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HCFC-141b; decomposition of HCFC; PES; canonical transition state theory.

The present study deals with the decomposition of haloalkoxy radical (CH₃CFClO) formed from 1,1-dichloro-1-fluoroethane (HCFC-141b) in the atmosphere. The sudy is performed using ab-initio quantum mechanical methods. Out of the three plausible pathways of decomposition of the titled species, the one that involved the C-C bond scission and the other occurring via Cl-atom elimination have been considered for detailed study. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at MP2 level of theory using 6-311G(d,p) basis set. Single point energy calculations have been performed at G2(MP2) level of theory. The path involving the Cl-elimination is found to be dominant and found to occur with a barrier height of 3.6 kcal mol^-1 whereas the C-C bond scission path proceeds with a barrier of 10.0 kcal mol^-1. The thermal rate constants for the above two decomposition pathways are evaluated using Canonical Transition State Theory (CTST) and these are found to be $2.9 \times 10^8 s^{−1}$ and $4.3 \times 10^5 s^{−1}$ for Cl-elimination and C-C bond scission respectively at 298 K and 1 atm. pressure. The existence of transition states on the corresponding potential energy surface is ascertained by the occurrence of only one imaginary frequency obtained during the frequency calculation. The Intrinsic Reaction Coordinate (IRC) calculation has also been performed to confirm the smooth connection of the TS to the reactant and the products.

Hari Ji Singh¹ Bhupesh Kumar Mishra¹

1. Department of Chemistry, DDU Gorakhpur University, Gorakhpur, 273 009, India

Published

September 2011

Manuscript received

23 September 2009

Manuscript revised

30 March 2011

Accepted

13 June 2011