  High-frequency electron paramagnetic resonance (HFEPR) and AC susceptibility
measurements are reported for a new high-symmetry Mn12 complex,
[Mn12O12(O2CCH3)16(CH3OH)4].CH3OH. The results are compared with those of other
high-symmetry spin S = 10 Mn12 single-molecule magnets (SMMs), including the
original acetate, [Mn12(O2CCH3)16(H2O)4].2CH3CO2H.4H2O, and the
[Mn12O12(O2CCH2Br)16(H2O)4].4CH2Cl2 & [Mn12O12(O2CCH2But)16(CH3OH)4].CH3OH
complexes. These comparisons reveal important insights into the factors that
influence the values of the effective barrier to magnetization reversal, Ueff,
deduced on the basis of AC susceptibility measurements. In particular, we find
that variations in Ueff can be correlated with the degree of disorder in a
crystal which can be controlled by desolvating (drying) samples. This
highlights the importance of careful sample handling when making measurements
on SMM crystals containing volatile lattice solvents. The HFEPR data
additionally provide important spectroscopic evidence suggesting that the
relatively weak disorder induced by desolvation strongly influences the quantum
tunneling interactions, and that it is under-barrier tunneling that is
responsible for a consistent reduction in Ueff that is found upon drying
samples. Meanwhile, the axial anisotropy deduced from HFEPR is found to be
virtually identical for all four Mn12 complexes, with essentially no measurable
reduction upon desolvation.
