Figure 3 . Linear relationship between the interaction energy ΔE int and Vmin values on the N atom of R-Ph-CN derivatives.
As for the deformation energy E def, which is defined as the energy required to distort each monomer from its optimized structure to that within the complex, its contribution is prominent in the studied pnicogen-bonded complexes, being about 28-30% of the interaction energy. According to the optimized geometries of the complexes, the relative magnitude for the structural deformation upon complexation can be estimated by the variation of the dihedral angle F-P-O-O of the PO2F molecule, which is 180°and 147.4 ° in the isolated monomer and Ph-CN…PO2F complex, respectively. The angle becomes smaller when the para -H atom in Ph-CN is replaced by the CH3 or NH2group, while the changes are the contrary when the substituent is F, Cl, Br, or CN group. As displayed in Figure 4(b), a good linear relationship is observed between the interaction energy of the complex and the dihedral angle of F-P-O-O with R2=0.997. It should be mentioned that in the earlier study of XO2F…NCH and XO2F…CNH (X=P, As) by Zhuo et al.[51], the deformation of the complexes is estimated by the changes of F-X…N/C angle. However, in our study, the linear correlation between the changes of P-O…N angle and the interaction energy of the complexes is not so good as that of the dihedral angle F-P-O-O of the PO2F molecule. This indicates that the well-chosen geometrical parameters of the complexes can be used to nicely reflect and describe the strength of the intermolecular interaction.