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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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References

  1. International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology 2nd edn (eds McNaught, A. D. and Wilkinson, A.) (Blackwell Scientific Publications, 1997); http://goldbook.iupac.org (2012).

  2. Bester, G. et al. Experimental imaging and atomistic modeling of electron and hole quasiparticle wave functions in InAs/GaAs quantum dots. Phys. Rev B. 76, 075338 (2007).

    Article  Google Scholar 

  3. Westcott, B. L., Gruhn, N. E., Michelsen, L. J. & Lichtenberger, D. L. Experimental observation of non-Aufbau behavior: photoelectron spectra of vanadyloctaethylporphyrinate and vanadylphthalocyanine. J. Am. Chem. Soc. 122, 8083–8084 (2000).

    Article  CAS  Google Scholar 

  4. Cloke, F. G. N., Green, J. C. & Kaltsoyannis, N. Electronic structure of [U2(μ2-N2)(η5-C5Me5)2(η8-C8H4(SiPr3i)2)2]. Organometallics 23, 832–835 (2004).

    Article  CAS  Google Scholar 

  5. Slipchenko, L. V., Munsch, T. E., Wenthold, P. G. & Krylov, A. I. 5-Dehydro- 1,3-quinodimethane: a hydrocarbon with an open-shell doublet ground state. Angew. Chem. Int. Ed. 43, 742–745 (2004).

    Article  CAS  Google Scholar 

  6. Sugawara, T., Komatsu, H. & Suzuki, K. Interplay between magnetism and conductivity derived from spin-polarized donor radicals. Chem. Soc. Rev. 40, 3105–3118 (2011).

    Article  CAS  Google Scholar 

  7. Kusamoto, T., Kume, S. & Nishihara, H. Realization of SOMO–HOMO level conversion for a TEMPO-dithiolate ligand by coordination to platinum(II). J. Am. Chem. Soc. 130, 13844–13845 (2008).

    Article  CAS  Google Scholar 

  8. Kobayashi, Y., Yoshioka, M., Saigo, K., Hashizume, D. & Ogura, T. Hydrogen-bonding-assisted self-doping in tetrathiafulvalene (TTF) conductor. J. Am. Chem. Soc. 131, 9995–10002 (2009).

    Article  CAS  Google Scholar 

  9. Gryn'ova, G., Barakat, J. M., Blinco, J. P., Bottle, S. E. & Coote, M. L. Computational design of cyclic nitroxides as efficient redox mediators for dye-sensitized solar cells. Chem. Eur. J. 18, 7582–7593 (2012).

    Article  CAS  Google Scholar 

  10. Coote, M. L., Lin, C. Y., Beckwith, A. L. J. & Zavitsas, A. A. A comparison of methods for measuring relative radical stabilities of carbon-centred radicals. Phys. Chem. Chem. Phys. 12, 9597–9610 (2010).

    Article  CAS  Google Scholar 

  11. Boyd, S. L. et al. A theoretical study of the effects of protonation and deprotonation on bond dissociation energies. J. Am. Chem. Soc. 117, 8816–8822 (1995).

    Article  CAS  Google Scholar 

  12. Campanelli, A. R., Domenicano, A. & Ramondo, F. Polar effects and structural variation in 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives: a quantum chemical study. J. Phys. Chem. A. 110, 10122–10129 (2006).

    Article  CAS  Google Scholar 

  13. Saracino, G. A. A. et al. Solvent polarity and pH effects on the magnetic properties of ionizable nitroxide radicals: a combined computational and experimental study of 2,2,5,5-tetramethyl-3-carboxypyrrolidine and 2,2,6,6-tetramethyl-4-carboxypiperidine nitroxides. J. Phys. Chem. A 106, 10700–10706 (2002).

    Article  CAS  Google Scholar 

  14. Hawker, C. J., Bosman, A. W. & Harth, E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem. Rev. 101, 3661–3688 (2001).

    Article  CAS  Google Scholar 

  15. Edeleva, M. V. et al. Imidazoline series with multiple ionizable groups as an approach for control of nitroxide mediated polymerization. J. Org. Chem. 76, 5558–5573 (2011).

