a ΔE bin,
ΔE int, and E def refers to
the total binding energy, interaction energy, and the deformation energy
of the binary complex obtained at the B2PLYP-D3/jun-cc-pVTZ level of
theory. Energies are given with the BSSE correction.
The interaction energy (ΔE int) and the binding
energy (ΔE bin) of
Ph-CN…PO2F amount to -26.85 and -19.25 kcal/mol,
respectively, which is stronger than those of
HCN…PO2F complex (-16.74 and -12.69 kcal/mol)
obtained at the same level (B2PLYP-D3/jun-cc-pVTZ). This is also
consistent with the negative MEPs on the N end of HCN (-32.44 kcal/mol)
and that of Ph-CN (-38.98 kcal/mol) calculated at B3LYP-D3/6-311+G(d,p)
level, which may be attributed to the conjugative effect of the phenyl
ring in Ph-CN. Although the actual values of
ΔE int and ΔE bin are
different for the studied complexes, the order of the relative energy
for the complexes is same, with the stability of the binary complexes
increases in the order of
CN<Br≈Cl<F<H<CH3<NH2.
This could possibly be explained by the fact that the nature of the
substituent attached to the ring influence the electron density on the
ring, thus affecting the binding energies of the complexes. The
NH2 group is a strong electron-donating substituent
through positive resonance effect, and has a larger enhancing effect
than the weak electron-donating CH3 group which has both
positive induction effect and hyperconjugation effect. The halogen
substituents (F, Cl, Br), however, exert a negative induction effect and
positive resonance effect to the R-Ph-CN molecule, and the calculated
N…P distance and interaction energies indicate that the former is
dominated for all the halogen substituents. All the halogen atoms are
shown as electron-withdrawing substituents and weaken the pnicogen bond
in comparison with Ph-CN…PO2F complexes. It is
noted that the order of Lewis basicity of R-Ph-CN
(Br<Cl<F) does not agree with the electronegativity
increase trend for halogen substituents. For example, for the
Cl-Ph-CN…PO2F and
F-Ph-CN…PO2F complexes, the stronger bonding
occurs for the later species. This may be arising from the larger
conjugative effect through the lone pair of the Cl atom with the π
electron of the aromatic ring, which results in the greater
Vs, min on the N atom of F-Ph-CN and larger interaction
in the F-Ph-CN…PO2F complexes.
By examining the interaction energies ΔE int and
the Vmin on the N atom of the substituted R-Ph-CN
derivatives, it can be seen that the strength of the interaction of the
binary complexes increases as the absolute value of Vmin,
N become larger. In fact, as shown in Figure 3, an acceptable linear
correlation is found between the ΔE int with
Vmin,N values with a coefficient of determination
R2=0.961, which suggests the electrostatically driven
nature of the pnicogen bonding interaction in stabilizing these
complexes. Besides, according to the geometrical analysis, a good linear
relationship exists between the binding energies
ΔE int and the N…P intermolecular distance
r(N…P) with R2=0.996, as can be seen in Figure
4(a).