Determination of the ¹⁴N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g_xx and the ¹⁴N hyperfine A_zz principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the ¹⁴N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P_xx and P_yy principal components of the ¹⁴N quadrupole tensor. The P_zz value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the ¹⁴N P_zz value directly, together with A _zz. This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g_xx direction to determine the principal P_xx and P_yy components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A_zz increases, while |P_zz| decreases with polarity, as predicted by Savitsky et al.