    Article  CAS  Google Scholar 

  16. Grob, C. A., Kaiser, A. & Schweizer, T. The transmission of polar effects. Part II. Helv. Chim. Acta 60, 391–399 (1977).

    Article  CAS  Google Scholar 

  17. Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H–Pu. J. Chem. Phys. 132, 154104 (2010).

    Article  Google Scholar 

  18. Knowles, P., Schütz, M. & Werner, H-J. in Modern Methods and Algorithms of Quantum Chemistry, Proceedings (ed. Grotendorst, J.) 2nd edn (John von Neumann Institute for Computing, 2000).

    Google Scholar 

  19. Verhoeven, J. W. & Pasman, P. The relative sign of through-bond and through-space interactions; ‘sigma assistance’ of cyclization and intramolecular hydrogen transfer. Tetrahedron 37, 943–947 (1981).

    Article  CAS  Google Scholar 

  20. Ohta, K., Closs, G. L., Morokuma, K. & Green, N. J. Stereoelectronic effects in intramolecular long-distance electron transfer in radical anions as predicted by ab initio MO calculations. J. Am. Chem. Soc. 108, 1319–1320 (1986).

    Article  CAS  Google Scholar 

  21. Forbes, M. D. E., Closs, G. L., Calle, P. & Gautam, P. Temperature dependence of the exchange coupling in polymethylene chain biradicals. Conclusions regarding the mechanism of the coupling. J. Phys. Chem. 97, 3384–3389 (1993).

    Article  CAS  Google Scholar 

  22. Dearman, H. H. & McConnell, H. M. Spin densities in several odd alternant radicals. J. Chem. Phys. 33, 1877 (1960).

    Article  CAS  Google Scholar 

  23. Kaptein, P., Van Leeuwen, P. W. N. M. & Huis, R. S–T± CIDNP from a thermally-generated diradical. Chem. Phys. Lett. 41, 264–266 (1976).

    Article  CAS  Google Scholar 

  24. Kirk, B. B., Harman, D. G. & Blanksby, S. J. Direct observation of the gas phase reaction of the cyclohexyl radical with dioxygen using a distonic radical ion approach. J. Phys. Chem. A 114, 1446–1456 (2010).

    Article  CAS  Google Scholar 

  25. Platz, J., Sehested, J., Nielsen, O. J. & Wallington, T. J. Atmospheric chemistry of cyclohexane: UV spectra of c-C6H11• and (c-C6H11)O2• radicals, kinetics of the reactions of (c-C6H11)O2• radicals with NO and NO2, and the fate of the alkoxy radical (c-C6H11)O•. J. Phys. Chem. A 103, 2688–2695 (1999).

    Article  CAS  Google Scholar 

  26. Hou, R., Gu, J. Xie, Y., Yi, X. & Schaefer, H. F. III, The 2′-deoxyadenosine- 5′-phosphate anion, the analogous radical, and the different hydrogen-abstracted radical anions: molecular structures and effects on DNA damage. J. Phys. Chem. B 109, 22053–22060 (2005).

    Article  CAS  Google Scholar 

  27. Stevenson, J. P., Jackson, W. F. & Tanko, J. M. Cyclopropylcarbinyl-type ring openings. Reconciling the chemistry of neutral radicals and radical anions. J. Am. Chem. Soc. 124, 4271–4281 (2002).

    Article  CAS  Google Scholar 

  28. Frisch, M. J. et al. Gaussian 09, revision B.01 (Gaussian Inc., 2010).

  29. Shao, Y. et al. Advances in quantum chemical methods and algorithms in the Q-Chem 3.0 program package. Phys. Chem. Chem. Phys. 8, 3172–3191 (2006).

    Article  CAS  Google Scholar 

  30. Werner, H-J. et al. MOLPRO 2009.1 (University College Cardiff Consultants Ltd, 2009).

  31. Schmidt, M. W. et al. General atomic and molecular electronic structure system. J. Comput. Chem. 14, 1347–1363 (1993).

    Article  CAS  Google Scholar 

  32. Merrick, J. P., Moran, D. & Radom, L. An evaluation of harmonic vibrational frequency scale factors. J. Phys. Chem. A 111, 11683–11700 (2007).

