Chemistry Reference
In-Depth Information
1.3 Intermolecular interactions
Themolecular packing ofMOMs results froma precise and subtle balance of several
intermolecular interactions within a narrow cohesion energy range of less than 1
eV molec
−
1
. This is the reason why crystal engineering is so powerful because this
balance can be intentionally modified but at the same time it implies that MOMs
are soft materials and that polymorphism is favoured. Detailed descriptions on
the fundamentals of interatomic and intermolecular interactions can be found in
many topics (see e.g., Kitaigorodskii, 1961). Here we briefly describe the relevant
interactions for MOMs and give a new approach supported on the nanoscience
perspective.
The interactions involved in the formation of molecular organic solids are of
dipole-dipole, hydrogen bonding and
-overlap types. When a permanent polar
molecule induces a dipole in a non-polar (but polarizable) molecule or when charge
fluctuation in a non-polar molecule induces a dipole in a non-polar surrounding
molecule, there is a weak attractive interaction between them, known as van der
Waals-London dispersion forces. The energies involved are typically less than
0.2 eV molec
−
1
and the interaction ranges are short, since forces vary as
D
−
6
,
where
D
represents the interatomic/intermolecular distances. The energies involved
in permanent dipole-permanent dipole interactions, known as Keeson forces, are
∼
π
5 eV molec
−
1
.
3
Hydrogen bonding is of electrostatic origin, where there is an attraction between
an H atom covalently bound to a donor D and a region with a high electronic
density over an atom A (D-H
0
.
A-X). Hydrogen-bonded architectures are abun-
dant in biological systems, which has motivated their exploitation in supramolec-
ular chemistry. Hydrogen bonding is responsible e.g., for the surprising physi-
cal properties of water and ice and for the DNA double-helix structure, which
is held together by N-H groups. Hydrogen bonds of the type (O-H
···
···
O) and
O) are usually termed
strong
(0.2-0.4 eV molec
−
1
) in order to dis-
tinguish them from the weaker (C-H
(N-H
···
···
···
π
) interactions (0.02-
0.2 eV molec
−
1
). The distinctive geometrical attributes of a hydrogen bond
D-H
O) and (O-H
A lengths, the hydrogen bond angle
D-H-A, the H-A-X angle and the planarity of the DHAX system. Typical values
for the H
···
A-X are the D
···
A and the H
···
···
A distance are 0.18-0.20 nm for N-H
···
O bonds and 0.16-0.18 nm
O bonds. D-H-A and H-A-X angles range around 150-160
◦
and
around 120-130
◦
, respectively.
for O-H
···
3
With the exception of He, solid noble gases crystallize in the face-centred-cubic Bravais lattice and their cohesive
energies (eV atom
−
1
) are 0.02 for Ne, 0.08 for Ar, 0.11 for Kr and 0.17 for Xe (Ashcroft & Mermin, 1976;
p. 401). Van der Waals-London forces are expressed by the Lennard-Jones potential
U
LJ
∝
C
R
D
−
12
−
C
A
D
−
6
,
where
C
R
and
C
A
stand for two constants multiplying the repulsive and attractive components, respectively.
