techniques. The rate constants of the reactions of CF with 1-butene, cis-2-butene, and trans-2-butene have been determined at 294 K. The CF(X2Π) radicals were generated by excimer laser multiphoton dissociation of CF2Br2 at 248 nm. Pseudo-first-order exponential decays of CF(X,v=0) were monitored by LIF (excitation at the P11 band head of the A2Σ+, v‘ = 1 ← X2Π, v‘‘ = 0 transition) at varying concentrations of the coreactant, always in very large excess over CF. Pressure effects were not observed. The rate coefficients, in units of cm3 molecule-1 s-1, can be expressed by the following Arrhenius equations: k(CF+C2H4) = (8 ± 1) × 10-13 exp{(−700 ± 96)/T}, k(CF+C3H6) = (9.1 ± 0.9) × 10-13 exp{(−80 ± 20)/T}, k(CF+i-C4H8) = (1.1 ± 0.1) × 10-12 exp{(400 ± 50)/T}, k(CF+2,3-dimethyl-2-butene) = (1.6 ± 0.5) × 10-12 exp{(720 ± 95)/T}, k(CF+allene) = (3.4 ± 0.4) × 10-12 exp{(−1037 ± 100)/T}, k(CF+1,3-butadiene) = (9 ± 2) × 10-12 exp{(−560 ± 140)/T}, k(CF+isoprene) = (1.2 ± 0.2) × 10-11 exp {(−190 ± 80)/T}, and k(CF+propyne) = (3 ± 1) × 10-12 exp{(−860 ± 200)/T}. For the other butenes, the following rate constants were measured at T = 294 K: k(CF+1-butene) = (8.28 ± 0.83) × 10-12, k(CF+cis-2-butene) = (5.22 ± 0.52) × 10-12, and k(CF+trans-2-butene) = (5.56 ± 0.56) × 10-12. For the alkenes, as well as for the alkanedienes, linear correlations were observed between the Arrhenius activation energy and the ionization potential (IP) of the corresponding hydrocarbon and likewise between the Arrhenius frequency factors and the IP. Ample evidence is presented that the title reactions are electrophilic cycloaddition reactions.