Meizhen Xu

Lithium ion complexation kinetics by cyclic and acyclic polyethers

Results of the ultrasonic absorption relaxation and nuclear magnetic resonance studies of the kinetics of alkali metal cation complexation by macrocycl ic 1 igands in a variety of nonaqueous solvents are reviewed. The effects of a competition between low permittivity solvent molecules, anions, and neutral macrocycles for the first coordination sphere of the lithium cation are clearly evident. utility of the Eigen-Winkler reaction mechanism for describing the ultrasonic absorption results is pointed out. techniques are used to explore the kinetics of lithium ion complexation by acyclic polyethers such as poly(ethy1ene oxide) in acetonitrile a striking insight emerges: The complexation kinetics reflect a localized cation- polyether interaction that is independent of the 1 ength of the polyether chain as one proceeds from triglyme to a poly(ethy1ene oxide) of 15,000 average molar w eight. complexation kinetic studies in neat 1 iquid poly(ethy1ene oxides) is discussed and possible applications of the results in developing an understanding of polymeric electrolytes in 1 ithium batteries is out1 ined. The When the same ultrasonic

Dielectric relaxation of chloroform and chloroform-cyclohexane mixtures at gigahertz and terahertz frequencies. The inertial term

Abstract Experimental values of the loss coefficient e″ of the complex permittivity e∗ = e′−J e″ for chloroform and for chloroform-cyclohexane mixtures at 25°C and in the frequency range ∼1 GHz to ∼5 THz are reported. These and previous literature data for chloroform are interpreted by the sum of one Debye and of one Rocard-Powles function.It appears that for data extending from low gigahertz frequencies to several terahertz use of a Debye function plus the Rocard-Powles function, the latter retaining the inertial term, can describe the frequency profile of the loss coefficient e″.

Anion solvation effects in polymer-electrolytes: NaSCN and LiSCN in PEO-400 and in PEG-400

Abstract Infrared spectra in the “CN-stretch” region of the SCN − anion, for NaSCN and LiSCN, dissolved in polyethylene oxide dimethylether (PEO-400) of average molar mass 400, show dramatic differences from the infrared spectra of the same electrolytes dissolved in polyethylene oxide glycol (PEG-400) of average molar mass 400. The differences are attributed to anion solvation by the -OH end of the PEG polymers. Similar comparison of the molar refraction R of the two electrolytes dissolved in the two solvents, also shows dramatic differences in the values and concentration dependence of the R NaSCN and of R LiSCN between the two polymer solutions. These differences are also attributed to anion H-bonding solvation by PEG-400. The different molecular status of the above electrolytes in the two solvents, should be considered, when planning their use as ion transport media in polymer-electrolyte batteries.

Molecular relaxation dynamics of poly(propylene oxide glycol)-400–LiClO4 systems

The dynamics of electrolytes dissolved in poly(propylene glycol)s are addressed by a combination of audiofrequency electrical conductivity measurements, IR spectra of the 4 mode of ClO4–, microwave and far-IR dielectric spectra, radiofrequency and UHF ultrasonic absorption spectra, and visible refractivities at the sodium doublet wavelength. The combination of IR and conductivity results indicates that the majority of the electrolyte exists in these low relative permittivity media as a solvent-separated ion pair and not as free ions. The high viscosity of the medium causes an overlap of solvent and solute relaxations that requires a new analysis of the molecular dynamics. The dielectric and ultrasonic spectra correlate with a similar average relexation time but with different distribution parameters. This is ascribed to a similar polymer chain response to the two perturbing functions although with different cooperative character of the response. The dielectric modulus approach is also examined, revealing serious errors committed by the improper use of this method of analysis. The calculated molar refractions suggest that the anion is solvated by the end protons of the polymer chains.

Molecular dynamics of macrocycle-metal ion complexes involving heteronuclear binding atoms

Combination of ultrasonic absorption relaxation spectra with infrared absorption spectra yields a clearer picture of the mechanism of complexation of metal ions by macrocycles in nonaqueous solutions. The Eigen-Winkler multistep reaction mechanism has consistently provided a suitable fit of the ultrasonic absorption data for systems such as monovalent sodium cation reacting with 18-crown-6 in acetonitrile. A macrocycle such as Kryptofix 22 having two nitrogen atoms in the ring experiences an exo-exo

Molecular dynamics of the dimerization of NaBø4 in 1,2-dimethoxyethane and tetrahydrofuran

exo-endo ;ft endo-endo change in conformation readily detected by ultrasonic techniques. Differences in complexation-decomplexation mechanisms deduced from ultrasonic and NMR data can be rationalized in terms of the limitations of the experimental techniques.

Experimental evidence of picosecond to femtosecond molecular motion of the macrocycles 12-crown-4 and 15-crown-5 in cyclohexane at 25°C

Abstract New experimental microwave dielectric relaxation, electrical conductance and ultrasonic relaxation kinetic data are reported for solutions of sodium tetraphenylborate (NaB o 4 ) dissolved in 1,2-dimethoxyethane (1,2-DME) and tetrahydrofuran (THF). 1,2-DME is a bidentate ether solvent, whereas THF is monodentate. NaB o 4 is strongly associated to ion pairs, and these ion pairs are dimerized to a considerable extent in both ether solvents.

Molecular dynamics and the structure of macrocycles —Solvent acetonitrile interactions

Values of the dielectric loss coefficient, e″, part of the complex permittivity, e*=e′−ie″, from UHF to terahertz (THz) frequencies, are reported for the macrocycles 12-crown-4 (12C4) and 15-crown-5 (15C5), in the solvent cyclohexane at 25°C. The coefficients of the real part of the complex permittivity e′ at UHF-microwave frequencies, and the refractive indices in the visible, at the sodium doublet (λ=589.3 nm), are also reported. The data are described by the sum of three Debye relaxation functions in the microwave region, and by three Rocard–Powles functions in the terahertz, far-IR range of frequencies. The molecular origin of the three Debye functions, following Vij and Hufnagel (J. Phys. Chem., 1991, 95, 6142), is attributed to rotation of the macrocycles along the molecular axes. The source of two Rocard–Powles functions is assigned to molecular librations while one describes a weak solvent absorption.

Molecular Structure and Dynamics of LiClO4-Polyethylene Oxide-400 (Dimethyl Ether and Diglycol Systems) at 25 .degree.C

Microwave complex permittivities, e✶ = e′ - Je″, are reported in the 1-90 GHz frequency range for the macrocycles 18-crown-6 (18C6) and 15C5 added to acetonitrile in stoichiometric proportions, in the solvent CCI4 at 25°C. Digitized infrared spectra of the ‘CN stretch’ ν2 vibration of acetonitrile for the same systems are reported in the 2300-2200 cm -1 spectral region. The macrocycle 12C4 added to CH3CN has also been investigated in the infrared. Both the dielectric relaxation and infrared results are interpreted in terms of macrocycle-acetonitrile interactions, probably involving a methyl-hydrogen to ethereal-oxygen interaction. These interactions with CH3CN diminish in strength according to the sequence: I8C6 > 15C5 > 12C4.