Magnetic and Structural Properties of an Octanuclear Cu(II) S=1/2 Mesoscopic Ring: Susceptibility and NMR Measurements

Magnetic susceptibility, 1H NMR and 63Cu NMR-NQR expts. on two slightly different species of the mol. S = 1/2 antiferromagnetic (AF) ring Cu8, [Cu8(dmpz)8(OH)8]·2C5H5N (Cu8P) and [Cu8(dmpz)8(OH)8]·2C6H5NO2 (Cu8N), are presented. The magnetic energy levels are calcd. exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic field. From the magnetic susceptibility measurements the authors est. the AF exchange coupling const. J ∼ 1000 K and the resulting gap Δ∼500 K between the ST = 0 ground state and the ST = 1 1st excited state. The 63,65Cu NQR spectra indicate four crystallog. inequivalent copper nuclei in each ring. From the combination of the 63Cu NQR spectra and of the 63Cu NMR spectra at high magnetic field, the authors est. the quadrupole coupling const. vQ of each site and the av. asymmetry parameter η of the elec.-field gradient tensor. The nuclear spin-lattice relaxation rate (NSLR) decreases exponentially on decreasing temp. for all nuclei studied. The gap parameter extd. from 63Cu NQR-NSLR is the same as for the susceptibility while a smaller value was obtained from the 63Cu NMR-NSLR in an external magnetic field of 8.2 T.