not only to see the interaction, but also to interpret its character and properties.
Different types of bonding can be revealed by various topological methods, each
based on different scalar fields [18]. This chapter deals with a new interpretative
tool, NCI (standing for Non-Covalent Interactions), for revealing non-covalent
interactions, which tries to bring some intuitive order into this fuzzy set of ideas.
18.1.1 Historical Framework
Over the years, different approaches have been developed to reveal chemical bonds.
Covalent bonds are intuitively represented using conventional Lewis structures [19].
Molecular Orbital (MO) theory has been very useful and successful for the theoretical
analysis of chemical reactions and chemical reactivity. The frontier orbital theory [20]
and the orbital symmetry rules of Woodward and Hoffman [21] are paradigmatic
examples of the possibilities of quantum chemistry within the MO theory.
To reduce the dimensionality of the problem, three-dimensional interpretative
approaches have been introduced. The conceptual density functional theory pioneered by Parr et al. [22] has been at the origin of very useful reactivity descriptors.
Another low dimensional approach has originally been developed by Bader: [23–
25] the topological approach. Within these approaches, 3D space is divided following the gradient of a scalar function into mutually disjoint regions. Bader’s
QTAIM (Quantum Theory of Atoms In Molecules) theory is based on the topological analysis of the density and provides an atomic picture of the system.
Chemical bonds and the underlying molecular graph may be traced by the analysis
of its bond critical points (BCPs, first order saddle points). Following the same
philosophy, ELF [26, 27] (Electron Localization Function) topological analysis
divides the space into chemically intuitive regions associated with electron pairs so
that electronic shells, bonds, and lone pairs are revealed. Also, purely electrostatic
interactions can be analyzed using electrostatic potential maps [28].
18.1.2 Weak Interactions
Chemical interactions between a protein and a drug, or a catalyst and its substrate,
self-assembly of nanomaterials [29, 30], and even some chemical reactions [31, 32],
are dominated by non-covalent interactions. This class of interactions spans a wide
range of binding energies, and encompasses hydrogen bonding, dipole-dipole
interactions and London dispersion [33] as well as more up to date interactions such
as halogen bonds, CH Á Á Á p and π Á Á Áπ interactions. Repulsive interactions, also
known as steric clashes, should not be disregarded either.
More specifically, non-covalent interactions are of paramount importance in
chemistry and especially in bio-disciplines [34, 35], since they set up the force field
scenario through which chemical species interact with each other without a significant electron sharing between them. They represent, in fact, the machinery
494
C. Narth et al.
Different types of bonding can be revealed by various topological methods, each
based on different scalar fields [18]. This chapter deals with a new interpretative
tool, NCI (standing for Non-Covalent Interactions), for revealing non-covalent
interactions, which tries to bring some intuitive order into this fuzzy set of ideas.
18.1.1 Historical Framework
Over the years, different approaches have been developed to reveal chemical bonds.
Covalent bonds are intuitively represented using conventional Lewis structures [19].
Molecular Orbital (MO) theory has been very useful and successful for the theoretical
analysis of chemical reactions and chemical reactivity. The frontier orbital theory [20]
and the orbital symmetry rules of Woodward and Hoffman [21] are paradigmatic
examples of the possibilities of quantum chemistry within the MO theory.
To reduce the dimensionality of the problem, three-dimensional interpretative
approaches have been introduced. The conceptual density functional theory pioneered by Parr et al. [22] has been at the origin of very useful reactivity descriptors.
Another low dimensional approach has originally been developed by Bader: [23–
25] the topological approach. Within these approaches, 3D space is divided following the gradient of a scalar function into mutually disjoint regions. Bader’s
QTAIM (Quantum Theory of Atoms In Molecules) theory is based on the topological analysis of the density and provides an atomic picture of the system.
Chemical bonds and the underlying molecular graph may be traced by the analysis
of its bond critical points (BCPs, first order saddle points). Following the same
philosophy, ELF [26, 27] (Electron Localization Function) topological analysis
divides the space into chemically intuitive regions associated with electron pairs so
that electronic shells, bonds, and lone pairs are revealed. Also, purely electrostatic
interactions can be analyzed using electrostatic potential maps [28].
18.1.2 Weak Interactions
Chemical interactions between a protein and a drug, or a catalyst and its substrate,
self-assembly of nanomaterials [29, 30], and even some chemical reactions [31, 32],
are dominated by non-covalent interactions. This class of interactions spans a wide
range of binding energies, and encompasses hydrogen bonding, dipole-dipole
interactions and London dispersion [33] as well as more up to date interactions such
as halogen bonds, CH Á Á Á p and π Á Á Áπ interactions. Repulsive interactions, also
known as steric clashes, should not be disregarded either.
More specifically, non-covalent interactions are of paramount importance in
chemistry and especially in bio-disciplines [34, 35], since they set up the force field
scenario through which chemical species interact with each other without a significant electron sharing between them. They represent, in fact, the machinery
494
C. Narth et al.