constantly and endlessly growing in almost all these related fields, so that the bibliography for this chapter cannot obviously be exhaustive.
Experimentally, they can be characterized using all flavours of spectroscopy
[29–38], while an equal huge number of papers [39–60] has been devoted to the
theoretical understanding and rationalization of these particular interactions, using
the whole computational and analysis arsenal developed over the past decades
(density functional theory (DFT), post Hartree-Fock methods, molecular orbital
theory, conceptual DFT, valence bond theory, energy decomposition, quantum
chemical topology…). For the very same reasons as those when dealing with
experimental progress, our references list about theoretical advances is far from
being comprehensive.
All these plethoric works have been in particular instrumental in the advent of
the retained definition of the halogen bond by the International Union of Pure and
Applied Chemistry (IUPAC): “A halogen bond occurs when there is evidence of a
net attractive interaction between an electrophilic region associated with a halogen
atom in a molecular entity and a nucleophilic region in another, or the same,
molecular entity” [1].
In a series of recent articles, we notably investigated the properties of halogen
bonds, and more generically of interactions between halogens and electronegative
atoms, whatever in intramolecular or intermolecular fashion [61–68], from the
electron density point of view (through conceptual DFT or Bader’s
atoms-in-molecules theory (QTAIM) [69, 70], which are complementary approaches, grounded on the same key-ingredient). Let us recall that the electron density is
the primary and fundamental observable of any chemical system and that it can be
obtained not only from theoretical calculations but also experimentally thanks to
X-ray diffraction. In this chapter, we will mainly focus on the QTAIM description
of halogen bond formation, that-is-to say we will follow selected QTAIM
descriptors along the reaction path leading to possible halogen-bond stabilized
complexes.
16.2 Theory
16.2.1 Basics of QTAIM: Atomic Basins and Energy
Decomposition
Bader’s atoms-in-molecules theory [69, 70] is based on the topology of the electron
density q ~ r
ð Þ and enables to partition the three-dimensional real space into
non-overlapping domains called “basins”. To achieve such separation, one looks at
the so-called gradient paths (GPs), which are the equivalent of field lines in classical
electromagnetism. A GP is defined as a curve C such that the electron density
gradient ~
rq ~ r
ð Þ (which can stem from theoretical calculations as well as from X-ray
diffraction experiments) is tangent to C at every C point. In general (non-nuclear
436
V. Tognetti and L. Joubert
Experimentally, they can be characterized using all flavours of spectroscopy
[29–38], while an equal huge number of papers [39–60] has been devoted to the
theoretical understanding and rationalization of these particular interactions, using
the whole computational and analysis arsenal developed over the past decades
(density functional theory (DFT), post Hartree-Fock methods, molecular orbital
theory, conceptual DFT, valence bond theory, energy decomposition, quantum
chemical topology…). For the very same reasons as those when dealing with
experimental progress, our references list about theoretical advances is far from
being comprehensive.
All these plethoric works have been in particular instrumental in the advent of
the retained definition of the halogen bond by the International Union of Pure and
Applied Chemistry (IUPAC): “A halogen bond occurs when there is evidence of a
net attractive interaction between an electrophilic region associated with a halogen
atom in a molecular entity and a nucleophilic region in another, or the same,
molecular entity” [1].
In a series of recent articles, we notably investigated the properties of halogen
bonds, and more generically of interactions between halogens and electronegative
atoms, whatever in intramolecular or intermolecular fashion [61–68], from the
electron density point of view (through conceptual DFT or Bader’s
atoms-in-molecules theory (QTAIM) [69, 70], which are complementary approaches, grounded on the same key-ingredient). Let us recall that the electron density is
the primary and fundamental observable of any chemical system and that it can be
obtained not only from theoretical calculations but also experimentally thanks to
X-ray diffraction. In this chapter, we will mainly focus on the QTAIM description
of halogen bond formation, that-is-to say we will follow selected QTAIM
descriptors along the reaction path leading to possible halogen-bond stabilized
complexes.
16.2 Theory
16.2.1 Basics of QTAIM: Atomic Basins and Energy
Decomposition
Bader’s atoms-in-molecules theory [69, 70] is based on the topology of the electron
density q ~ r
ð Þ and enables to partition the three-dimensional real space into
non-overlapping domains called “basins”. To achieve such separation, one looks at
the so-called gradient paths (GPs), which are the equivalent of field lines in classical
electromagnetism. A GP is defined as a curve C such that the electron density
gradient ~
rq ~ r
ð Þ (which can stem from theoretical calculations as well as from X-ray
diffraction experiments) is tangent to C at every C point. In general (non-nuclear
436
V. Tognetti and L. Joubert