Conformational Study of the Structure of Free 18-Crown-6

Al-Jallal, Nada A.; Al-Kahtani, A. A.; El-Azhary, Adel A.

doi:10.1021/jp050133c

Cited by 42 publications

(46 citation statements)

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“…It is probable that this effect might be significant for the larger 18c6-alkali metal cation complexes with a larger and more flexible ring and consequently less energy gap between the possible conformations. 26,30,73 All bands reported in ref 40 of the Raman spectrum of the Na + complex in the methanol solution phase, Table 3, are observed in the current study, also in the Raman spectrum for the same methanol solution phase, and at a position difference of not more than 2 cm -1 . This is also observed for the Li + complex, Table 2, except for bands at 252, 262, and 579 cm -1 that are not observed in the current study.…”

Section: Resultssupporting

confidence: 73%

Experimental and Theoretical Study of the Vibrational Spectra of 12-Crown-4−Alkali Metal Cation Complexes

2006

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The vibrational, Raman, and IR, spectra of the five 12-crown-4 (12c4) complexes with Li+, Na+, K+, Rb+, and Cs+ alkali metal cations were measured. Except for a small shift of the position of some bands in the vibrational spectra of the Li+ complex, the vibrational spectra of the five complexes are so similar that it is concluded that the five complexes exist in the same conformation. B3LYP/6-31+G* force fields were calculated for six of the eight predicted conformations in a previous report (J. Phys. Chem. A 2005, 109, 8041) of the 12c4-Li+, Na+, and K+ complexes that are of symmetries higher than the C1 symmetry. These six conformations, in energy order, are of C4, Cs, Cs, C(2v), C(2v), and Cs symmetries. Comparison between the experimental and calculated vibrational frequencies assuming any of the above-mentioned six conformations shows that the five complexes exist in the C4 conformation. This agrees with the fact that the five alkali metal cations are larger than the 12c4 ring cavity. The B3LYP/6-31+G* force fields of the C4 conformation of the Li+, Na+ and K+ complexes were scaled using a set of eight scale factors and the scale factors were varied so as to minimize the difference between the calculated and experimental vibrational frequencies. The root-mean-square (rms) deviations of the calculated frequencies from the experimental frequencies were 7.7, 5.6, and 5.1 cm(-1) for the Li+, Na+, and K+ complexes, respectively. To account for the earlier results of the Li+ complex that the Cs conformation is more stable than the C4 conformation by 0.16 kcal/mol at the MP2/6-31+G* level, optimized geometries of the complex were calculated for the C4 and Cs conformations at the MP2/6-311++G** level. The C4 conformation was calculated to be more stable than the Cs conformation by 0.13 kcal/mol.

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Section: Resultssupporting

confidence: 73%

Experimental and Theoretical Study of the Vibrational Spectra of 12-Crown-4−Alkali Metal Cation Complexes

2006

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“…The optimized geometries of the free 18c6 and the 18c6‐ammonium complex are shown in Figures and . The structure of the free 18c6 resembles the previously found structures (see e.g., Al‐Kahtani and Al‐Jallal) with two ether oxygens pointing outside the cavity and four ether oxygens pointing inward. The slight differences in the structure of the present study in comparison to the most stable conformation in Refs.…”

Section: Resultssupporting

confidence: 80%

“…The slight differences in the structure of the present study in comparison to the most stable conformation in Refs. [49,50] are due to the effects of dispersion interactions and to differences in technical details of the calculations. As expected, the complex geometry with the ammonium ion placed in the center of the crown ether is more symmetric than the free 18c6.…”

Section: Geometrymentioning

confidence: 99%

Structure and stability of supramolecular crown ether complexes

2015

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Despite the fact that the complexation of ammonium cations with ionophores like crown ethers plays an important role in biological and industrial processes, there is still a lack of theoretical methods to reproduce or even predict the host-guest complex structures or their thermodynamic stabilities in an accurate manner. Hence, the development of ionophores has often relied on a trial-and-error approach and the synthetic efforts associated with this have been enormous, so far. Therefore, theoretical methods for the reliable prediction of binding affinities of crown ether derivatives with ammonium ions would be an indispensable tool for the rational design of new receptors with tailored properties. Here, we suggest a computationally efficient but still accurate theoretical approach. It is tested for a model system consisting of 18-crown-6 ether and an ammonium cation, but is invented for application to much larger complexes. The accuracy of various approximate quantum-chemical methods, based on density functional theory (DFT) and many-body perturbation theory, is evaluated against the gold standard CCSD(T) in the basis set limit as internal reference. An important aspect is the consideration of dispersion interactions in DFT methods, for which the dispersion-correction by Grimme was employed. For all selected methods, the basis-set dependence of calculated interaction energies was investigated.

