Timothy Ramadhar - Accurate Prediction of Experimental Free Energy of Activation Barriers for the Aliphatic-Claisen Rearrangement through DFT Calculations

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      Publication Details (including relevant citation   information):

      Timothy R. Ramadhar, Robert A. Batey. Computational and   Theoretical Chemistry. 2011, 976,   167-182.

      Abstract:

      A benchmarking study was performed to determine DFT methods that   can accurately predict free energy of activation barriers   (ΔG) for the aliphatic-Claisen   rearrangement. Accurate experimental ΔG  values from eight gas-phase literature reactions were used for   validation. Previously applied density functionals for this   system and its variants, along with the Minnesota 2005/2006/2008   functionals, were tested using the 6-31+G** basis set. On   B3LYP/6-31+G** geometries, M08-HX afforded the best results and   gave a seven-fold increase in accuracy over the energies   predicted by the robust B3LYP functional, which is routinely used   for Claisen rearrangements. An investigation of whether   improvements in ΔG predictions could occur   by optimizing structures with newer functionals then became of   interest. For geometry validation, optimization and   Bigeleisen-Mayer KIE calculations with M05/6-31+G** and   M05-2X/6-31+G** for the rearrangement of allyl vinyl ether were   performed and compared to previous experimental data, and both   were found to predict new valid Claisen transition structures.   All structures were then reoptimized, and it was found that   M05-2X/6-31+G** geometries afforded worthwhile enhancement, where   M08-HX/6-31+G**//M05-2X/6-31+G** yielded the most accurate and   reliable results with a mean unsigned error (MUE) of 0.3 kcal/mol   relative to experimental ΔG values. These   results should prove useful for Claisen rearrangement studies,   and this is the first instance that the Minnesota 2008   functionals have been applied to the study of sigmatropic   reactions.

      Address (URL): http://dx.doi.org/10.1016/j.comptc.2011.08.022