Thermal Variation of the EPR Linewidth of new Phosphoniumyl Free Radicals for NMR Magnetometers

The temperature dependence of the EPR linewidth of two phosphoniumyl radicals is investigated between 193K and 353K. The observed behaviour is interpreted through a simple model which assumes that the main contribution to the EPR linewidth arises from the anisotropic component of the large hyperfine coupling of the unpaired electron with the phosphorus nucleus. However the non resolved hyperfine coupling with the protons of the ortho methyl substituents is shown to contribute significantly to the broadening of the EPR linewidths in the high temperature range. The EPR features of the two radicals are discussed to evaluate their usefulness in low field NMR magnetometers.