Low pressure rate coefficients of the reactions of O(3P) with CF3I and CH3I in the gas phase

M. TERUEL AND J.N. CROWLEY

Los Cocos, Córdoba

Conferencia; 9th Latin American Conference on Physical Organic Chemistry (CLAFQO); 2007

CLAFQO

In this work absolute rate coefficients for the gas-phase reactions of :                                               CF3I  + O(3P) à Products                                               CH3I + O(3P) à Products  have been measured at 298 K using a discharge flow tube coupled to a resonance fluorescence detection system to monitor O(3P) atoms. The experiments were carried out under pseudo-first-order conditions with [O(3P)]o << [CF3l]o or [CH3l]o . The oxygen atoms were generated by an electrodeless microwave discharge on a 2% O2/He mixture and were introduced to the main flow through a side port at the upstream end of the flow tube. The alkyl iodide was introduced into the tube at different distances from the detection system by a sliding injector. Experiments were performed under laminar flow conditions with efficient radial mixing, using Helium as carrier gas. More than 95 % of the total gas flow is given by the carrier gas, which thus determines the physical properties of the system. The concentrations of the reactants in the tube were calculated by measuring the corresponding partial flows and the total pressure. The linear flow velocities of the reactants varied from 13-16 m s-1. The concentration of O(3P) was monitored by atomic resonance fluorescence (RF). The fluorescence signal, which is proportional to the oxygen atom concentration, was detected with a photomultiplier tube, amplified and displayed on an oscilloscope. The rate constant values observed are (4.7 ± 0.7) ´ 10-12 and   (2.1 ± 0.3) ´ 10-11 cm3 molecule-1 s-1, respectively. This is the first determination of the rate constant at 298 K measured by discharge flow-resonance fluorescence technique (DF-RF) at low pressure conditions (1.6-3.3Torr) that was compared with our PLP-RF previous results as well as other determinations in the literature. The effect of substituent atoms or groups on the overall rate constants is analyzed in comparison with other alkanes in the literature. Acknowledgements We thank D. Hölscher for his assistance in the determination of the I2 impurity in the RI samples. We would like to thank the BMBF (AFO2000-AFOHAL) and the EU (Thaloz) for financial support. .