3 Results and Discussion

3.1 Substitution Effect on the pnicogen bond

3.1.1 MEPs analysis

To better understood the structure of the complexes, the MEP analysis was performed for the selected molecules. The MEP maps of PO2F and Ph-CN are illustrated in Figure 1, in which the most positive regions are designated by a red color. Due to the similar profile as Ph-CN, the MEP maps of a series of substituted benzonitrile R-Ph-CN (R=F, Cl, Br, CH3, NH2, CN) we studied are all given in Figure S1 of Supporting Information. The values of the most positive and negative electrostatic potential (Vmax and Vmin) in the isolated monomers are listed in Table 1. It is observed that there is a π-hole (red region) on the outermost portion of the P surface, which are perpendicular to the molecular framework. The π-hole value (Vmax) is 61.59 kcal/mol for the P atom in PO2F monomer, and this enables PO2F to form a π-hole pnicogen bond as a strong Lewis acid with R-Ph-CN compounds. As for the isolated R-Ph-CN molecules, the electron-donating group shifts the electron density towards the CN side, and this results in the potential associated with the N atom becomes more negative. The absolute value of Vmin on the N atom increases with respect to substituent R in the order of CN<Br≈Cl<F<H<CH3≈NH2. Here, the power of electron-withdrawn of the halogen substituents is a little different from the previous report for small molecules[49].