Absolute Rate Coefficients over Extended Temperature Ranges and Mechanisms of the CF(X-2 Pi) Reactions with F-2, Cl-2 and O-2

The absolute rate coefficients of the reactions of the carbyne-radical CF(X2P, v=0) with O2, F2 and Cl2 have been measured over extended temperature ranges, using pulsed laser photodissociation / laser -induced fluorescence (PLP-LIF) techniques. The CF(X2P) radicals were generated by KrF excimer laser 2-photon photolysis of CF2Br2 at 248 nm and the real-time exponential decays of CF(X2P, u=0) at varying coreactant concentrations, in large excess, were monitored by LIF (A2S+, u’=1 ¬ X2P, u”=0 transition). The experimental bimolecular rate coefficients of the CF(X 2P) reactions with F2 and C l2 can be described by simple Arrhenius expressions : k F2(295 K - 408 K) = (1.5 ± 0.2) ´ 10-11 exp[-(370 ± 40)K/T] cm3 molecule-1 s-1; and kCl2(295 K – 392 K) = (6.1 ± 2.1) ´ 10-12 exp[+(280 ± 120)K/T]. The kF2(T) and kCl2(T) results can be rationalized in terms of direct halogen-atom abstraction reactions in which the radical character of CF dominates; a quantum chemical CBSQ// BHandHLYP/6-311G(d,p) study confirms that the ground state reactants CF(X2P) + F2(X1S) connect directly with the ground state products CF2(X1A1) + F(²P) via a nearly barrierless F-atom abstraction route. The rate coefficient of CF(X2P) + O2 can be represented by a two-term Arrhenius expression: kO2(258 K - 780 K) = 1.1 ´ 10-11 exp(-850 K/T) + 2.3 ´ 10-13 exp(500 K/T), with a standard deviation of 5%. The first term dominat es at higher temperatures T and the second at lower T where a negative temperature dependence is observed (<290 K). Quantum chemical comput ations at the CBS-QB3 and CCSD(T)/aug-cc-pVDZ levels of theory show that the kO2(T) behaviour is consistent with a change of the dominant ratedetermining mechanism from a carbyne-type insertion into the O-O bond at high T to a radical-radical combination at low T.

Publication year:2009

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Authors from:Higher Education