Magnetization barrier reduction in Mn$_{12}$ single-molecule magnets

High-frequency electron paramagnetic resonance (HFEPR) and AC susceptibility data will be presented for a new high-symmetry Mn$_{12}${\-}Ac complex, [Mn$_{12}$O$_{12}$(OAc)$_{16}$(MeOH)$_{4}$]$_{~}\cdot $~MeOH, in which the acetic acid solvent is replaced by a single methanol. The results are compared with those of several other Mn$_{12}$ single-molecule magnets (SMMs), including Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. AC susceptibility studies indicate that Mn$_{12}${\-}Ac$_{~}\cdot _{~}$MeOH has a relatively large effective barrier, $U_{eff}$~$\sim $~74 K, in comparison to Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. Meanwhile, EPR studies suggest more-or-less identical zero-field-splitting parameters for the two complexes. Based on these findings, we discuss the factors that can lead to reductions in $U_{eff}$ in various Mn$_{12}$ SMMs.