These electron density redistributions lead to one of the most interesting characteristics that non-covalent interactions evidenced in recent years: their capability
to modulate the intrinsic properties of the interacting subunits by changing their
donor or acceptor electron capacities [38–40]. In general, the Lewis base has its
intrinsic acidity increased after the formation of the complex, due to the charge
transfer towards the Lewis acid, whose basicity concomitantly increases too.
Depending on the intensity of this charge transfer mechanism, the increase of the
intrinsic acidity of the base can be dramatic. Typical bases like aniline, for instance,
become stronger Bronsted acids than phosphoric acid, whereas 1H-tetrazole
becomes a nitrogen Bronsted acid stronger than perchloric acid [39]. One important
consequence of these acidity enhancements is the possibility of having spontaneous
proton transfers between a Lewis base and a Lewis acid, with the spontaneous
formation of an ion-pair in the gas phase [41, 42], or the formation of polymeric
structures for ditopic monomer presenting one end that acts as a Lewis base
whereas the other acts a Lewis acid [43]. Another signature of the non-covalent
interactions is cooperativity (or anticooperativity), that is the reinforcement (or
weakening) of non-covalent interactions when two or more functionalities coincide
within the same molecular assembly, and is closely related with the aforementioned
electron density redistributions [44–75].
Therefore, the analysis of the electron density distribution of the molecular
aggregates held together through non-covalent interaction can provide valuable
information on both their properties and how these change with respect to the
isolated building blocks. This analysis can be carried out by means of different
techniques, the most popular of which are the Quantum Theory of Atoms in
Molecules (QTAIM) theory [76, 77], the Electron Localization Function
(ELF) [78–80] and the Natural Bond Orbital (NBO) method [81].
The QTAIM approach relies on a topological analysis of the electron density,
ρ(r), allowing to build up the molecular graphs as the ensemble of the critical points
of ρ(r). These include maxima associated with the position of the nuclei, bond and
ring critical points associated with the existence of a bond between two atoms or a
ring, and the bond paths, defined as lines of maximum density which connect two
maxima through a first-order saddle point called bond critical point (BCP). In
general, the electron density at the BCPs offers direct information on the strength of
the bond and the values of the so-called energy density provides information about
the covalent or electrostatic nature of the interaction.
The ELF theory allows one to divide the physical space into regions dominated
by an opposite spin pair or by a single electron. These electronic domains can be
seen as a generalization of the ideas of Lewis, so that the valence shell of a
molecule consists of two types of basin: disynaptic (or polysynaptic) basins, which
belong to two (or more) atomic valence shells and the monosynaptic ones (typically
lone-pairs), which belong to only one valence shell, and therefore correspond to
nonbonding valence density. The basin populations also offer interesting information on the strength and characteristics of the bonds.
17 Charge Transfer in Beryllium Bonds and Cooperativity …
463
to modulate the intrinsic properties of the interacting subunits by changing their
donor or acceptor electron capacities [38–40]. In general, the Lewis base has its
intrinsic acidity increased after the formation of the complex, due to the charge
transfer towards the Lewis acid, whose basicity concomitantly increases too.
Depending on the intensity of this charge transfer mechanism, the increase of the
intrinsic acidity of the base can be dramatic. Typical bases like aniline, for instance,
become stronger Bronsted acids than phosphoric acid, whereas 1H-tetrazole
becomes a nitrogen Bronsted acid stronger than perchloric acid [39]. One important
consequence of these acidity enhancements is the possibility of having spontaneous
proton transfers between a Lewis base and a Lewis acid, with the spontaneous
formation of an ion-pair in the gas phase [41, 42], or the formation of polymeric
structures for ditopic monomer presenting one end that acts as a Lewis base
whereas the other acts a Lewis acid [43]. Another signature of the non-covalent
interactions is cooperativity (or anticooperativity), that is the reinforcement (or
weakening) of non-covalent interactions when two or more functionalities coincide
within the same molecular assembly, and is closely related with the aforementioned
electron density redistributions [44–75].
Therefore, the analysis of the electron density distribution of the molecular
aggregates held together through non-covalent interaction can provide valuable
information on both their properties and how these change with respect to the
isolated building blocks. This analysis can be carried out by means of different
techniques, the most popular of which are the Quantum Theory of Atoms in
Molecules (QTAIM) theory [76, 77], the Electron Localization Function
(ELF) [78–80] and the Natural Bond Orbital (NBO) method [81].
The QTAIM approach relies on a topological analysis of the electron density,
ρ(r), allowing to build up the molecular graphs as the ensemble of the critical points
of ρ(r). These include maxima associated with the position of the nuclei, bond and
ring critical points associated with the existence of a bond between two atoms or a
ring, and the bond paths, defined as lines of maximum density which connect two
maxima through a first-order saddle point called bond critical point (BCP). In
general, the electron density at the BCPs offers direct information on the strength of
the bond and the values of the so-called energy density provides information about
the covalent or electrostatic nature of the interaction.
The ELF theory allows one to divide the physical space into regions dominated
by an opposite spin pair or by a single electron. These electronic domains can be
seen as a generalization of the ideas of Lewis, so that the valence shell of a
molecule consists of two types of basin: disynaptic (or polysynaptic) basins, which
belong to two (or more) atomic valence shells and the monosynaptic ones (typically
lone-pairs), which belong to only one valence shell, and therefore correspond to
nonbonding valence density. The basin populations also offer interesting information on the strength and characteristics of the bonds.
17 Charge Transfer in Beryllium Bonds and Cooperativity …
463