Issue 21, 2009

Absolute rate coefficients over extended temperature ranges and mechanisms of the CF(X2Π) reactions with F2, Cl2 and O2

Abstract

The absolute rate coefficients of the reactions of the carbyne-radical CF(X2Π, ν = 0) with O2, F2 and Cl2 have been measured over extended temperature ranges, using pulsed-laser photodissociation–laser-induced fluorescence (PLPLIF) techniques. The CF(X2Π) radicals were generated by KrF excimer laser 2-photon photolysis of CF2Br2 at 248 nm and the real-time exponential decays of CF(X2Π, ν = 0) at varying coreactant concentrations, in large excess, were monitored by LIF (A2Σ+, ν′ = 1 ← X2Π, ν″ = 0 transition). The experimental bimolecular rate coefficients of the CF(X2Π) reactions with F2 and Cl2 can be described by simple Arrhenius expressions: kF2(295–408 K) = (1.5 ± 0.2) × 10−11 exp[−(370 ± 40)K/T] cm3 molecule−1 s−1; and kCl2(295–392 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 CBS-Q//BHandHLYP/6-311G(d,p) study confirms that the ground state reactants CF(X2Π) + F2(X1Σ) connect directly with the ground-state products CF2(X1A1) + F(2P)via a nearly barrierless F-atom abstraction route. The rate coefficient of CF(X2Π) + O2 can be represented by a two-term Arrhenius expression: kO2(258–780 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 dominates at higher temperatures T and the second at lower T where a negative temperature dependence is observed (<290 K). Quantum chemical computations 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 rate-determining mechanism from a carbyne-type insertion into the O–O bond at high T to a radical–radical combination at low T.

Graphical abstract: Absolute rate coefficients over extended temperature ranges and mechanisms of the CF(X2Π) reactions with F2, Cl2 and O2

Article information

Article type
Paper
Submitted
11 Nov 2008
Accepted
20 Feb 2009
First published
23 Mar 2009

Phys. Chem. Chem. Phys., 2009,11, 4319-4325

Absolute rate coefficients over extended temperature ranges and mechanisms of the CF(X2Π) reactions with F2, Cl2 and O2

B. Vetters, B. Dils, T. L. Nguyen, L. Vereecken, S. A. Carl and J. Peeters, Phys. Chem. Chem. Phys., 2009, 11, 4319 DOI: 10.1039/B819984A

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