17.1 Introduction
Non-covalent interactions play a crucial role in nature, because they are responsible
for the organization of practically all molecular assemblies, either of natural origin
like DNA, or of artificial origin, such as the so-called metal-organic-frameworks
(MOFs) [1]. They hold together the so-called ‘‘soft materials’’, characterized by
binding energies of the order of the thermal energy. This definition includes
polymers, colloids and surfactants [1, 2]. Hence, not surprisingly, the detailed
characterization of the noncovalent interactions turns out to be essential for the
development of new molecular materials. In particular, the last two decades have
witnessed a massive development of new materials of potential interest for technological applications based on molecular assemblies [3, 4]. Over the same period,
the interest on non-covalent interactions has continuously increased, leading to the
characterization of a large variety of them, differing in energy or nature [5]
Remarkably enough, some of them are almost as strong as conventional covalent
linkages [6], thus going beyond the conventional view which describes the noncovalent interactions as weak interactions between closed-shell systems with no
electron sharing. The most paradigmatic example is the inter- and intramolecular
hydrogen bond; but after its early recognition, similar closed-shell interactions, such
as the dihydrogen [6, 7], halogen [8–10], pnicogen [11–16], chalcogen [17, 18],
beryllium bonds [19, 20] or tetrel interactions [21–23] have been found to contribute to the stability of different molecular assemblies.
It is worth noting that, in this astonishing development, modeling based on the
use of quantum chemistry techniques has played a crucial role. Modeling helps to
save huge amounts of money by providing clues to understand the mechanisms
involved in the material activity and opens the possibility of designing new
materials, with better performances, in very powerful synergy with experiment
[24–35].
One of the signatures of the non-covalent interactions mentioned above is the
polarization of the electron density of the closed-shell system behaving as a Lewis
base towards the closed-shell moiety which acts as a Lewis acid. Again hydrogen
bonds AH···B constitute a paradigmatic example, in which there is an interaction
between the lone-pair of the hydrogen bond acceptor B towards the σAH* antibonding molecular orbital of the hydrogen bond donor [36]. This electronic density
redistribution is mirrored in the lengthening of the A–H bond and the red shift of
the A–H stretching frequency, which characterize this kind of non-covalent interactions [37]. This mechanism reaches its maximum intensity in the so-called dative
bonds, in which a significant charge transfer from the Lewis base towards the Lewis
acid takes place. The difference between dative bonds and the other much weaker
non-covalent interactions mentioned above is essentially quantitative rather than
qualitative.
462
K. Mykolayivna Lemishko et al.
Non-covalent interactions play a crucial role in nature, because they are responsible
for the organization of practically all molecular assemblies, either of natural origin
like DNA, or of artificial origin, such as the so-called metal-organic-frameworks
(MOFs) [1]. They hold together the so-called ‘‘soft materials’’, characterized by
binding energies of the order of the thermal energy. This definition includes
polymers, colloids and surfactants [1, 2]. Hence, not surprisingly, the detailed
characterization of the noncovalent interactions turns out to be essential for the
development of new molecular materials. In particular, the last two decades have
witnessed a massive development of new materials of potential interest for technological applications based on molecular assemblies [3, 4]. Over the same period,
the interest on non-covalent interactions has continuously increased, leading to the
characterization of a large variety of them, differing in energy or nature [5]
Remarkably enough, some of them are almost as strong as conventional covalent
linkages [6], thus going beyond the conventional view which describes the noncovalent interactions as weak interactions between closed-shell systems with no
electron sharing. The most paradigmatic example is the inter- and intramolecular
hydrogen bond; but after its early recognition, similar closed-shell interactions, such
as the dihydrogen [6, 7], halogen [8–10], pnicogen [11–16], chalcogen [17, 18],
beryllium bonds [19, 20] or tetrel interactions [21–23] have been found to contribute to the stability of different molecular assemblies.
It is worth noting that, in this astonishing development, modeling based on the
use of quantum chemistry techniques has played a crucial role. Modeling helps to
save huge amounts of money by providing clues to understand the mechanisms
involved in the material activity and opens the possibility of designing new
materials, with better performances, in very powerful synergy with experiment
[24–35].
One of the signatures of the non-covalent interactions mentioned above is the
polarization of the electron density of the closed-shell system behaving as a Lewis
base towards the closed-shell moiety which acts as a Lewis acid. Again hydrogen
bonds AH···B constitute a paradigmatic example, in which there is an interaction
between the lone-pair of the hydrogen bond acceptor B towards the σAH* antibonding molecular orbital of the hydrogen bond donor [36]. This electronic density
redistribution is mirrored in the lengthening of the A–H bond and the red shift of
the A–H stretching frequency, which characterize this kind of non-covalent interactions [37]. This mechanism reaches its maximum intensity in the so-called dative
bonds, in which a significant charge transfer from the Lewis base towards the Lewis
acid takes place. The difference between dative bonds and the other much weaker
non-covalent interactions mentioned above is essentially quantitative rather than
qualitative.
462
K. Mykolayivna Lemishko et al.