Magnetic Properties and Low-Field ESR of Organic Free Radicals, Monochloroporphyrexide and Porphyrexide

Abstract

Abstract The static magnetic susceptibility, and ESR and proton NMR spectra of powder samples of monochloroporphyrexide were measured above 1 K and compared with those of porphyrexide. A broad maximum in the static susceptibility of monochloroporphyrexide, indicating an antiferromagnetic interaction, appeared at 22.5 K. The relative susceptibilities of the two radicals obtained from the ESR signal intensities and the proton NMR shifts agree qualitatively with the static susceptibilities. In the low-field ESR spectra (135 MHz) of each radical, broadening of the g=2 absorption line and distinct appearance of the g=4 absorption line were found in the temperature region below Tm, the temperature at which the susceptibility is maximum. The anisotropy of the g-value was estimated from Q-band ESR spectra. Furthermore, the unpaired electron-spin distribution over the ring was estimated from the X-band ESR and the proton NMR spectra. The results are interpreted assuming that the magnetic behavior of the two radicals is due to alternating antiferromagnetic Heisenberg linear chains with alternating parameters, a=0.4 and a=0.6, respectively.