  The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its
role in causing anomalous dielectric properties are investigated. Normal
coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs)
is employed to distinguish disorder contributions from classical and
quantum-mechanical vibrational contributions. The analysis relies on
multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational
frequencies extracted from the temperature dependence of the ADPs are in good
agreement with THz spectroscopic data. Aspects of the static disorder revealed
by this work, primarily tilting and displacement of the molecules, are compared
with corresponding results from previous, much more in-depth and time-consuming
Monte Carlo simulations; their salient findings are reproduced by this work,
demonstrating that the faster NCA approach provides reliable constraints for
the interpretation of diffuse scattering. The dielectric properties of PCNB can
thus be rationalized by an interpretation of the temperature-dependent ADPs in
terms of thermal motion and molecular disorder. The use of atomic displacement
parameters in the NCA approach is nonetheless hostage to reliable neutron data.
The success of this study demonstrates that state-of-the-art single-crystal
Laue neutron diffraction affords sufficiently fast the accurate data for this
type of study. In general terms, the validation of this work opens up the field
for numerous studies of solid-state molecular disorder in organic materials.
