Unconventional Heisenberg spin triangle in magnetic molecule {V15}:: A proton nuclear magnetic resonance study

We present results of H-1 nuclear magnetic resonance (NMR) and relaxation in polyoxovanadate cluster {V15} with formula K-6[V15As6O42].9(H2O). The data in {V15} are compared with the published data in {V6} with the formula [CN3H6](4)Na-2[H-4{V3L}(2)P4O4].14H(2)O, which contains two independent spin triangles. Temperature T and field H dependence of the NMR linewidth are well explained by the magnetic dipolar interaction between the proton nuclei and vanadium ion spins in both compounds. T and H dependence of proton relaxation rate T-1(-1) show great differences in the two compounds. The difference indicates a completely different spin dynamics for the two Heisenberg spin-triangle systems whereby in {V15} the fluctuations of the V-IV (s=1/2) in the central triangle are dominated by the interlayer coupling with the V-IV spins in the two upper and lower hexagons. (C) 2003 American Institute of Physics.