TEMPO in Solution, Melted, Solid, and Adsorbed on a Silica Surface: A Paramagnetic NMR Study

TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) is a stable radical that is phenomenologically interesting and of high importance in synthesis and catalysis. This radical has been studied extensively with EPR and used as a spin tag for biological materials and to probe hydrogen bonding. The NMR properties of TEMPO have not yet been reported. This contribution describes that all ¹H and ¹³C NMR signals of TEMPO are visible in the paramagnetic NMR spectra. The ¹H NMR spectra of TEMPO solutions have a chemical shift range of 50 ppm and require only 8 scans. The ¹³C NMR signals are localized within a chemical shift range of 2600 ppm and are obtained within 30 minutes. Samples prepared with different solvents and TEMPO concentrations have been investigated, and it has been demonstrated that all solvents undergo chemical shift changes due to the presence of TEMPO and adduct formation in the case of CDCl₃. The impact of TEMPO on the ³¹P chemical shift of PPh₃ is studied. Higher concentrations of TEMPO in solution lead to narrower signals and the molten substance, measured at 60°C, represents the culmination of this trend. It is demonstrated with paramagnetic solid-state NMR that TEMPO adsorbs on a silica surface and displays the chemical shift and linewidth features of a dilute solution of TEMPO.