Chapter 9
Molecular Flexibility and Material
Properties
9.1 Single-Particle or Extended Scheme
Among molecular deformation, the easiest is the twisting around a single bond, which
mostly has the axial symmetry. The reorientation of methyl groups has long been
a subject of extensive studies as typical dynamics capable in condensed states [1].
Most experimental results have successfully been analyzed within a single-particle
scheme. Namely, reasonable descriptions are possible of the dynamics as a motion
of the entity that senses an averaged potential, a kind of mean-field (molecular-field).
The intramolecular potential dominates the potential while surrounding molecules
also contribute. It is, however, interesting to note that some literature claims the
necessity of an extended scheme [2].
When a twisting body is large and anisotropic like a phenyl ring, a characteristic
frequency becomes smaller than 100 cm
−1 , a typical upper bound of the external
lattice vibrations in molecular crystals. The twisting degree strongly couples with
lattice degrees of freedom, as discussed in Sect. 6.3. Since the lattice vibration intrinsically exhibits the so-called dispersion, an extended scheme is necessary to correctly
describe the molecular deformation.
9.2 Glass Transitions
The intramolecular motional degrees can be involved in phase transitions or glass
transitions. The example introduced in Sect. 8.1.2 concerns the conformational
freezing in organic conductors. As other examples of a glass transition arising
from the freezing-in of an intramolecular motion, we can list those in crystals of
trans-azobenzene (C 6 H 5 –N=N–C 6 H 5 ), trans-stilbene (C 6 H 5 –CH=CH–C 6 H 5 ), and
related charge-transfer compounds [3–5]. The orientational disorder is reported for
© The Editor(s) (if applicable) and The Author(s), under exclusive license
to Springer Nature Singapore Pte Ltd. 2020
K. Saito, Chemical Physics of Molecular Condensed Matter,
Lecture Notes in Chemistry 104,
https://doi.org/10.1007/978-981-15-9023-8_9
177
Molecular Flexibility and Material
Properties
9.1 Single-Particle or Extended Scheme
Among molecular deformation, the easiest is the twisting around a single bond, which
mostly has the axial symmetry. The reorientation of methyl groups has long been
a subject of extensive studies as typical dynamics capable in condensed states [1].
Most experimental results have successfully been analyzed within a single-particle
scheme. Namely, reasonable descriptions are possible of the dynamics as a motion
of the entity that senses an averaged potential, a kind of mean-field (molecular-field).
The intramolecular potential dominates the potential while surrounding molecules
also contribute. It is, however, interesting to note that some literature claims the
necessity of an extended scheme [2].
When a twisting body is large and anisotropic like a phenyl ring, a characteristic
frequency becomes smaller than 100 cm
−1 , a typical upper bound of the external
lattice vibrations in molecular crystals. The twisting degree strongly couples with
lattice degrees of freedom, as discussed in Sect. 6.3. Since the lattice vibration intrinsically exhibits the so-called dispersion, an extended scheme is necessary to correctly
describe the molecular deformation.
9.2 Glass Transitions
The intramolecular motional degrees can be involved in phase transitions or glass
transitions. The example introduced in Sect. 8.1.2 concerns the conformational
freezing in organic conductors. As other examples of a glass transition arising
from the freezing-in of an intramolecular motion, we can list those in crystals of
trans-azobenzene (C 6 H 5 –N=N–C 6 H 5 ), trans-stilbene (C 6 H 5 –CH=CH–C 6 H 5 ), and
related charge-transfer compounds [3–5]. The orientational disorder is reported for
© The Editor(s) (if applicable) and The Author(s), under exclusive license
to Springer Nature Singapore Pte Ltd. 2020
K. Saito, Chemical Physics of Molecular Condensed Matter,
Lecture Notes in Chemistry 104,
https://doi.org/10.1007/978-981-15-9023-8_9
177