1.2 Intermolecular Interaction
21
molecules. In such cases, however, it is reminded that other contributions may intrinsically exist in the related potential. For example, the torsional (twisting) potential
of a biphenyl molecule (C 6 H 5 –C 6 H 5 ) is determined by not only the interactions
between non-bonded atoms expressed by the atom—atom potential (mainly of the
repulsion between ortho hydrogen atoms) but also the conjugation of π-electrons
through the central C–C bond.
1.2.5.3 Computer Learning
Both ways described in the previous two sections are based on theories even though
their levels of reliability may differ. Reflecting the significant improvement in computational power and the progress in information science (for artificial intelligence, i.e.,
AI), recently, an attempt to find interatomic potentials capable of reproducing input
data (such as crystal structures at various temperatures for many compounds) has
been reported [9]. The plots of resultant interatomic potentials against interatomic
separation are different from those proposed in other ways, such as the LennardJones and Buckingham-type potentials. Even a maximum can be recognized. Their
practical applicability for molecular materials is to be assessed in the future.
1.2.6 Other Interactions
1.2.6.1 Hydrogen Bond
Hydrogen bond (H-bond) is described as “a form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative
atom” [10]. The “definition” implies that the hydrogen bond is strongly related to
electrostatic interaction. This relationship is undoubtedly the case. However, the saturation in the number of the H-bond, suggests a partial character of valence bonds.
Depending on the separation between the electronegative atoms, the potential energy
curve takes a single-minimum or double-well form. Since the hydrogen nucleus is
the smallest in mass, the so-called quantum effect is most significant in H-bonds [11,
12].
5 Indeed, the substitution of hydrogen with deuteron results in a shift of location
closer to the nearby electronegative atom. Notable changes in properties of phase
transitions happen when the mechanism involves the dynamics or disordering of
hydrogen atoms [14, 15]. There exist compounds, the deuteration of which induces
a new phase transition [16, 17]. Conversely, the deuteration is useful in clarifying
the mechanism of structural phase transitions.
The strength (the energy gain) ranges from ca. a few to more than a hundred kJ
mol
−1 , which are in between those of intermolecular dispersion interaction and (nor5 The situation is the same in methanes, in which the phase relation is significantly altered by
deuteration [13], though the molecular symmetry also changes upon partial deuterations.
21
molecules. In such cases, however, it is reminded that other contributions may intrinsically exist in the related potential. For example, the torsional (twisting) potential
of a biphenyl molecule (C 6 H 5 –C 6 H 5 ) is determined by not only the interactions
between non-bonded atoms expressed by the atom—atom potential (mainly of the
repulsion between ortho hydrogen atoms) but also the conjugation of π-electrons
through the central C–C bond.
1.2.5.3 Computer Learning
Both ways described in the previous two sections are based on theories even though
their levels of reliability may differ. Reflecting the significant improvement in computational power and the progress in information science (for artificial intelligence, i.e.,
AI), recently, an attempt to find interatomic potentials capable of reproducing input
data (such as crystal structures at various temperatures for many compounds) has
been reported [9]. The plots of resultant interatomic potentials against interatomic
separation are different from those proposed in other ways, such as the LennardJones and Buckingham-type potentials. Even a maximum can be recognized. Their
practical applicability for molecular materials is to be assessed in the future.
1.2.6 Other Interactions
1.2.6.1 Hydrogen Bond
Hydrogen bond (H-bond) is described as “a form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative
atom” [10]. The “definition” implies that the hydrogen bond is strongly related to
electrostatic interaction. This relationship is undoubtedly the case. However, the saturation in the number of the H-bond, suggests a partial character of valence bonds.
Depending on the separation between the electronegative atoms, the potential energy
curve takes a single-minimum or double-well form. Since the hydrogen nucleus is
the smallest in mass, the so-called quantum effect is most significant in H-bonds [11,
12].
5 Indeed, the substitution of hydrogen with deuteron results in a shift of location
closer to the nearby electronegative atom. Notable changes in properties of phase
transitions happen when the mechanism involves the dynamics or disordering of
hydrogen atoms [14, 15]. There exist compounds, the deuteration of which induces
a new phase transition [16, 17]. Conversely, the deuteration is useful in clarifying
the mechanism of structural phase transitions.
The strength (the energy gain) ranges from ca. a few to more than a hundred kJ
mol
−1 , which are in between those of intermolecular dispersion interaction and (nor5 The situation is the same in methanes, in which the phase relation is significantly altered by
deuteration [13], though the molecular symmetry also changes upon partial deuterations.