Theoretical Studies On The Influences Of Pnicogenbonded With Substitution And Cooperativity

Noncovalent interactions are widely existed in many molecular systems, which play a typical role in supramolecular chemistry, crystal engineering, and materials self-assembly. The hydrogen bond and halogen bond, the most important ones, have been widely studied over many years. Recently, the pnicogen bond has been recognized as a new and important type of intramolecular and intermolecular interaction. The pnicogen bond is Lewis acid-Lewis base interaction in which a pnicogen atom(N, P, or As) acts as the Lewis acid. As a new conceivable molecular linker, the pnicogen bond will play an important role in crystal engineering and supermolecular chemistry.In this work, the nature and characteristics of some typical P···π pnicogen bonds and cation···π interactions were studied systematically. The influences of hydrogen, halogen, and lithium bonds on n-type and π-type pnicogen bonds were also investigated. The second-order M?ller-Plesset perturbation theory, the quantum theory of “atoms in molecules”(QTAIM), natural bond orbitals(NBO) as well as molecular formation density difference(MFDD) method were used in this study to deepen the nature of the interactions considered. The work mainly contained three sections:1. Ab initio calculations were carried out in a systematic investigation of P···π pnicogenbonded 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 the enhancing effects on the strength of P···π interactions. The electron-donating group in the electron donor leads to strengthening while the electron-withdrawing one leads to weakening the P···π interactions. The studied P···π and Br···π interactions are similar and have typical ″closed-shell″ non-covalent character in nature. Moreover, the analyses of natural bond orbital(NBO) and density difference of molecular formation(MFDD) indicated that charge transfer and polarization also play important roles in the P···π interactions. The substituents have direct effects on the values of molecular electrostatic potential, the charge transfer amount and polarization extent of the P···π interaction is as well relevant to the substituents.2. The cooperative effects of some noncovalent interactions(hydrogen and halogen bonds, etc) on n-type and π-type pnicogen bonds were investigated at the MP2 level of theory. All calculations involved the molecular electrostatic potential(MEP), MEP at the nuclei, electron density and its Laplacian were used to study the nature and properties of P···π and P···N interactions in dimers FH2P···C6H5CN and trimers FH2P···C6H5CN···XCN(X = H, Br, and Li). The aim is to explore the influences of different types and positions of noncovalent interactions on the pnicogen bonds. The results indicated that the P···π and P···N interactions in dimers and trimers can be classified as "closed-shell" and noncovalent interactions. The MEP at the nuclei, topological properties based on the electron density and its Laplacian can be taken as important parameters to measure the strength of pnicogen bond. The introduction of XCN(X = H, Br, Li) in the molecule C6H5 CN could lead to the change of the most negative electrostatic potentials and MEPs at the nuclei of nitrogen atom. In the FH2P···C6H5CN···XCN(geometry a and b) termolecular complexes, the hydrogen, halogen, and lithium bond have the ability to weaken the P···π and P···N interactions, while the hydrogen bond can enhance the P···N pnicogen bond in the FH2P···C6H5CN···XCN(geometry c). That is to say, the positive or negative cooperativity exists in the different geometry of termolecular complexes. The topological properties at the critical points based on the electron density and its Laplacian change regularly with the addition of molecule XCN in C6H5CN···PH2F bimolecular complex.3. The nature of the cation···π interactions between alkali metal cations(Li+, Na+, and K+) and five-membered heteroaromatic rings(furan(C4H4O), thiophene(C4H4S), pyrrole(C4H5N)) has been investigated at MP2(full)/6-311++G(d, p) level. The studied cation···π interactions can be classified as "closed-shell" and noncovalent interactions. The size of cation and the character of heteroaromatic rings are two influential factors that affect the geometry, frequency, interaction energy and electron density of cation···π interactions. The interaction energies become more negative in the order π···Li+ > π···Na+ > π···K+, and C4H4O···M+ < C4H4S···M+ < C4H5N···M+. The different role of C4H4 O, C4H4 S, and C4H5 N as the electron donor has been evaluated. The results indicate that sulfur atom of C4H4 S plays a certain role in the cation···π interactions except the CC π bond, which is different from C4H4 O and C4H5 N.The innovation in this thesis:1. It was explicit that the effects of substituents in both pnicogen bond donor and acceptor on the molecular electrostatic potential, the charge transfer amount and polarization extent of the P···π interaction.2. It was found that the MEP at the nuclei and topological properties at the critical points based on the Laplacian of electron density could be applied as measures of the strengths of pnicogen bond. They have good linear relationships with the interaction energies.3. It was clear that two influential factors affect the interactions between alkali metal cations and heteroaromatic rings, the size of cation and the character of heteroaromatic rings.