(b)
Figure 4. Relationship
between interaction energy and the geometrical parameters in
R-Ph-CN…PO2F complexes, including (a) the
N…P intermolecular distance r(N…P), and (b) the dihedral
angle F-P-O-O.
3.1.3 AIM and NBO analyses
The quantum theory of AIM[52] is useful to
analyze various intermolecular interactions. The N…P pnicogen
bond is characterized by the existence of an intermolecular BCP between
N atom of R-Ph-CN and the P atom of PO2F. Values of the
corresponding topological parameters are gathered in Table 3. The
Laplacian ∇2ρ of the BCPs are all positive, and this
indicates a depletion of the electron density, which is common in
closed-shell kind of interactions in the complexes[53]. It is known
that the signs of ∇2ρBCP and
HBCP can be used to characterize the strength of
intermolecular interaction and describe the covalent properties of a
bond [54]. In the
complexes studied herein, the positive
∇2ρBCP and negative
HBCP at the P…N BCP in all cases indicate that
the P…N pnicogen bonding can be classified as medium strength
interaction with partially covalent characteristics[55]. It is
observed from Table 3 that the value of HBCP become more
negative, as the interaction energy of
R-Ph-CN…PO2F increases, indicating the more
covalent character in the P…N interactions. As seen in Figure 5,
there is a good linear relationship between binding distance
r(N…P) and bond critical point electron density
(ρN…P).
Therefore, it is accepted that the electron density at the BCPs is a
good measurement to estimate the strength of the pnicogen bond of the
complexes we studied. Similar results have been observed in some
H-bonded complexes, but it is the logarithm of the electron density that
participated in the linear relationship[56].
Table 3. The AIM topological parameters at the N…P BCPs
and the natural bond orbital analysis of the
R-Ph-CN…PO2F binary complexes.