Magnetic and structural properties of an octanuclear Cu(II) S=1/2 mesoscopic ring:: Susceptibility and NMR measurements

Magnetic susceptibility, H-1 NMR and Cu-63 NMR-NQR experiments on two slightly different species of the molecular S = 1/2 antiferromagnetic (AF) ring Cu8, [Cu-8(dmpz)(8)(OH)(8)].2C(5)H(5)N (Cu8P) and [Cu-8(dmpz)(8)(OH)(8)].2C(5)H(5)NO(2) (Cu8N), are presented. The magnetic energy levels are calculated exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic field. From the magnetic susceptibility measurements we estimate the AF exchange coupling constant J similar to 1000 K and the resulting gap Delta similar to 500 K between the S-T = 0 ground state and the S-T = 1 first excited state. The Cu-63,Cu-65 NQR spectra indicate the presence of four crystallographically inequivalent copper nuclei in each ring. From the combination of the Cu-63 NQR spectra and of the Cu-63 NMR spectra at high magnetic field, we estimate the quadrupole coupling constant nu(Q) of each site and the average asymmetry parameter eta of the electric-field gradient tensor. The nuclear spin-lattice relaxation rate (NSLR) decreases exponentially on decreasing temperature for all nuclei investigated. The gap parameter extracted from Cu-63 NQR-NSLR is the same as for the susceptibility while a smaller value is obtained from the Cu-63 NMR-NSLR in an external magnetic field of 8.2 T.