Studies On The P···π Interaction Between Benzene And Three Substituted Phosphides

Noncovalent interactions play an important role in molecular recognition, crystal engineering and biological system. Pnicogen bond is a new noncovalent interactions, and has a certain directionality. It has been attracting attention of many experimental and theoretical studies. In the pnicogen bonding interaction, a pnicogen atom（N, P, As, or Sb） acts as a Lewis acid and interacts with an electron donor molecule. The P···π interaction, which use π electron as electron donor, is one kind of pnicogen bond. Therefore, it is important to research the nature of P···π interaction.In this work,we mainly research the P···π interaction between Benzene and substituted Phosphides by the quantum chemistry calculation. The geometries and the frequencies of the monomers and the complexes were calculated with the Gaussian 09 program package. The energy decomposition was performed by GAMESS suite of programs. The charge transfer and orbital interaction were analyzed using the NBO software. The molecular electrostatic potentials and topological properties were computed by WFA and AIMALL programs, The main research work is consist of two parts as follows:1. In the first part, we discusses the P···π interaction between the center of Benzene and the P（As） atom in MR₃（M=P, As; R=F, Cl, Br, H, CH₃, NH₂, OH, CHO, CN, NO₂）. According to the analysis of the molecular electrostatic potential（MEP）, it is fond that the P···π interaction occurs between the negative region of MEP in the benzene and the positive region of MEP on the P（As） atom in MR₃. In the C6H6···MR₃ complexes, when R=H, CH₃, OH, NH₂, CHO, CN, NO₂, there is a positive correlation between the interaction energy and the molecular electrostatic potential of the P（As） atom. But this correlation becomes opposite when R=F, Cl, Br,. So the electrostatic attraction is not significant for the stabilization of the complex. Considering the energy decomposition, the exchange energy represents the largest component in all of the complexes examined, followed by dispersion energy and electrostatic energy, and then the polarization energy. The orbital interaction is the main origin of the P···π interaction in the studied complexes. By analyzing the frontier molecular orbitals, we found that the interaction energy is well correlated with the energy gap between the HOMO of the electron donor and LUMO of the electron acceptor. In the most of complexes, there is only one kind of charge transfer that is from the bonding orbital of C₆H₆ to the anti-bonding orbital of MR₃. However, in the complexes of C6H6···PF3, C6H6···AsF3, C6H6···P（OH）3 the charge transfers are different. There are two ways of the charge transfer, one is from the bonding orbital of C₆H₆ to the anti-bonding orbital of the lone pairs of P（As） atom, another one is from the anti-bonding orbital of the lone pairs of P（As） atom to the anti-bonding orbital of C6H6. The charge transfer of the first way is the major mode. According to the topological analysis of the electron density, it is obviously found that the interaction energy is proportional to the electron density at the critical point of the P···π bond. That is to say, the larger value of ρb in P···π bond, the stronger the bond is, and the greater the Interaction energy is. The analysis of Laplacian reveals that the studied P···π interactions are typically “closed-shell” noncovalent interactions.2. In the second part, we discusses the P···π interaction between the P atom of the PH₃ and the center of C6H₃R₃（R=F, Cl, Br, CN, CHO, NO₂）. According to the molecular electrostatic potential, the P···π interaction is formed by the negative region of MEP in the C6H₃R₃ with the positive region of MEP on the P atom in the PH₃. In the C6H₃R₃···PH₃ complexes, when R= F, Cl, Br, there is a positive correlation between the interaction energy and the ability of electron withdrawing of the R group. But a negative correlation was found when the substituted groups were CN, CHO, NO₂.Thus, considering only the electrostatic interaction is not sufficient to explain the nature of these P···π interactions. Compared with the interaction energy of C6H6···PH₃, the interaction energies in the complexes of substituted benzene increase slightly. In the components of interaction energy, the exchange energy is the largest factor, the ratio of dispersion energy increase, ratio of the electrostatic energy decrease, and the polarization energy still has the smallest percentage. The orbital interaction is the main origin of the P···π interaction in the studied complexes. By analyzing the frontier molecular orbitals, we found that the interaction energy is well correlated with the energy gap between the HOMO of the electron donor and LUMO of the electron acceptor. On the same of front section, in the most of complexes, there is only one kind of charge transfer that is from the bonding orbital of C6H₃R₃ to the anti-bonding orbital of PH₃. However, in the complexes of C6H₃（CN）3···PH₃ the charge transfers is different. There are two ways of the charge transfer, one is from the bonding orbital of C6H₃（CN）3 to the anti-bonding orbital of the P-H bond of PH₃, another one is from the bonding orbital of the lone pairs of P atom to the anti-bonding orbital of C6H₃R₃. The charge transfer of the first way is the major mode. According to the topological analysis of the electron density, it is obviously found that the interaction energy is proportional to the electron density at the critical point of the P···π bond. That is to say, the larger value of ρb in P···π bond, the stronger the bond is, and the greater the interaction energy is. The analysis of Laplacian reveals that the studied P···π interactions are typically “closed-shell” noncovalent interactions.