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Electron-nuclear quadrupole interaction in trimeric antiferromagnetic dusters with half-integer ion spins is considered. The electronic shell of the exchange cluster is shown to produce electric field gradient at the nuclei. The antisymmetric exchange interaction of Dzialoshinskii leads to the flattening of the electric field gradient at the nuclei, located in geometrically non-equivalent positions due to distortions. The parameters of the quadrupole hamiltonian depend upon the external magnetic field. The magnetic field, parallel to the plane of the cluster, reduces the antisymmetric exchange interaction and re-establishes the geometric non-equivalence of the ions. The hyperfine fields on MSssbauer nuclei is examined. Noncollinear spin structure of the trimeric cluster is predicted. The theory explains qualitatively the peculiarities of MSssbauer spectra of iron threenuclear carboxylate crystals. INTRODUCTIONIn the preceding paper [1] tetrahedral clusters with spin-coupled ground states have been examined. The special form of the electron-nflclear quadrupole interaction when there is orbital degeneracy of exchange multiplets has been considered. When S= 0 the problem is not complicated by the influence of the electronic spin and hyperfine spin-spin electron-nuclear interaction. Hyperfine interaction is important for systems with S# 0. Therefore trimeric clusters of ions with half-integer spins possessing degenerate orbital multiplets and non-zero ground state spin will be considered below.

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