Finding the Reactive Electron in Paramagnetic Systems: A Critical Evaluation of Accuracies for EPR Spectroscopy and Density Functional Theory Using 1,3,5-Triphenyl Verdazyl Radical as a Testcase

Abstract

One of the biggest challenges in studying catalytic reactions is characterizing intermediate states and identifying reaction pathways. Oftentimes, intermediate states with unpaired electrons are formed which provide an opportunity to study the compound via electron paramagnetic resonance (EPR). Combining EPR with density functional theory (DFT) represents a powerful synergistic approach to accomplish these goals. Once the catalytic intermediates and reaction pathway are known, rate-limiting steps critical to parameters like overpotential and turnover number may be identified and eliminated. In this study 1,3,5-triphenyl verdazyl is examined using continuous-wave-EPR, electron nuclear double resonance and DFT as an instructive example of how theory and experiment can complement each other to find the reactive electron. The methods and concomitant analysis have been presented in didactic fashion and with emphasis on the strengths and weaknesses of the methods.

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Acknowledgments

We thank Annette Schäfermeier (University of Bonn) for her help with the synthesis of 1,3,5-triphenyl verdazyl. We also thank Dr. Ragnar Björnsson for many helpful conversations regarding computational approaches. This project was funded by the Max Planck Institute for Chemical Energy Conversion.

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Correspondence to Maurice van Gastel.

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Electronic Supplementary Information (ESI) available: cartesian coordinates [Å] or the model geometries used in the calculations, Mulliken charge and spin populations and singly occupied molecular orbital. (DOCX 81 kb)

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Barilone, J., Neese, F. & van Gastel, M. Finding the Reactive Electron in Paramagnetic Systems: A Critical Evaluation of Accuracies for EPR Spectroscopy and Density Functional Theory Using 1,3,5-Triphenyl Verdazyl Radical as a Testcase. Appl Magn Reson 46, 117–139 (2015). https://doi.org/10.1007/s00723-014-0627-2

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Keywords

  • Electron Paramagnetic Resonance
  • Electron Paramagnetic Resonance Spectrum
  • Spin Density
  • Unpaired Electron
  • Central Ring