(646a) Quantum Mechanics Study Of Decomposition Of CF3OCF3 And (CF3CF2)2O Catalyzed By AlF3

Quantum Mechanics Study of Decomposition of CF₃OCF₃

and (CF₃CF₂)₂O catalyzed by AlF₃

Bangwu Jiang, David J. Keffer, and Brian J. Edwards

Department of Chemical Engineering

University of Tennessee

Knoxville, TN 37996-2200

Abstract

Perfluoropolyethers (PFPEs) form a class of lubricants, broadly applied in different environments. While the physical properties of these PFPEs are intrinsically stable, they decompose under certain conditions, for example, heating in the presence of metal surfaces and Lewis acids, which form if unreacted fluorine is present in the lubricant and attacks the metal.  In addition to tribological properties, chemical and thermal stability are key properties for PFPEs to be utilized in different extreme conditions.  The simplest compound that can provide information regarding the thermal and chemical stability of PFPEs is CF₃OCF₃.  Decomposition of CF₃OCF₃ in the presence of AlF₃ was investigated by Pancansky et. al¹. through quantum mechanics; a transition structure was proposed to describe this reaction. In this work we re-investigated the decomposition of CF₃OCF₃ and extended the study to include a longer chain PFPE, (CF₃CF₂)₂O, in the presence of AlF₃ at two different theory levels through quantum mechanics. Contrary to reports in the literature, we find that the catalyst, AlF₃, not only attacks the oxygen atom, but also functions as a transfer agent of fluorine atoms from one carbon and another one. A new transition state structure with a significantly lower energy barrier is proposed.  The two transition state structures are given below:

        Structure proposed by Pancansky et. al¹                       New transition structure in our work

From quantum mechanical calculations, we report the activation barrier and the heat of reaction.   Statistical Mechanics is applied to estimate the reaction rate constants as a function of temperature according to the transition state theory.  We examine the reactivity as a function of chain length and draw connections to lubricant stability.

Acknowledgements:

            This work has been supported by Air Force Office of Scientific Research through contract # FA 9550-05-1-0342. The authors wish to acknowledge resources of the Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the DOE under Contract DE-AC05-00OR22725. Reference:

            (1)        Pacansky, J.; Waltman, R. J. Journal of Fluorine Chem. 1997, 83, 41.