Single molecule magnets:: High frequency electron paramagnetic resonance study of two isomeric forms of an Mn12 molecule

Different crystallographic forms of the single molecule magnet [Mn12O12(O2CR)(16)(H2O)(4)] (complex 1) with a given R substituent have been isolated. The two different isomeric forms of the p-methylbenzoate complex crystallize as [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)].(HO2CC6H4-p-Me) (complex 2) and [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)].3H(2)O (complex 3). In complex 2, one Mn-III ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 2 and 3 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 2 compared with that of complex 3. Complex 2 likely has a larger tunneling matrix element and this explains why this complex shows an out-of-phase ac peak (chi(M)) in the signal in the 2-3 K region, whereas complex 3 has its chi(M) peak in the 4-7 K range, i.e., the rate of tunneling of magnetization is greater in complex 2 than complex 3. High frequency electron paramagnetic resonance (HFEPR) experiments were performed on both isomers. Computed simulations of the experimental HFEPR data yield spin Hamiltonian parameters for both complexes. (C) 2002 American Institute of Physics.