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Theoretical Studies On The Structures And Properties Of Some Weak Intermolecular Interactions

Posted on:2013-07-21Degree:MasterType:Thesis
Country:ChinaCandidate:M ZhuFull Text:PDF
GTID:2231330395453719Subject:Physical chemistry
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Intermolecular noncovalent interactions have a vital role to play in a variety of chemical,physical, and biological phenomena. The hydrogen bond (HB) has been the object of severalexperimental and theoretical investigations. LiB resembles HB to a great extent. Although theconcept of LB has been accepted and closely related to many fields, studies of LB interactionsare relatively rare compared to those of HB interactions. The lithium bond (LB) was firstproposed in1959by Shigorin, theoretically predicted in1970by Kollman et al., andexperimentally confirmed in1975by Ault and Pimentel. Comparisons between LB and HB,there are still significant differences in the binding energies and geometries of complexes. Theintermolecular interactions between π-electron complexes and Lewis acids have attractedincreasing attention in recent years, particularly the aromatic hydrogen bond π-HB (where aπ-electron system acts as an electron donor) because such interactions play a key role incertain chemical reactions, particularly those involving aromatic rings.In this thesis, the lithium bond and the hydrogen bond, as well as the interactions in thesupermolecules are studied by the Quantum Chemistry methods. The nature of theintermolecular interaction in LB and HB are invested by the the quantum theory of “atoms inmolecules”(QTAIM). All calculations involved the optimization, frequency analysis and theenergies of the intermolecular interactions were performed with the Gaussian03programpackage. And topological analyses of electron density were carried out by the “atoms inmolecules”(AIM)2000program and AIMALL program. To illustrate better theintermolecular interactions, σ and π orbitals were separated and the π-electron densities wereobtained by using the GTA-2010program, which was developed by the authors and registeredat the Quantum Chemistry Program Exchange (QCPE, register number QCPE-661). Then, theelectron localization function (ELF) analyses were performed by the TopMod package. Thereare mainly contained three sections:1. The optimized geometries and the energies of the complexes C6H6···LiX (X=OH,NH2, F, Cl, Br, NC, CN) were studied under the MP2/aug-cc-pVDZ level. The topologicalproperties and energy properties at the lithium bond critical points in the C6H6···LiXcomplexes displayed the characters of “closed-shell” and noncovalent interactions. Theinteractions between the π electrons of benzene and LiX are stronger than those between the σelectrons of benzene and LiX. Hence, the intermolecular interaction in the C6H6···LiX complexes are mainly attributed to the π-type interaction. The electron localization function(ELF) analysis indicates that the formation of these lithium bonds leads to the reduction of theELF π-electron density and volume.2. The intermolecular interactions existing at three different sites betweenphenylacetylene and LiX (X=OH, NH2, F, Cl, Br, CN, NC) have been investigated by meansof the MP2/aug-cc-pVDZ level. At each site, the lithium-bonding interactions withelectron-withdrawing groups (-F,-Cl,-Br,-CN,-NC) were found to be stronger than thosewith electron-donating groups (-OH and-NH2). The intermolecular interactions in theC6H5C≡CH···LiX complexes can be mainly attributed to the π-type interaction. The QTAIMstudies have shown that these lithium-bond interactions display the characteristics of“closed-shell” noncovalent interactions, and the molecular formation density difference(MFDD) indicates that electron transfer plays an important role in the formation of the lithiumbond. For each site, linear relationships have been found between the topological properties atthe BCP (the electron density ρb, its Laplacian2ρb, and the eigenvalue λ3of the Hessianmatrix) and the lithium bond length d(Li-bond). The shorter the lithium bond lengthd(Li-bond), the larger ρb, and the stronger the π···Li bond. The shorter d(Li-bond), the larger2ρb, and the greater the electrostatic character of the π···Li bond.3. The intermolecular interactions in the dimers of m-nisoldipine polymorphism werestudied by B3LYP calculations and quantum theory of “atoms in molecules”(QTAIM) studies.Four geometries of dimers were obtained: dimer I (a-dimer, O···H-N), dimer II (b-dimer,O···H-N), dimer III (b-dimer, π-stacking-c), and dimer IV (b-dimer, π-stacking-p). Theinteraction energies of the four dimers are along the sequence of II> I> III> IV. Theintermolecular distances of the interactions follow the order: I (O···H-N)<II(O···H-N), andIII(π-stacking)<IV(π-stacking). Both the O···H-N hydrogen-bonding and π-stackinginteractions belong to weak noncovalent interactions. The O···H-N hydrogen-bondinginteractions with more electrostatic characters are stronger than the π-stacking interactions.The strength of the weak interactions decreases in the order: I> II> III> IV, and theelectrostatic character decreases along the sequence: I> II> III> IV.4. The optimized geometries and the energies of the complexes C6H6···HX (X=OH, NH2,F, Cl, Br, NC, CN) were studied at the MP2/aug-cc-pVDZ level. Comparedbenzene-containing hydrogen bonds with lithium bonds, it can be found that LiBs are morestable than the corresponding HBs; the geometries are different due to the different molecular electrostatic potential, as well as the different molecular graphs in AIM. The interactionsbetween the π electrons of benzene and HX are stronger than those between the σ electrons ofbenzene and HX. Hence, the intermolecular interaction in the C6H6···HX complexes aremainly attributed to the π-type interaction.The innovations in this thesis:1. To illustrate better the intermolecular interactions, σ and π orbitals of benzene orphenylacetylene were separated and the π-electron densities were obtained by using theGTA-2010program. The intermolecular interactions studied in this thesis can be mainlyattributed to the π-type interaction. The molecular graphs of σ-complexes and π-complexeswere shown by AIM2000program. It can be proved clearly the nature of π-type lithium bondand hydrogen bond.2. The TopMod program was used to study the nature of the π-electron in the C6H6···LiX(X=OH, NH2, F, Cl, Br, NC, CN) complexes. Compared with the πC6H6monomer, theformations of the lithium bond in complexes lead to the reduction of the ELF π-electrondensity, and the reduction of the π-electron volume.3. Assessment of intermolecular interactions at three sites of the arylalkyne inphenylacetylene-containing lithium-bonded complexes is studied firstly. The geometries andthe energies of the intermolecular interactions show that the interactions in three sites can bestable existed.4. The interactions in the m-nisoldipine polymorphism dimers were theoretical studies.There are two kinds of the intermolecular interactions: hydrogen-bonding and π-stacking. Theinteraction energies of the four dimers are along the sequence of II> I> III> IV. The O···H-Nhydrogen-bonding interactions with more electrostatic characters are stronger than theπ-stacking interactions.
Keywords/Search Tags:intermolecular interaction, lithium bonding, QTAIM study, ELF analysis, π-stacking interaction
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