Theoretical Study On The Recognition Of A Series Of Crown Ethers Compounds For The Alkali Metal Ions

For the separation of a particular alkali metal ion, membrane transport using acrown ether derivative is a convenient, effective, and low-energy-consuming method. Insupramolecular chemistry, the appealing crown ethers as a synthesis of macrocyclic hostare easy to be prepared. They display a unique charm especially in the recognition ofcations and neutral molecules. The paper applied the quantum chemistry mothods andsystematically studied the following four systems:(1) The molecular recognition of theN-butyl-aza-15-crown-5ethers and the alkali metal cations: Li~+and Na~+;(2) Themolecular recognition of the bis-3-benzo-crown ethers and the alkali metal cations: Na~+,K~+, Rb~+and Cs~+;(3) The molecular recognition of crown ether-bridged Z-stilbenes withalkali metal cations: Li~+, Na~+, K~+, Rb~+and Cs~+;(4) The fluorescent molecular devicesbearing stilbene-4.Firstly, a series of lariat crown ether based on N-butyl-aza-15-crown-5, which havethe same parent macrocycle but with a different terminal group in sidearm, have beencalculated at the DFT//B3LYP/6-31G*level in Gaussian03package program. Theoptimized geometries of two conformers (“V”-shaped sandwich conformer A and thelinear structural conformer B) for their cation complex are obtained from theory in gasphase. In addition, the NBO analysis and the IR spectra indicate the interaction betweenthe host-guest molecules in detail. As a result, a discrepant conclusion obtained that themain interaction between the LCEs and cations (Li~+and Na~+) is the cation-dipole, andthe cation-π is negligible in gas phase.Secondly, the binding interactions of bis-3-benzo-15-crown-5ethers andbis-3-benzo-18-crown-6ethers (neutral hosts) with series of alkali metal cations Na~+, K~+,Rb~+and Cs~+(charged guests) have been investigated using quantum chemical densityfunctional theory (DFT). Different optimized structures, binding energies and variousthermodynamic parameters of free crown ethers and their metal cations complexes havebeen obtained based on B3LYP functional using mixed basis set (C, H, O, Na~+and K~+using6-31g, and the heavier cation Rb~+and Cs~+using Effective core potentials (ECPs)).The optimized geometric structures are performed natural bond orbital (NBO) analysis.The main types of driving force host-guest interactions are investigated, the electrondonating O offers lone pair electrons to the contacting LP*(1-center valence antibondlone pair) orbitals of metal cations. The bis-3-benzocrown ethers are assumed sandwichlike conformations, considering the binding energies to gauge the exact interactions with alkali cations. It is found that there are two different types of complexes: one is tight ionpair and the other is separated ion pair.Thirdly, a density functional theory (DFT) based on the interaction of alkali metalcations (Li~+, Na~+, K~+, Rb~+and Cs~+) with crown ether-bridged Z-stilbene has beeninvestigated. The minimum energy structures, binding energies, and variousthermodynamic parameters of free ligands and their metal cations complexes have beendetermined with B3LYP functional using mixed basis set (C, H, O, Li~+, Na~+and K~+using6-31~+G(d), and the heavier cations: Rb~+and Cs~+using LANL2DZ). The optimizedgeometric structures are performed natural bond orbital (NBO) analysis. The main typesof driving force host-guest interactions are investigated, the electron donating O offerslone pair electrons to the contacting LP*(1-center valence lone pair) of alkali metalcations. In addition, the maximum absorption wavelength is studied on thetime-dependent density functional theory (TD-DFT), the calculated results indicate thatthe wavelength is blue shifted when alkali cations incorporating into the ether rings. It isfound that the crown ether-bridged Z-stilbene is good candidate for moderate alkali metalcations’ recognition regent.At lastly, the basic chromophore stilbene derivatives were combined with lithiumcation in gas phase. The geometric parameters and electronic properties of the groundstate (S0) and first excited state (S1), HOMO-LUMO gaps (DE), as well as thethermodynamic parameters were determined with the B3LYP functional using6-31~+G(d).Additionally, the lowest excitation energies (E g), the maximal absorption wavelength,and the emission spectra were studied, by means of the time-dependent densityfunctional theory (TD-DFT). The calculated results indicated stilbene-4was fluorescentresponse to lithium, while its two isomers showed different fluorescence effect: afterbonding with lithium, the spectrum of (Z)-isomer was blue shifted accompaniedfluorescence enhancement effect; in reverse, the spectrum of (E)-isomer was red shiftedaccompanied fluorescence quenching effect. These shift resulted from the intramolecularcharge transitions (ICT). Moreover, the spectra appeared large Stokes shift.