Keep Your TEMPO Up: Nitroxide Radicals as Sensors of Intermolecular Interactions

Abstract

This study examines experimental data on the influence of the surrounding medium and non-covalent interactions on the isotropic hyperfine coupling constant, A_iso(¹⁴N), of the stable nitroxide radical 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in solution. The data were used to identify a density functional theory functional/basis set combination that accurately reproduces the experimental A_iso(¹⁴N) values. The variations in A_iso(¹⁴N) due to external factors are two orders of magnitude greater than the accuracy of its experimental measurements, making A_iso(¹⁴N) a highly sensitive experimental probe for quantifying these effects. Additionally, it was found that the proton-accepting ability of the N-O^• moiety in TEMPO resembles that of the P=O moiety, enabling the simultaneous formation of two equally strong hydrogen bonds.

Keywords: EPR; TEMPO; hydrogen bonding; hyperfine coupling constant; non-covalent interactions.

Figure 1

EPR spectrum of 5 × 10⁻⁴ M (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in toluene at 300 K. A_iso(¹⁴N) = 1.55 mT, g = 2.0023.

Figure 2

The model structures of the selected 1:1 complexes of TEMPO with (a) water, (b) methanol, and (c) 2-methylpropan-2-ol. Oxygen atoms are represented in red, nitrogen atoms in blue, carbon atoms in grey, and exchangeable protons in white.

Figure 3

The model structures of the 1:1 complexes of TEMPO with (a) phenol, (b) 2,5-dinitrophenol, and (c) 2,4,6-trinitrophenol. Oxygen atoms are represented in red, nitrogen atoms in blue, carbon atoms in grey, and exchangeable protons in white.

Figure 4

The model structures of the complexes of TEMPO with strong proton donors. (a) Hydrogen fluoride. (b) Two molecules of hydrogen fluoride. (c) Pyridine-H⁺ cation. (d) 2,2′-bipyridine-H⁺ cation. (e) 1,10-phenanthroline-H⁺ cation. Oxygen atoms are represented in red, nitrogen atoms in blue, carbon atoms in grey, fluorine atoms in cyan, and exchangeable protons in white.

Figure 5

The model structure of the 1:1 complexes of TEMPO with iodo 2,2,2-trifluoroacetate in hexane. The calculated A_iso(¹⁴N) value in this complex is 1.8635 mT. The calculated A_iso(¹⁴N) value of TEMPO in hexane is 1.4996 mT. Oxygen atoms are represented in red, the nitrogen atom in blue, carbon atoms in grey, and the iodine atom in purple.