Multifrequency electron spin-echo envelope modulation: The determination of nitro group ¹⁴N hyperfine and quadrupole interactions of DPPH in frozen solution

Two- and three-pulse electron spin-echo modulations from ¹⁴N in the nitro groups of DPPH in toluene at 77 K have been measured at electron spin excitation frequencies of 4.45, 4.94, 5.47, 6.68, and 9.10 GHz. Analysis of the frequencies, amplitudes, and linewidths of the modulation components, together with their variation with excitation frequency, establishes that the ¹⁴N hyperfine interactions are approximately isotropic and enables the evaluation of the isotropic coupling. Distinctive spectral trends apparent in multifrequency ESEEM are employed to identify the cancellation of hyperfine and Zeeman interactions; modulation components accordingly assigned as pure quadrupole frequencies afford accurate determination of quadrupole coupling constants. Multifrequency ESEEM is shown to be a vital tool of general applicability in ¹⁴N ESEEM spectroscopy.