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 sligh tly 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 calculat ed exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic f ield. From the magnetic susceptibility measurements we estimate the AF exch ange coupling constant J similar to 1000 K and the resulting gap Delta simi lar 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 crystallo graphically inequivalent copper nuclei in each ring. From the combination o f 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 a verage asymmetry parameter eta of the electric-field gradient tensor. The n uclear spin-lattice relaxation rate (NSLR) decreases exponentially on decre asing temperature for all nuclei investigated. The gap parameter extracted from Cu-63 NQR-NSLR is the same as for the susceptibility while a smaller v alue is obtained from the Cu-63 NMR-NSLR in an external magnetic field of 8 .2 T.