EXCHANGE INTERACTIONS IN TRINUCLEAR BASIC CHROMIUM(III) CLUSTERS - DIRECT OBSERVATION OF THE MAGNETIC SPECTRUM BY INELASTIC NEUTRON-SCATTERING

Incoherent inelastic neutron scattering spectra are reported for salts of the complex [Cr3O(OOCCH3)6(OH2)3]+. The data are consistent with predominantly antiferromagnetic coupling between pairs of chromium ions. The complete spectrum of transitions between successive states with total spin S = 1/2, 3/2, 5/2, 7/2, and 9/2 has been observed for the first time. Splittings of the ground state S = 1/2 have been observed directly and attributed to lowering of symmetry of the triangular cluster. For the chloride salt the data confirm that two sets of complex cations with different degrees of symmetry lowering are present in the crystal, at least at the lowest temperatures used (T = 1.4 to 50 K). In principle, the relationship of J values for the symmetry-lowered case could be described as ''isosceles,'' with J(ab) > J(bc) = J(ac) or J(ab) < J(bc) = J(ac); or as ''scalene,'' with all three J values different. We find that, for at least one of the two sets of metal ion clusters, the scalene case applies, with J values of - 11.5 +/- 0.2, -10.5 +/- 0.2, and -9.7 +/- 0.2 cm-1. Using appropriate linear combinations of J values, analogous to symmetry coordinates for a triatomic molecule, these values can be regarded as being midway between the two limiting isosceles cases. With these parameters, the Heisenberg spin-only coupling model accounts well for the energies and intensities of the magnetic transitions.