No abstract

EPR spectra have been recorded in very high fields up to 25 T, and at high frequency up to 525 GHz, on a polycrystalline sample of ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔•2CH 3 COOH•4H 2 O ͑Mn12ac͒, a molecular cluster behaving like a nanomagnet. The simulation of the spectra has provided an accurate determination of the parameters of the spin HamiltonianThe presence of the fourth-order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to quantum tunneling between degenerate M states of the ground Sϭ10 multiplet of the cluster. The term in (S ϩ 4 ϩS Ϫ 4 ) is responsible for the transverse magnetic anisotropy and plays a crucial role in the mechanism of quantum tunneling. The highfrequency-EPR spectra have shown its presence and quantified it. ͓S0163-1829͑97͒08237-4͔

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me(6)tren)Cl](ClO(4)) (1) and [Ni(Me(6)tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental D(expt) value (energy difference between the M(s) = ± 1 and M(s) = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm(-1). The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M(s) = ± 1 and M(s) = 0 components of approximately -600 cm(-1). Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D(theor) between -100 and -200 cm(-1)) is obtained.