NANOSECOND MOLECULAR-DYNAMICS AND VIBRATIONAL-SPECTRA OF LI+-CHAIN POLYETHERS IN ACETONITRILE

被引:26
作者
ESCHMANN, J
STRASSER, J
XU, M
OKAMOTO, Y
EYRING, EM
PETRUCCI, S
机构
[1] POLYTECH INST NEW YORK,WEBER RES INST,LONG ISL CTR,ROUTE 110,FARMINGDALE,NY 11735
[2] UNIV UTAH,DEPT CHEM,SALT LAKE CITY,UT 84112
关键词
D O I
10.1021/j100373a009
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Ultrasonic relaxation spectra in the 1-500-MHz frequency range for LiCIO4 or LiAsF6 (0.03 to ∼ 1 M) added to open-chain polyethers (triglyme) in molar ratio R = 1, or added to poly(ethylene oxide) in molar ratio RPEO = [(-CH2-CH2-O-)]/[Li+] = 4, in the solvent acetonitrile at 25 °C are reported. The spectra were interpreted by the sum of two Debye relaxation processes. The remarkable finding is that the relaxation times are independent of the chain length in going from triglyme at R = 1 to a poly(ethylene oxide) of 15 000 average molar weight at RPEO = 4. The observed processes appear to reflect a localized cation-polyether interaction (here dubbed the "ether moiety effect"). The relaxations are interpreted by an Eigen-Winkler mechanism in which a cation-ether contact is followed by the polyether chain wrapping around the Li+ ion. Specific effects in the ultrasonic absorption amplitudes differentiating the spectra of triglyme from the polyether spectra are reported. Also specific anion effects appear at concentrations higher than 0.3 M, probably reflecting competition between anion and polyether for the first coordination sphere positions around Li+. The extent of these effects varies with the nature of the anion, with AsF6- appearing to interfere more than ClO4- does. Infrared spectra between 800 and 900 cm-1 of the same systems show cation-ether vibrations due to cage effects. The infrared spectral envelopes of both the polyethers alone and the polyether added to Li+ are deconvoluted by three Gaussian-Lorentzian bands. For triglyme a weak band at 879 cm-1 is strongly enhanced in absorbance and shifted to 870 cm-1 when Li+ is present. This band is attributed to wrapping of the polyether glyme around the cation. Since no significant infrared band enhancement is observed for poly(ethylene oxide) in acetonitrile, it is possible that the same configuration is hindered for long polyether chains. Ultrasonic and infrared spectra for the cyclic polyether 12-crown-4 in the presence of Li+ in acetonitrile are reported for comparison. For 12C4 + Li+, the ultrasonic spectrum can be interpreted by the sum of two Debye relaxation processes. The first at lower frequencies has a lower relaxation frequency (by a factor of ∼4) than the one present in triglyme. This may reflect the more rigid ring structure of 12C4 opposing the entrance of the cation and thus altering (enthalpically or entropically) the activation free energy of the complexation process. The infrared spectrum of 12C4 in the 800-900-cm-1 region can be deconvoluted by four Gaussian-Lorentzian bands, two of which dominate the absorbance. Addition of Li+ in molar ratio R = [macrocycle] / [Li+] = 0.5 shifts dramatically both the position and relative intensity of the infrared bands of the crown ether. © 1990 American Chemical Society.
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页码:3908 / 3916
页数:9
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