A comprehensive analysis of P···π pnicogen bonds: substitution effects and comparison with Br···π halogen bonds.

Ab initio calculations were carried out in a systematic investigation of P···π pnicogen-bonded complexes XH2P···C2H2/C2H4 and FH2P···C2R2/C2R4 for X = H, CH3, OH, CN, Br, Cl, NO2, F, and R = F, CH3, as well as corresponding Br···π halogen-bonded complexes XBr···C2H2. Both the electron-withdrawing and electron-donating substituents in the electron acceptor have enhancing effects on the strength of P···π interactions. The electron-donating group in the electron donor leads to strengthening while the electron-withdrawing group leads to weakening of P···π interactions. The studied P···π and Br···π interactions are similar and are typically "closed-shell" non-covalent character in nature. Moreover, analyses of natural bond orbital and density difference of molecular formation indicated that charge transfer and polarization also play important roles in P···π interactions. The substituents have direct effects on the molecular electrostatic potential, and the charge transfer amount and extent of polarization of the P···π interaction are also specific to each substituent.