meat curing. Nitrites in plant food have also been subject of research since their
presence in food has been thought to be associated with an increased risk of
gastrointestinal cancer and methemoglobinemia in infants [7].
Physico-chemical properties of the nitrite group are associated with the nature of
the chemical bonds binding the O atom to NO group and detailed knowledge of
their electronic structure is very important. The nature of chemical bonds is usually
studied in the Hilbert space within the molecular orbital theory. A more sophisticated approach, Quantum Chemical Topology [8], performed in the real space,
invokes such concepts of gradient dynamical theory as gradient path, critical point,
attractor and its basin. The most popular applications are: topological analysis of
electron density, ρ(r) proposed by Bader [9], topological analysis of Electron
Localisation Function (ELF), η(r) [10–15] and topological analysis of Electron
Localizability Indicator (ELI), ϒ
σ (r) [16, 17]. Our previous studies [18–28] have
shown that topological analysis is an effective tool for studying the nature of the
N–O (N–O1) bond in the –ONO (–O1–N=O2) group.
In order to understand diverse nature of N–O bonds in nitrites, a detailed
information about changes in bond topological structure during the N–O bond
dissociation is essential. Analysis of evolution of ELF field using topological
approach, during dissociation of covalent homopolar C–C bond in C 2 H 6 and
covalent dative N–B bond in H 3 BNH 3 , was reported by Krokidis et al. [29]. On the
basis of his research the Bonding Evolution Theory (BET) was formulated [30–32].
This methodology combines topological analysis of ELF and catastrophe theory of
Thom [33]. The reaction mechanism is analysed along the energy profile connecting the stationary points on PES using the intrinsic reaction coordinate (IRC) of
Fukui [34]. Along reaction pathway, a system experiences a series of structural
stability domains (SSDs) within all the critical points are hyperbolic and separated
by catastrophes where at least one critical point is non hyperbolic. The bifurcation
catastrophes are identified according to Thom’s classification [33] which gives
access to their unfolding, a compact polynomial expression which contains all the
information about how ELF may change as the control parameters change. This
way, a chemical reaction is viewed as a sequence of SSDs connected by catastrophes describing chemical events such as bond forming/breaking processes,
creation/annihilation of lone pairs and other types of electron pair rearrangements.
The results of BET analysis for FONO show four SSDs (I–IV), characterised by
different number of the local maxima (attractors) of ELF field. For example, the
N–O1 bond dissociation of the F–O1–N=O2 (syn) molecule studied at the
CCSD/aug-cc-pVTZ//CCSD(T)/aug-cc-pVTZ shows the topologies as presented in
Fig. 19.1. For short N–O1 distances only the bonding disynaptic attractor and basin
V(N,O1) are observed. Such an ELF-topology (I) corresponds to the ‘normal’
covalent bond. ‘Normal’ covalent bond is a chemical bond A–B, represented by the
disynaptic attractor V(A,B) in topological analysis of ELF, with any value of its
basin population. Further elongation of the N–O1 bond results in electron density
redistribution and the change of local properties of ELF in the region of valence
interaction between the N and O1 atoms. As a result, two non-bonding monosynaptic attractors are observed, instead of the single V(N,O1) attractor. These are: the
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S. Berski and A.J. Gordon
presence in food has been thought to be associated with an increased risk of
gastrointestinal cancer and methemoglobinemia in infants [7].
Physico-chemical properties of the nitrite group are associated with the nature of
the chemical bonds binding the O atom to NO group and detailed knowledge of
their electronic structure is very important. The nature of chemical bonds is usually
studied in the Hilbert space within the molecular orbital theory. A more sophisticated approach, Quantum Chemical Topology [8], performed in the real space,
invokes such concepts of gradient dynamical theory as gradient path, critical point,
attractor and its basin. The most popular applications are: topological analysis of
electron density, ρ(r) proposed by Bader [9], topological analysis of Electron
Localisation Function (ELF), η(r) [10–15] and topological analysis of Electron
Localizability Indicator (ELI), ϒ
σ (r) [16, 17]. Our previous studies [18–28] have
shown that topological analysis is an effective tool for studying the nature of the
N–O (N–O1) bond in the –ONO (–O1–N=O2) group.
In order to understand diverse nature of N–O bonds in nitrites, a detailed
information about changes in bond topological structure during the N–O bond
dissociation is essential. Analysis of evolution of ELF field using topological
approach, during dissociation of covalent homopolar C–C bond in C 2 H 6 and
covalent dative N–B bond in H 3 BNH 3 , was reported by Krokidis et al. [29]. On the
basis of his research the Bonding Evolution Theory (BET) was formulated [30–32].
This methodology combines topological analysis of ELF and catastrophe theory of
Thom [33]. The reaction mechanism is analysed along the energy profile connecting the stationary points on PES using the intrinsic reaction coordinate (IRC) of
Fukui [34]. Along reaction pathway, a system experiences a series of structural
stability domains (SSDs) within all the critical points are hyperbolic and separated
by catastrophes where at least one critical point is non hyperbolic. The bifurcation
catastrophes are identified according to Thom’s classification [33] which gives
access to their unfolding, a compact polynomial expression which contains all the
information about how ELF may change as the control parameters change. This
way, a chemical reaction is viewed as a sequence of SSDs connected by catastrophes describing chemical events such as bond forming/breaking processes,
creation/annihilation of lone pairs and other types of electron pair rearrangements.
The results of BET analysis for FONO show four SSDs (I–IV), characterised by
different number of the local maxima (attractors) of ELF field. For example, the
N–O1 bond dissociation of the F–O1–N=O2 (syn) molecule studied at the
CCSD/aug-cc-pVTZ//CCSD(T)/aug-cc-pVTZ shows the topologies as presented in
Fig. 19.1. For short N–O1 distances only the bonding disynaptic attractor and basin
V(N,O1) are observed. Such an ELF-topology (I) corresponds to the ‘normal’
covalent bond. ‘Normal’ covalent bond is a chemical bond A–B, represented by the
disynaptic attractor V(A,B) in topological analysis of ELF, with any value of its
basin population. Further elongation of the N–O1 bond results in electron density
redistribution and the change of local properties of ELF in the region of valence
interaction between the N and O1 atoms. As a result, two non-bonding monosynaptic attractors are observed, instead of the single V(N,O1) attractor. These are: the
530
S. Berski and A.J. Gordon