Coordination Number of Li+ in Nonaqueous Electrolyte Solutions Determined by Molecular Rotational Measurements

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• 作者：Kaijun Yuan ; Hongtao Bian ; Yuneng Shen ; Bo Jiang ; Jiebo Li ; Yufan Zhang ; Hailong Chen ; Junrong Zheng
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：April 3, 2014
• 年：2014
• 卷：118
• 期：13
• 页码：3689-3695
• 全文大小：342K
• 年卷期：v.118,no.13(April 3, 2014)
• ISSN：1520-5207

文摘

The coordination number of Li⁺ in acetonitrile solutions was determined by directly measuring the rotational times of solvent molecules bound and unbound to it. The CN stretch of the Li⁺ bound and unbound acetonitrile molecules in the same solution has distinct vibrational frequencies (2276 cm^鈥? vs 2254 cm^鈥?). The frequency difference allows the rotation of each type of acetonitrile molecule to be determined by monitoring the anisotropy decay of each CN stretch vibrational excitation signal. Regardless of the nature of anions and concentrations, the Li⁺ coordination number was found to be 4鈥? in the LiBF₄ (0.2鈥? M) and LiPF₆ (1鈥? M) acetonitrile solutions. However, the dissociation constants of the salt are dependent on the nature of anions. In 1 M LiBF₄ solution, 53% of the salt was found to dissociate into Li⁺, which is bound by 4鈥? solvent molecules. In 1 M LiPF₆ solution, 72% of the salt dissociates. 2D IR experiments show that the binding between Li⁺ and acetonitrile is very strong. The lifetime of the complex is much longer than 19 ps.