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Wednesday, April 26 • 10:30am - 10:50am
Computational Investigations Of 1,1 Elimination Transition State Geometries And Post-Transition State Complexes Of Hydrofluorocarbons Using Single And Multi-Reference Techniques

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Chlorofluorocarbons (CFCs) were banned by the Montreal Protocol in 1987 due to their contribution to ozone depletion in the stratosphere through chlorine radical reactions. Their replacements, hydrochlorofluorocarbons (HCFCs), are less harmful because of their more reactive H-C bonds. We present a computational investigation of the singlet potential energy surfaces of specific HCFCs which could undergo either a 1,1-HF or 1,1-HCl elimination forming a carbene and then undergoing rearrangement into an alkene. Transition state geometries are reported for 1,1 HF and HCl elimination from CH3CHFCl, CHCl2CHCl2, and similar HCFCs using various levels of theories including Density Functional Theory (DFT), Complete-Active-Space-Consistent-Field theory (CASSCF), and Møller–Plesset perturbation theory (MP). These show that there is a very loose transition state, and in many cases DFT fails to predict a viable transition state geometry because of the late transition state with highly stretched bonds. There are also clear energy differences between these theories. We show that the transition state geometry does not lead to the dissociated products, but a “post transition state complex” with a weak interaction between HF or HCl and the carbene. This new mechanism for 1,1 HF or HCl elimination may explain the differences between experimental results and previous computational studies.

Wednesday April 26, 2017 10:30am - 10:50am PDT
123 Zeis Hall