electrostatic potential for the H 2 O and C 6 H 5 F molecules calculated separately at
0.001 au electron density molecular surfaces. The distributions of the EP for both
species are in line with the observations presented earlier here and concerning the
electron charge distribution.
There is another interesting observation for the C 6 H 5 F–H 2 O complex
(Fig. 15.1). For all O–H, C–H and C–C covalent bonds one can see the continuous
regions of the negative laplacian surrounding the corresponding bond critical
points; with one exception of the C–F bond where there is the region of positive
laplacian between two corresponding nuclei; even the positive ∇
2
ρ BCP value is
detected for the C–F bond critical point. This may indicate that the C–F bond is
mostly ionic in nature and that there is the polarization of C–F bond with the
concentration of the electron charge density at the fluorine centre. The polarization
of C–F bond (the percentage of the electron charge density calculated at F-centre)
evaluated within NBO approach is equal to 72.8 %. The QTAIM integrated charges
of carbon and fluorine in this bond are equal to +0.472 au and −0.714 au,
respectively.
The aim of this chapter is to show, on the basis of several examples, how the
location of the bond path may be useful to characterize, define and/or verify the
specific, considered interaction. Mainly the QTAIM approach [4–7] is considered
here; however sometimes there are also references to other methods and concepts as
for example; the Natural Bond Orbitals (NBO) method [21, 22] or the σ-hole
concept [25–27]. This is worth to note that the results presented hereafter are mainly
based on the MP2/aug-cc-pVTZ level of calculations; those results are taken from
earlier studies or the calculations were carried out especially for the purposes of this
chapter. Consequently the QTAIM calculations were performed on the
MP2/aug-cc-pVTZ wave functions. The binding energies (E bin ’s) were calculated
as differences between the energy of the complex and the sum of energies of
monomers optimized separately and they were corrected for the basis set superposition error (BSSE) by the counterpoise method [28]. Since the NBO method is
based on the Hartree-Fock method thus the corresponding NBO results, i.e.
orbital-orbital interactions or atomic charges, if presented, are based on the
HF/aug-cc-pVTZ//MP2/aug-cc-pVTZ level. Hence there is rather not indicated the
level of calculations for the next systems discussed hereafter; unless the results
presented were obtained within other levels of calculations.
15.2 The Case of Halogen Bond
The halogen bond interaction is one of the most interesting phenomena analyzed
during the last few decades. It was found that the halogen atoms (designated later
here as X), especially if connected with carbon (in C–X bonds), often play a role of
electron acceptors (Lewis acid centers) interacting with the Lewis bases, i.e. with
the electron rich species. This seems to be strange since halogen atoms are commonly known as the electronegative centers. There were various trials to explain
404
S.J. Grabowski
0.001 au electron density molecular surfaces. The distributions of the EP for both
species are in line with the observations presented earlier here and concerning the
electron charge distribution.
There is another interesting observation for the C 6 H 5 F–H 2 O complex
(Fig. 15.1). For all O–H, C–H and C–C covalent bonds one can see the continuous
regions of the negative laplacian surrounding the corresponding bond critical
points; with one exception of the C–F bond where there is the region of positive
laplacian between two corresponding nuclei; even the positive ∇
2
ρ BCP value is
detected for the C–F bond critical point. This may indicate that the C–F bond is
mostly ionic in nature and that there is the polarization of C–F bond with the
concentration of the electron charge density at the fluorine centre. The polarization
of C–F bond (the percentage of the electron charge density calculated at F-centre)
evaluated within NBO approach is equal to 72.8 %. The QTAIM integrated charges
of carbon and fluorine in this bond are equal to +0.472 au and −0.714 au,
respectively.
The aim of this chapter is to show, on the basis of several examples, how the
location of the bond path may be useful to characterize, define and/or verify the
specific, considered interaction. Mainly the QTAIM approach [4–7] is considered
here; however sometimes there are also references to other methods and concepts as
for example; the Natural Bond Orbitals (NBO) method [21, 22] or the σ-hole
concept [25–27]. This is worth to note that the results presented hereafter are mainly
based on the MP2/aug-cc-pVTZ level of calculations; those results are taken from
earlier studies or the calculations were carried out especially for the purposes of this
chapter. Consequently the QTAIM calculations were performed on the
MP2/aug-cc-pVTZ wave functions. The binding energies (E bin ’s) were calculated
as differences between the energy of the complex and the sum of energies of
monomers optimized separately and they were corrected for the basis set superposition error (BSSE) by the counterpoise method [28]. Since the NBO method is
based on the Hartree-Fock method thus the corresponding NBO results, i.e.
orbital-orbital interactions or atomic charges, if presented, are based on the
HF/aug-cc-pVTZ//MP2/aug-cc-pVTZ level. Hence there is rather not indicated the
level of calculations for the next systems discussed hereafter; unless the results
presented were obtained within other levels of calculations.
15.2 The Case of Halogen Bond
The halogen bond interaction is one of the most interesting phenomena analyzed
during the last few decades. It was found that the halogen atoms (designated later
here as X), especially if connected with carbon (in C–X bonds), often play a role of
electron acceptors (Lewis acid centers) interacting with the Lewis bases, i.e. with
the electron rich species. This seems to be strange since halogen atoms are commonly known as the electronegative centers. There were various trials to explain
404
S.J. Grabowski