    Article  CAS  Google Scholar 

  33. Henry, D. J., Sullivan, M. B. & Radom, L. G3-RAD and G3X-RAD: modified Gaussian-3 (G3) and Gaussian-3X (G3X) procedures for radical thermochemistry. J. Chem. Phys. 118, 4849–4860 (2003).

    Article  CAS  Google Scholar 

  34. Curtiss, L. A., Raghavachari, K., Redfern, P. C., Baboul, A. G. & Pople, J. A. Gaussian-3 theory using coupled cluster energies. Chem. Phys. Lett. 314, 101–107 (1999).

    Article  CAS  Google Scholar 

  35. Chan, B., Deng, J. & Radom, L. G4(MP2)-6X: a cost-effective improvement to G4(MP2). J. Chem. Theory Comput. 7, 112–120 (2011).

    Article  CAS  Google Scholar 

  36. Izgorodina, E. I. et al. Should contemporary density functional theory methods be used to study the thermodynamics of radical reactions? J. Phys. Chem. A 111, 10754–10768 (2007).

    Article  CAS  Google Scholar 

  37. Steinfeld, J. I., Francisco, J. S. & Hase, W. L. Chemical Kinetics and Dynamics (Prentice Hall, 1989).

    Google Scholar 

  38. Hodgson, J. L., Roskop, L. B., Gordon, M. S., Lin, C. Y. & Coote, M. L. Side reactions of nitroxide-mediated polymerization: N–O versus O–C cleavage of alkoxyamines. J. Phys. Chem. A 114, 10458–10466 (2010).

    Article  CAS  Google Scholar 

  39. Blinco, J. P. et al. Experimental and theoretical studies of the redox potentials of cyclic nitroxides. J. Org. Chem. 73, 6763–6771 (2008).

    Article  CAS  Google Scholar 

  40. Glaesermann, K. R. & Schmidt, M. W. On the ordering of orbital energies in high-spin ROHF. J. Phys. Chem. A 114, 8772–8777 (2010).

    Article  Google Scholar 

  41. Schoening, K-U., Fischer, W., Hauck, S., Dichtl, A. & Kuepfert, M. Synthetic studies on N-alkoxyamines: a mild and broadly applicable route starting from nitroxide radicals and aldehydes. J. Org. Chem. 74, 1567–1573 (2008).

    Article  Google Scholar 

  42. Cooks, R. G. & Wong, P. S. H. Kinetic method of making thermochemical determinations: advances and applications. Acc. Chem. Res. 31, 379–386 (1998).

    Article  CAS  Google Scholar 

  43. Cooks, R. G., Patrick, J. S., Kotiaho, T. & McLuckey, S. A. Thermochemical determinations by the kinetic method. Mass Spectrom. Rev. 13, 287–339 (1994).

    Article  CAS  Google Scholar 

  44. Ervin, K. M. Microcanonical analysis of the kinetic method: the meaning of the ‘effective temperature’. Int. J. Mass Spectrom. 195/196, 271–284 (2000).

    Article  Google Scholar 

  45. Jones, C. M. et al. Gas-phase acidities of the 20 protein amino acids. Int. J. Mass Spectrom. 267, 54–62 (2007).

    Article  CAS  Google Scholar 

  46. Thomas, M. C., Mitchell, T. W. & Blanksby, S. J. A comparison of the gas phase acidities of phospholipid headgroups: experimental and computational studies. J. Am. Soc. Mass Spectrom. 16, 926–939 (2005).

    Article  CAS  Google Scholar 

  47. Borden, W. T. Qualitative methods for predicting the ground states of non-Kekule hydrocarbons and the effects of heteroatom substitution on the ordering of the electronic states. Mol. Cryst. Liq. Cryst. 232, 195–218 (1993).

    Article  CAS  Google Scholar 

  48. Kusamoto, T., Kume, S. & Nishihara, H. Cyclization of TEMPO radicals bound to metalladithiolene induced by SOMO–HOMO energy-level conversion. Angew. Chem. Int. Ed. 49, 529–531 (2010).

    Article  CAS  Google Scholar 

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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, 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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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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