  New Mn12 single-molecule magnets (SMM's) with the composition
[Mn12O12(O2CR)16(H2O)4] were prepared and structurally characterized. Complexes
of this type can exhibit two out-of-phase ac magnetic susceptibility signals,
one in the 4-7 K region and the other in the 2-3 K region. The origin of the
two magnetization relaxation processes was systematically examined. The X-ray
structures of two isomeric forms of a Mn12 SMM are given. One complex has an
abnormal Jahn-Teller distortion axis. The magnetization hysteresis loops for
the two isomers are quite different.
