ABSOLUTE RATE-CONSTANT MEASUREMENTS OF CF(X2-PI) REACTIONS .1. REACTIONS WITH O2, F2, CL2, AND NO

Rate coefficients of the elementary reactions of CF(X2II, upsilon = 0) with O2, F2, Cl2, and NO at T = 294 K and p = 2-10 Torr (He or Ar bath gas) have been determined for the first time, using laser photodissociation/laser induced fluorescence (LIF) techniques. The CF(X2II) radicals were generated by excimer laser photolysis of CF2Br2 at 248 nm. Exponential decays of CF(X, upsilon = 0) were monitored by LIF (A(2)SIGMA+, upsilon' = 1 <-- X2II, upsilon" = 0 transition) at varying concentrations of the reactant, in very large excess over CF. The rate constant for each of the title reactions was found to be independent of pressure in the 2-10-Torr range: k(O2) = (1.6 +/- 0.2) x 10(-12); k(F2) = (3.9 +/- 0.4) x 10(-12); k(Cl2) = (1.7 +/- 0.2) x 10(-11), and k(NO) = (2.1 +/- 0.2) x 10(-11) cm3 molecule-1 s-1. Besides, an upper limit k less-than-or-equal-to 10(-14) was established for the reactions of CF(X) with H2, CH4, C2H2, CF4, and CO2. In general, the rate coefficients obtained for the investigated CF(X2II) reactions are close to literature data on CCl and CBr analogues but far below reported rate constants of corresponding CH(X2II) processes, specifically regarding reactions with closed-shell molecules. The exceptional stability and concomitantly low reactivity of CF-relative to CH-can be attributed to strong back-donation from one of the fluorine 2p-pi orbitals into the vacant carbon 2p-pi orbital.