  The variation with microwave frequency and temperature of previously reported
anomalous peaks in the EPR spectra of Mn12-acetate, under large transverse
fields, reveals that the molecular easy magnetization axes are tilted with
respect to the global symmetry direction. More importantly, on the basis of the
angle-dependence of fine structures observed in the EPR spectra we infer that
the tilt distribution must be discrete, as was previously suspected from
studies which demonstrated the presence of a locally varying rhombic anisotropy
[S. Hill et al., Phys. Rev. Lett. 90, 217204 (2003)]. The tilts are confined to
two orthogonal planes, and the distribution extends up to ~1.7 degrees away
from the the global easy (z-) axis. We ascribe the tilting to the
hydrogen-bonding effect associated with the disordered acetic acid solvent
molecules. The effect is considerably larger than deduced from x-ray
diffraction analyses. These data constitute the sought-after evidence for the
presence of transverse fields in Mn12-acetate, and provide a possible
explanation for the lack of selection rules in the resonant quantum tunneling
behavior seen in low-temperature hysteresis experiments for this S = 10 system.
