  The molecular dynamics in the ambient-pressure solid phase of
(1,1,2)-trichloroethane is studied by means of broadband dielectric
spectroscopy and molecular dynamics simulations. The dielectric spectra of
polycrystalline samples obtained by crystallization from the liquid phase
exhibit, besides a space-charge relaxation associated with accumulation of
charges at crystalline domain boundaries, two loss features arising from
dipolar molecular relaxations. The most prominent and slower of the two loss
features is identified as a configurational leap of the molecules which
involves a simultaneous change in spatial orientation and structural
conformation, namely between two isomeric forms (gauche$^+$ and gauche$^-$) of
opposite chirality. In this peculiar dynamic process, the positions of the
three chlorine atoms in the crystal lattice remain unchanged, while those of
the carbon and hydrogen atoms are modified. This dynamic process is responsible
for the disorder observed in an earlier x-ray diffraction study and confirmed
by our simulation, which is present only at temperatures relatively close to
the melting point, starting 40 K below. The onset of the disorder is visible as
an anomaly in the temperature dependence of the dc conductivity of the sample
at exactly the same temperature. While the slower relaxation dynamics (combined
isomerization/reorientation) becomes increasingly more intense on approaching
the melting point, the faster dynamics exhibits significantly lower but
constant dielectric strength. Based on our molecular dynamics simulations, we
assign the faster relaxation to large fluctuations of the molecular dipole
moments, partly due to large-angle librations of the chloroethane species.
