the BCP (or NNA) of the triple C ≡ C bond mimics the one-center Lewis base.
Since within the NBO approach the Lewis type structures are considered thus three
π CC → σ CH
* intermolecular orbital-orbital interactions were found here, each of the
energy amounting 0.4 kcal/mol and each corresponding to the overlap of CC πbond orbital of benzene and the antibonding CH orbital of acetylene. The high C 6v
structure symmetry results in the same electron charge distribution symmetry thus
as it was pointed out earlier here, there are six equivalent C…H bond paths between
acetylene and benzene as well as the equivalent charges for benzene atoms are
detected (NBO charges for C and H atoms are equal to −0.208 au and +0.209 au,
respectively; the corresponding charges for isolated benzene are equal to −0.204 au
and +0.204 au).
For the C 6 H 6 …HF complex there is the H…BCP bond path between the HF
molecule and the BCP of the CC bond of benzene; thus it is the similar situation to
that one occurring for the C 2 H 2 …HF complex. The interaction is stronger here than
for the complex of benzene with acetylene since the binding energy is equal to
−3.3 kcal/mol; the following characteristics were found for the BCP of the mentioned above H…BCP bond path; ρ BCP = 0.010 au and ∇
2
ρ BCP = 0.031 au. One can
see that the C 6v symmetry is broken here; two carbon atoms are distinguished since
the BCP of the path between them mimics the one-center Lewis base for this
complex. The charges of those carbon atoms in the C 6 H 6 –HF complex are the most
negative ones since they are equal to −0.225 au while the remaining C-charges
amount −0.204 au; besides the intermolecular H…C distances are smallest for these
carbons, 2.503 Å, while the remaining H…C (HF-benzene) distances amount 2.724
−2.929 Å. This means that the bond path between HF and C 6 H 6 molecules shows
the preferable H…C interactions. What is the reason of such an orientation of the
HF molecule in the C 6 H 6 –HF complex? Probably for this structure the electrostatic
interactions between benzene and HF are more attractive than in a case of the C 6v
symmetry complex. However the difference is not significant; the
MP2/aug-cc-pVTZ calculations show that the transition state C 6v symmetry C 6 H 6 –
HF complex is higher in energy than the corresponding complex being in the
energetic minimum only by 0.1 kcal/mol. The single π CC → σ CH
*
orbital-orbital
interaction is observed for the C 6 H 6 –HF complex with the interaction energy of
1.1 kcal/mol; the CC bond π-orbital corresponds here to the CC BCP which is
connected with the H-atom attractor of HF molecule (see Fig. 15.20).
Figure 15.20 presents also the C 5 H 5
−
…HF complex where the F–H…π
hydrogen bond is assisted by the negative charge. It results in the stronger C 5 H 5
− –
HF interaction since the binding energy is equal to −16.3 kcal/mol. The molecular
graph shows the single H…C bond path where the following characteristic of the
corresponding BCP were calculated; ρ BCP = 0.028 au and ∇
2
ρ BCP = 0.056 au. The
total electron energy density, H BCP , at this bond critical point is equal to -0.002 au
what may be explained by the partly covalent in nature corresponding H…C
interaction. Also in this case, similarly as it was found for the C 6 H 6 –HF complex,
the bond path shows the preferable H…C interaction since the charge of the corresponding C-atom of the C 5 H 5
− anion is equal to −0.433 au while for the
remaining carbons of the anion this charge is situated in a range between −0.350 au
424
S.J. Grabowski
Since within the NBO approach the Lewis type structures are considered thus three
π CC → σ CH
* intermolecular orbital-orbital interactions were found here, each of the
energy amounting 0.4 kcal/mol and each corresponding to the overlap of CC πbond orbital of benzene and the antibonding CH orbital of acetylene. The high C 6v
structure symmetry results in the same electron charge distribution symmetry thus
as it was pointed out earlier here, there are six equivalent C…H bond paths between
acetylene and benzene as well as the equivalent charges for benzene atoms are
detected (NBO charges for C and H atoms are equal to −0.208 au and +0.209 au,
respectively; the corresponding charges for isolated benzene are equal to −0.204 au
and +0.204 au).
For the C 6 H 6 …HF complex there is the H…BCP bond path between the HF
molecule and the BCP of the CC bond of benzene; thus it is the similar situation to
that one occurring for the C 2 H 2 …HF complex. The interaction is stronger here than
for the complex of benzene with acetylene since the binding energy is equal to
−3.3 kcal/mol; the following characteristics were found for the BCP of the mentioned above H…BCP bond path; ρ BCP = 0.010 au and ∇
2
ρ BCP = 0.031 au. One can
see that the C 6v symmetry is broken here; two carbon atoms are distinguished since
the BCP of the path between them mimics the one-center Lewis base for this
complex. The charges of those carbon atoms in the C 6 H 6 –HF complex are the most
negative ones since they are equal to −0.225 au while the remaining C-charges
amount −0.204 au; besides the intermolecular H…C distances are smallest for these
carbons, 2.503 Å, while the remaining H…C (HF-benzene) distances amount 2.724
−2.929 Å. This means that the bond path between HF and C 6 H 6 molecules shows
the preferable H…C interactions. What is the reason of such an orientation of the
HF molecule in the C 6 H 6 –HF complex? Probably for this structure the electrostatic
interactions between benzene and HF are more attractive than in a case of the C 6v
symmetry complex. However the difference is not significant; the
MP2/aug-cc-pVTZ calculations show that the transition state C 6v symmetry C 6 H 6 –
HF complex is higher in energy than the corresponding complex being in the
energetic minimum only by 0.1 kcal/mol. The single π CC → σ CH
*
orbital-orbital
interaction is observed for the C 6 H 6 –HF complex with the interaction energy of
1.1 kcal/mol; the CC bond π-orbital corresponds here to the CC BCP which is
connected with the H-atom attractor of HF molecule (see Fig. 15.20).
Figure 15.20 presents also the C 5 H 5
−
…HF complex where the F–H…π
hydrogen bond is assisted by the negative charge. It results in the stronger C 5 H 5
− –
HF interaction since the binding energy is equal to −16.3 kcal/mol. The molecular
graph shows the single H…C bond path where the following characteristic of the
corresponding BCP were calculated; ρ BCP = 0.028 au and ∇
2
ρ BCP = 0.056 au. The
total electron energy density, H BCP , at this bond critical point is equal to -0.002 au
what may be explained by the partly covalent in nature corresponding H…C
interaction. Also in this case, similarly as it was found for the C 6 H 6 –HF complex,
the bond path shows the preferable H…C interaction since the charge of the corresponding C-atom of the C 5 H 5
− anion is equal to −0.433 au while for the
remaining carbons of the anion this charge is situated in a range between −0.350 au
424
S.J. Grabowski