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“…38 Although the position of these exothermic peaks should depend on cooling rate, the higher temperatures of phase transitions for 18C6-KO 2 than for pure 18C6 may be due to the presence of KO 2 particles, which could help phase transition or nucleation of the coexisting 18C6 matrix.…”

Section: Resultsmentioning

confidence: 99%

An Ionic Liquid State Composed of Superoxide Radical Anions and Crownether-Coordinated Potassium Cations

Kitada

Ishikawa

Fukami

et al. 2017

J. Electrochem. Soc.

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We report an "ionic liquid state" substance composed of superoxide radical anions (O 2 -) and crownether-coordinated potassium cations. A binary mixture of 18-crown-6-ether (18C6) and potassium superoxide (KO 2 ) becomes liquid above ca. 38 • C. The coordination of 18C6 to K + and the presence of O 2 -were confirmed in the liquid state by Raman spectroscopy measurements. Below 38 • C, however, the 18C6-KO 2 mixture does not form a complex but undergoes phase separation to parent phases of solid 18C6 and KO 2 , because the ion-dipole interaction between K + and 18C6 is overwhelmed by the ion-ion interaction between K + and O 2 -. In other words, the pair of O 2 -and 18C6-coordinated K + only appears above the melting point of pure 18C6, to form an "ionic liquid state". Among radical-containing ionic liquids, the liquid 18C6-KO 2 mixture has high fluidity and comparable conductivity. , aprotic solvents were used to dissolve KO 2 . For example, a highly polar aprotic solvent, dimethylsulfoxide (DMSO), was used with a cyclic ionophore dicyclohexyl-18-crown-6-ether (D18C6) to prepare KO 2 solution. 5The highest concentration of O 2 -in organic solution, however, is only 0.15 mol dm -3 in D18C6-containing DMSO solution, i.e. KO 2 :D18C6:DMSO = 1:2:100 by mole.5 O 2 -solutions can also be obtained chemically or electrochemically in aprotic solvents including ionic liquids, but the concentrations are small. 6-10 Therefore, drastic increase in superoxide radical concentration is of interest; for this purpose, designing novel superoxide radical compounds in liquid state is necessary.Examples of known superoxides are alkaline and alkaline-earth metal superoxides, of which magnetism have attracted attention. 11-15These counter cations have small ionic radii and high charge densities, i.e. they were "hard acids" to give strong cation-anion interactions and melting points much higher than RT. From this viewpoint, larger counter cations need to be designed. There are several superoxides using large cations such as quaternary ammonium cations, but their liquid properties are missing due to rapid decomposition. 16,17 An alternative approach for novel "superoxide liquids" may be coordination of metal cations by ethers. This is because ethers were judged to be more stable to O 2 -attack than organic carbonates, 18 which attracted attention as metal-air battery electrolytes. [19][20][21][22][23] It is also because ethers have been used to prepare so-called "solvate ionic liquids", where ether-coordinated metal cations are component cations. 24-28Our preliminary results showed that an equimolar mixture of 18-crown-6-ether (18C6) and potassium superoxide (KO 2 ) becomes liquid above 41• C, without sizable decomposition of O 2 -, while equimolar mixtures of KO 2 and other ethers such as glymes and D18C6 rapidly decomposed. 29 In this work, we study the detailed physico- * Electrochemical Society Member. z E-mail: kitada.atsushi.3r@kyoto-u.ac.jp chemical properties of 18C6-KO 2 mixture: in addition to our earlier work, 29 we performed ...

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Conformational Study of the Structure of Free 18-Crown-6

Cited by 42 publications

References 50 publications

Experimental and Theoretical Study of the Vibrational Spectra of 12-Crown-4−Alkali Metal Cation Complexes

Experimental and Theoretical Study of the Vibrational Spectra of 12-Crown-4−Alkali Metal Cation Complexes

Structure and stability of supramolecular crown ether complexes

An Ionic Liquid State Composed of Superoxide Radical Anions and Crownether-Coordinated Potassium Cations

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