Thermal decomposition of haloethanols: single pulse shock tube and ab initio studies B. Rajakumar, 1 K.P.J. Reddy 2 and E. Arunan 1, 3 1 Department of Inorganic and Physical Chemistry 2 Department of Aerospace Engineering Indian Institute of Science Bangalore, INDIA 560 012 3 Author for correspondance: arunan@ipc.iisc.ernet.in Abstract. This paper reports single pulse shock tube and ab initio studies on thermal decom- position of 2-fluoro and 2-chloroethanol at T = 1000-1200 K. Both molecules have HX (X = F/Cl) and H2O molecular elimination channels. The CH3CHO formed by HX elimination is chemically active and undergoes secondary decomposition resulting in the formation of CH4, C2H6, and C2H4. A detailed kinetic simulation indicates that the formation of C2H4 could not be quantitatively explained as arising exclusively from secondary CH3CHO decomposition. Con- tributions from primary radical processes need to be considered to explain C2H4 quantitatively. Ab initio calculations on HX and H2O elimination reactions from the haloethanols at HF, MP2, and DFT levels with various basis sets up to 6/311++G** are reported. It is pointed out that due to strong correlations between A and Ea, comparison of these two parameters between experimental and theoretical results could be misleading. 1 Introduction Recently 2-fluoroethanol has been suggested as a potential replacement for chlorofluoro- carbons [1]. There has been no experimental or theoretical investigations on its thermal stability and hence a single pulse shock tube investigation was carried out in our labora- tory. Since submitting the abstract for this conference, results from this work have been published. [2] This paper summarizes the results and also expands on the comparison of experimental and theoretical rate parameters for HF and H 2 O elimination reactions. In addition, it gives preliminary results on thermal decomposition of 2-chloroethanol. The experimental results on 2-chloroethanol show significant similarities to that of 2- fluoroethanol, suggesting that the secondary reactions involved in both systems are quite similar, wide infra. The HX (X = F/Cl) elimination reactions from haloethanes have attracted enormous interest as they offer an excellent example for a truely unimolecular reaction. [3] From the early days, single pulse shock tubes have been used in these studies. [4] Thermal rate constants yielded a pre- exponential factor of 10 13 for a large number of such reactions. Earlier empirical models used this factor to conclude that, the transition state (TS) for HX elimination reactions should be ‘tight’. [5] On the other hand, ab initio calculations on these reactions yielded pre- exponential factors that were typically an order of mag- nitude larger, suggesting that the TS is ‘looser’, than predicted from experimental A factor. However, theoretical activation energies were larger as well. Our ab initio calcula- tions on chlorofluorethanes [6], suggested that within a narrow range of T (100-200 K), both experimental and theoretical rate parameters would reproduce the measured rate constants. Experimental results on haloethanols are useful as they give rate parameters