Determination of the relative signs of the hyperfine coupling constants for the phosphonenitroxyl radical from the ESR spectra in weak fields

Abstract

The direct method for the measurement of tile signs of hyperfine coupling constants using paramagnetic shifts in the NMR spectra of free radicals is frequently not possible on account of extensive broadening of the NMR lines. The signs of these constants can be determined from an analysis of the dependence of the relaxation width of the hyperfine components of the ESR spectrum on the nuclear quantum number [ 1, 21. In free radicals, the anisotropy of the paraineters in the spin-Hamiltonian is often so small that such an analysis is based on measurements of ESR linewidths which lie beyond tile limits for the experimental accuracy of the measurements. In these cases information on tile relative signs of the hyperfine constants can be obtained more reliably from ESR spectra recorded at low frequencies [3]. In the present work, the ESR spectrum of an aqueous acetone solution of nitroxyl with a phosphonesulfinate group [4] has been investigated at a frequency of 426.5 MHz: CHa ! -O2S -- N -- C -- P (O) (OEt)a. O" CI-t a In recording the spectra we made use of the method of electron -nuclear double resonance (ENDOR) which, in weak fields, is characterized by a higher sensitivity than the ESR method which normally employs an autodyne generator [5]. In order to interpret the restilts, the energy spectrum of the phosphonenitroxyl radical was calculated in the representation of the total angular momentum of the molecule. The radical under consideration possesses an electron spin S = 1/2, which interacts with the two magnetic nuclei: 3~ P(I~ = I/2) and 14N(12 = 1) with different hyperfine coupling constants a and a', respectively. The observed spectrum is described by the spin-Hamiltonian