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Switching radical stability by pH-induced orbital conversion

Nature Chemistry volume 5, pages 474–481 (2013)Cite this article

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Abstract

In most radicals the singly occupied molecular orbital (SOMO) is the highest-energy occupied molecular orbital (HOMO); however, in a small number of reported compounds this is not the case. In the present work we expand significantly the scope of this phenomenon, known as SOMO–HOMO energy-level conversion, by showing that it occurs in virtually any distonic radical anion that contains a sufficiently stabilized radical (aminoxyl, peroxyl, aminyl) non-π-conjugated with a negative charge (carboxylate, phosphate, sulfate). Moreover, regular orbital order is restored on protonation of the anionic fragment, and hence the orbital configuration can be switched by pH. Most importantly, our theoretical and experimental results reveal a dramatically higher radical stability and proton acidity of such distonic radical anions. Changing radical stability by 3–4 orders of magnitude using pH-induced orbital conversion opens a variety of attractive industrial applications, including pH-switchable nitroxide-mediated polymerization, and it might be exploited in nature.

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Figure 1: Examples of organic and organometallic systems with SOMO–HOMO conversion.
Figure 2: Distonic radical anions, a new class of species with the SOMO–HOMO conversion.
Figure 3: pH switching of radical stability.
Figure 4: Assessing the σ-assistance effect.
Figure 5: Experimental proof of the BDE switching by pH-induced orbital conversion.
Figure 6: SOMO–HOMO conversion in various substrates.

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Acknowledgements

We acknowledge financial support from the Australian Research Council (ARC) Centre of Excellence for Free-Radical Chemistry and Biotechnology, an ARC Future Fellowship (to M.L.C.), an Australian Postdoctoral Award (to D.L.M.), allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure and useful discussions with R. D. Amos, M. G. Banwell, P. M. W. Gill, J. Ho, R. Kobayashi, C. Y. Lin, M. J. Monteiro and J. C. Poutsma.

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Authors and Affiliations

  1. Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra, 0200, Australian Capital Territory, Australia

    Ganna Gryn'ova & Michelle L. Coote

  2. Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, School of Chemistry, University of Wollongong, Northfields Avenue, Wollongong, 2522, New South Wales, Australia.

    David L. Marshall & Stephen J. Blanksby

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  1. Ganna Gryn'ova
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Contributions

G.G. and M.L.C. designed the project and co-wrote the manuscript, with assistance from the other authors. G.G. carried out the computational studies, and G.G. and M.L.C. analysed the computational results. S.J.B. and D.L.M. designed and analysed the experimental studies, which were carried out by D.L.M.

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Correspondence to Michelle L. Coote.

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Gryn'ova, G., Marshall, D., Blanksby, S. et al. Switching radical stability by pH-induced orbital conversion. Nature Chem 5, 474–481 (2013). https://doi.org/10.1038/nchem.1625

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