title: Switching radical stability by pH-induced orbital conversion.
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.
authors.affiliation: 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.
authors.forename: Ganna
authors.initials: G
authors.lastname: Gryn'ova
authors.forename: David L
authors.initials: DL
authors.lastname: Marshall
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Blanksby
authors.forename: Michelle L
authors.initials: ML
authors.lastname: Coote
mesh_terms: D005609:Free Radicals / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007221:Industry; D008958:Models, Molecular; D008968:Molecular Conformation; D009930:Organic Chemicals / Q000737:chemistry; D011522:Protons; D013816:Thermodynamics
chemical_list: D005609:Free Radicals; D009930:Organic Chemicals; D011522:Protons