Theoretical Study On A Seriers Of Crown Ethers Metal Complexes And Molecular Switch

Supramolecular chemistry is one of the most popular and fastest-growing areas in the current and the future and also in quite a long time. The paper applied the quantum chemistry mothods and systematically studied the following three systems: (1) DFT study for a series of less-symmetrical crown ethers and the molecular recognition of their complexes with alkali metal cations: Na⁺, K⁺; (2) The molecular recognition between an allosteric switch moleculer: 2, 2'-bipyridyl-3, 3'-15-crown-5 and the alkali metal cations (Na⁺, K⁺) and W(CO)₄ fragment. (3)Theoretical study of switchable crown ethers with incorporated azobenzene moiety and the molecular recognition between their cis-isomers and alkali metal cation : Li⁺, Na⁺, K⁺, Rb⁺.Firstly, a series of ring-contracted (14-crown-5, 17-crown-6) and ring-enlarged (16-crown-5, 17-crown-5, 19-crown-6, 20-crown-6) crown ethers and their complexes with alkali-metal cations Na⁺ and K⁺ had been explored using density functional theory (DFT) at B3LYP/6-31G(d) level in order to reveal the effects of the methylene-chain length in a crown ether. The nucleophilicity of all crown ethers had been investigated by the Fukui functions. The quantum chemistry parameters, such as the energy gap (ΔE), the highest occupied molecular orbital energy (EHOMO) and the lowest unoccupied molecular orbital energy (ELUMO) for less-symmetrical crown ethers and symmetrical frameworks (15-crown-5, 18-crown-6) had been calculated. In addition, the thermodynamic energies of complexation reactions had also been studied. The results of the DFT calculations show that the methylene-chain length plays an important role in determining the structure characters of the crown ethers and also strongly influences the properties of the ethers. Some of the calculated results are in a good agreement with the experimental values.Secondly, a classical model of"molecular machine"which acts as an ON-OFF switch for 2, 2'-bipyridyl-3, 3'-15-crown-5 (= L ) has been theoretically studied. It is highly important to understand the mechanism of this switch. The alkali-metal cations (Na⁺ and K⁺) and W(CO)₄ fragment are introduced to coordinate with the different active sites of L, respectively. The density functional theory (DFT) method is employed for understanding the stereochemical structural natures and thermodynamic properties of all the target molecules at B3LYP/6-31G(d) and SDD level, together with the corresponding effective core potential (ECP) for tungsten (W). The fully optimized geometries have been performed with real frequencies which indicate the minima states. The nucleophilicity of L has been investigated by the Fukui functions. The natural bond orbital (NBO) analysis is used to explore the origin of the intermolecular charge-transfer interactions. The calculated results show that the most significant interaction is the interaction between the lone pair electrons of electron-donating oxygens in the crown ether part of L and the RY* (1-center Rydberg) or LP* (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Na⁺ and K⁺), and for W(CO)₄L is the interaction between lone pair electrons of N atoms in the 2, 2'-bipyridine unit and the LP (1-center valence lone pair) or RY* orbitals of transition-metal tungsten (W). In addition, the binding energies, enthalpies and Gibbs free energies have been studied for L and its complexes. The calculated results of allosterism displayed by L are in a good agreement with the experimental results.Thirdly, a series of crown ethers containing the azobenzene moiety incorporated into the crowns of various sizes (Cr(O6), Cr(O7) and Cr(O8)) and their corresponding alkali metal cation (Li⁺, Na⁺, K⁺, Rb⁺) complexes have been theoretically studied. The density functional theory (DFT) method is employed for understanding the stereochemical structural natures and thermodynamic properties for all the target molecules at B3LYP/6-31G(d) and LANL2DZ level for cation Rb⁺. The fully optimized geometries have been performed with real frequencies which indicate the minima states. In addition, the atomic bond lengths between metal cation and oxygen atoms, atomic torsion angles and thermodynamic energies for complexes have been studied. The natural bond orbital (NBO) analysis is used to explore the origin of the internal forces and the intermolecular interactions for the metal complexes. The calculated results show that the most significant interaction is the interaction between the lone pair electrons of electron-donating oxygens in the cis-forms of azobenzen crown ethers (cis-ACEs) and the LP* (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Li⁺, Na⁺, K⁺ and Rb⁺). The electronic spectra for the cis-ACEs: cis-Cr(O₆), cis-Cr(O₇) and cis-Cr(O₈) are obtained by the time dependent density functional theory (TDDFT) at B3LYP/6-31G(d) level. The spectra of the cis-isomers show broadπ→π* (S₀→S₂) absorption bands with wavelength 310-340 nm but weaker n→π* (S₀→S₁) bands with wavelength 480-490 nm. The calculated results are in a good agreement with the experimental results.