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).