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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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).
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).
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).
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).
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).
Sugawara, T., Komatsu, H. & Suzuki, K. Interplay between magnetism and conductivity derived from spin-polarized donor radicals. Chem. Soc. Rev. 40, 3105–3118 (2011).
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).
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).
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).
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).
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).
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).
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).
Hawker, C. J., Bosman, A. W. & Harth, E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem. Rev. 101, 3661–3688 (2001).
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).
Grob, C. A., Kaiser, A. & Schweizer, T. The transmission of polar effects. Part II. Helv. Chim. Acta 60, 391–399 (1977).
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).
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).
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).
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).
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).
Dearman, H. H. & McConnell, H. M. Spin densities in several odd alternant radicals. J. Chem. Phys. 33, 1877 (1960).
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).
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).
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).
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).
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).
Frisch, M. J. et al. Gaussian 09, revision B.01 (Gaussian Inc., 2010).
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).
Werner, H-J. et al. MOLPRO 2009.1 (University College Cardiff Consultants Ltd, 2009).
Schmidt, M. W. et al. General atomic and molecular electronic structure system. J. Comput. Chem. 14, 1347–1363 (1993).
Merrick, J. P., Moran, D. & Radom, L. An evaluation of harmonic vibrational frequency scale factors. J. Phys. Chem. A 111, 11683–11700 (2007).
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).
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).
Chan, B., Deng, J. & Radom, L. G4(MP2)-6X: a cost-effective improvement to G4(MP2). J. Chem. Theory Comput. 7, 112–120 (2011).
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).
Steinfeld, J. I., Francisco, J. S. & Hase, W. L. Chemical Kinetics and Dynamics (Prentice Hall, 1989).
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).
Blinco, J. P. et al. Experimental and theoretical studies of the redox potentials of cyclic nitroxides. J. Org. Chem. 73, 6763–6771 (2008).
Glaesermann, K. R. & Schmidt, M. W. On the ordering of orbital energies in high-spin ROHF. J. Phys. Chem. A 114, 8772–8777 (2010).
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).
Cooks, R. G. & Wong, P. S. H. Kinetic method of making thermochemical determinations: advances and applications. Acc. Chem. Res. 31, 379–386 (1998).
Cooks, R. G., Patrick, J. S., Kotiaho, T. & McLuckey, S. A. Thermochemical determinations by the kinetic method. Mass Spectrom. Rev. 13, 287–339 (1994).
Ervin, K. M. Microcanonical analysis of the kinetic method: the meaning of the ‘effective temperature’. Int. J. Mass Spectrom. 195/196, 271–284 (2000).
Jones, C. M. et al. Gas-phase acidities of the 20 protein amino acids. Int. J. Mass Spectrom. 267, 54–62 (2007).
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).
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).
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).
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.
The authors declare no competing financial interests.
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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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