Burba Christopher M, Frech Roger
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Rm 208, Norman, Oklahoma 73019, USA.
J Phys Chem B. 2005 Aug 11;109(31):15161-4. doi: 10.1021/jp058045f.
Ionic association in nonaqueous electrolytes containing LiPF6 was investigated with infrared absorption spectroscopy. The spectral intensity of the nondegenerate nu1 mode of the PF6- anion was found to be sensitive to ion pairing. Although the nu(1) mode of an isolated PF6- anion is only Raman active, coordination of Li+ to PF6- destroys the octahedral symmetry of the anion and results in nu1 becoming simultaneously IR and Raman active. When the dielectric constant of the solvent is increased, the IR-intensity of the nu1 band decreases because ion pairing is not favored in high dielectric solvents. Spectroscopic studies of solutions containing LiPF6 in diglyme show that ion pairing is also affected by specific cation-solvent interactions. The diglyme-containing solutions contain significantly fewer ion pairs than expected on the basis of the solvent's dielectric constant. It is thought that diglyme:LiPF6 solutions consist mostly of "spectroscopically free" PF6- anions because the sixfold coordination of Li+ by two diglyme molecules hinders Li+...PF6- ion pairing.
利用红外吸收光谱研究了含LiPF6的非水电解质中的离子缔合。发现PF6-阴离子的非简并ν1模式的光谱强度对离子配对敏感。虽然孤立的PF6-阴离子的ν(1)模式仅具有拉曼活性，但Li+与PF6-的配位破坏了阴离子的八面体对称性，导致ν1同时具有红外和拉曼活性。当溶剂的介电常数增加时，ν1带的红外强度降低，因为在高介电常数溶剂中离子配对不受青睐。对二甘醇二甲醚中含LiPF6溶液的光谱研究表明，离子配对也受特定阳离子-溶剂相互作用的影响。含二甘醇二甲醚的溶液中离子对的数量明显少于基于溶剂介电常数预期的数量。据认为，二甘醇二甲醚-LiPF6溶液主要由“光谱学上自由的”PF6-阴离子组成，因为两个二甘醇二甲醚分子对Li+的六重配位阻碍了Li+...PF6-离子配对。
