Figure 5. The linear correlation between intermolecular distance r(N…P) and the electron density (ρb) at N…P bond critical point in R-Ph-CN…PO2F complexes.
The NBO analysis has been conducted to characterize the orbital-orbital interaction E (2) and the charge transfer in the complexes studied herein, and the results are summarized in Table 3. As expected, there is an electron charge transfer shift from the Lewis base unit R-Ph-CN to the Lewis acid unit PO2F. As a result, PO2F gains electron density and becomes negatively charged, while R-Ph-CN base loses electron density and becomes positively charged. The charge transfer (CT) is expressed as the net natural population analysis (NPA) charge on all the atom of the electron donor molecule (R-Ph-CN). As displayed in Figure 6(a), there is a good linear relationship between the total charge transfer amount and the binding distance r(N…P) with R2=0.997. The amount of charge transfer is 0.193 e for the Ph-CN…PO2F complex, which is larger than the 0.125 e charge transfer computed for the HCN…PO2F complex obtained at the same B3LYP-D3/6-311+G** level. As pointed by Grabowski [38], the deformation energy is strongly related to the electron charge transfer, which is often ignored in investigating the intermolecular interactions. Here, for the dimers we studied, an excellent linear relationship is found between the deformation energy and the charge transfer CT with R2=0.977, as seen in Figure 6(b). For each complex, there are two dominant orbital interactions of Lp(N) →p*(P) and Lp(N) →σ*(P-O) in these pnicogen-bonded complexes, where Lp(N) denotes the lone pair orbital on N atom, and p*(P) represents the empty p(π) orbital on P atom, and σ*(P-O) denotes the P-O antibonding orbital. The schematic diagrams of the orbital interactions in Ph-CN…PO2F complex are displayed in Figure 7. As indicated from the second-order perturbation energies in the last three columns of Table 3, the contributions of the Lp(N) →p*(P) orbital interaction ranges from 100.7 to 142.1 kcal/mol, which is far greater than that of Lp(N) →σ*(P-O). Similar to the case of tetrel bond[49], this is a feature of strong pnicogen-bond. It is evident from Table 3 that theE (2) energy of R-Ph-CN…PO2F complexes follows the same order of the strength of the interactions.