Low-temperature theory of proton. NMR in the molecular antiferromagnetic ring Fe10

The proton nuclear spin-lattice relaxation rate 1/T-1 in the molecular "ferric wheel" Fe-10(OCH3)(20)(C2H2O2Cl)(10) (Fe10) is calculatcd in the framework of the linear responce theory, starting from the unperturbed eigenvalues and cigenstates of the magnetic ring. The dipole-dipole superhyperfine interaction between the proton nuclear spins and the electronic spins of the magnetic ions is shown to induce transitions between states \S, M] which differ by 1 both in the quantum number S (the total spin of the ring) and, in M (the projection of the total spin along the quantization axis). The theory predicts that level crossing between the states \S+1, -(S+1)] and \S, -S] induced by all applied magnetic field should be accompanied by a strong enhancement of T-1(-1), in good agreement with resent, low-temperature NMR experimental data on Fe10.