Article | Published:

Switching radical stability by pH-induced orbital conversion

Nature Chemistry volume 5, pages 474481 (2013) | Download Citation

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.

  • Compound C19H19N2O2S4

    Tetrathiafulvalene nitronyl nitroxide

  • Compound C24H32N2NiO2S82•2-

    2,2,6,6-Tetramethylpiperidine-N-oxyl nickel(II) dithiolene

  • Compound C28H28N4O8S16

    Tetrakis(ammonium tetrathiafulvalene-2-carboxylate)

  • Compound C10H17NO3•-

    4-Carboxylate-2,2,6,6-tetramethylpiperidine-N-oxyl

  • Compound C10H18NO3

    4-Carboxy-2,2,6,6-tetramethylpiperidine-N-oxyl

  • Compound C5H8O4•-

    5-Peroxyl pentanoate

  • Compound C5H9O4

    5-Peroxyl pentanoic acid

  • Compound C14H21N4O

    2-(4-(Dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl

  • Compound C5H8O3•-

    5-Alkoxyl pentanoate

  • Compound C5H9O3

    5-Alkoxyl pentanoic acid

  • Compound C11H18O3•-

    4-Carboxylate-2,2,6,6-tetramethylcyclohexyl-1-oxyl

  • Compound C11H19O3

    4-Carboxy-2,2,6,6-tetramethylcyclohexyl-1-oxyl

  • Compound C9H18NO

    2,2,6,6-Tetramethyl piperidine-N-oxyl

  • Compound C14H25NO3•-

    4-(5-Pentanoate)-2,2,6,6-tetramethylpiperidine-N-oxyl

  • Compound C11H21NO3•-

    2,2,6,6-Tetramethylpiperidine-N-oxyl ethanoate

  • Compound C11H20NO3-

    1-Methoxy-2,2,6,6-tetramethyl piperidine-4-carboxylate

  • Compound C11H23NO5P-

    1-Methoxy-2,2,6,6-tetramethyl piperidin-4-yl methyl phosphate

  • Compound C10H20NO5S-

    1-Methoxy-2,2,6,6-tetramethyl piperidin-4-yl sulfate

  • Compound C11H20NO2S-

    1-Methoxy-2,2,6,6-tetramethyl piperidine-4-carbothioate

  • Compound C10H20NO2-

    1-Methoxy-2,2,6,6-tetramethylpiperidin-4-olate

  • Compound C10H13N5O10P22-

    ((2R,3S,5R)-5-(2-Amino-6-oxo-1,6-dihydro-9H-purin-9-yl)-3-((hydroxyoxidophosphoryl)oxy)tetrahydrofuran-2-yl)methyl hydrogen phosphate

  • Compound C10H13N5O11P22-

    ((2R,3S,4R,5R)-5-(2-Amino-6-oxo-1,6-dihydro-9H-purin-9-yl)-4-hydroxy-3-((hydroxyoxidophosphoryl)oxy)tetrahydrofuran-2-yl)methyl hydrogen phosphate

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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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  1. Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 0200, 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, New South Wales 2522, Australia.

    • David L. Marshall
    •  & Stephen J. Blanksby

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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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Michelle L. Coote.

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https://doi.org/10.1038/nchem.1625

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