  High-field electron paramagnetic resonance (HFEPR) spectra were collected at
several frequencies for single crystal [Zn3.91Ni0.09(hmp)4(dmb)4Cl4] (1), where
dmb is 3,3-dimethyl-1-butanol and hmp is the monoanion of
2-hydroxymethylpyridine. This crystal is isostructural to [Ni4(hmp)4(dmb)4Cl4]
(2), which has been characterized to be a single-molecule magnet (SMM) with
fast quantum tunneling of its magnetization (QTM). The single NiII ion
zero-field-splitting (zfs) parameters D [= -5.30(5) cm-1] and E [= +/-1.20(2)
cm-1] in the doped complex 1 were evaluated by rotation of a crystal in three
planes. The easy-axes of magnetization associated with the single-ion zfs
interactions were also found to be tilted 15 degrees away from the
crystallographic c-direction. This inclination provides a possible explanation
for the fast QTM observed for complex 2. The single-ion zfs parameters are then
related to the zfs parameters for the Ni4 molecule by irreducible tensor
methods to give D = -0.69 cm-1 for the S = 4 ground state of the SMM, where the
axial zfs interaction is given by DSz2.
