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

## TL;DR

This paper uses paramagnetic NMR to study the stable radical TEMPO in various states and shows how it affects chemical shifts and signal linewidths.

## Contribution

The first detailed paramagnetic NMR study of TEMPO's 1H and 13C signals across multiple physical states and solvent interactions.

## Key findings

- All 1H and 13C NMR signals of TEMPO are visible in paramagnetic NMR spectra with narrow chemical shift ranges.
- TEMPO causes solvent chemical shift changes and forms adducts in CDCl3.
- Solid-state NMR shows TEMPO adsorbs on silica surfaces with dilute solution-like features.

## Abstract

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.

Paramagnetic species can often be measured with EPR and NMR spectroscopy. This is demonstrated using the important radical TEMPO. All 1H and 13C NMR signals of TEMPO could be recorded and assigned. The impact of TEMPO on the chemical shifts of solvent signals and a phosphine is described.