Structural Heterogeneity and Unique Distorted Hydrogen Bonding in Primary Ammonium Nitrate Ionic Liquids Studied by High-Energy X-ray Diffraction Experiments and MD Simulations

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• 作者：Xuedan Song ; Hiroshi Hamano ; Babak Minofar ; Ryo Kanzaki ; Kenta Fujii ; Yasuo Kameda ; Shinji Kohara ; Masayoshi Watanabe ; Shin-ichi Ishiguro ; Yasuhiro Umebayashi
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：March 8, 2012
• 年：2012
• 卷：116
• 期：9
• 页码：2801-2813
• 全文大小：628K
• 年卷期：v.116,no.9(March 8, 2012)
• ISSN：1520-5207

文摘

Liquid structure and the closest ion鈥搃on interactions in a series of primary alkylammonium nitrate ionic liquids [C_nAm⁺][NO₃^{鈥?/sup>] (n = 2, 3, and 4) were studied by means of high-energy X-ray diffraction (HEXRD) experiments with the aid of molecular dynamics (MD) simulations. Experimental density and X-ray structure factors are in good accordance with those evaluated with MD simulations. With regard to liquid structure, characteristic peaks appeared in the low Q (Q: a scattering vector) region of X-ray structure factors S(Q)'s for all ionic liquids studied here, and they increased in intensity with a peak position shift toward the lower Q side by increasing the alkyl chain length. Experimentally evaluated SQ_peak(r_max) functions, which represent the S(Q) intensity at a peak position of maximum intensity Q_peak as a function of distance (actually a integration range r_max), revealed that characteristic peaks in the low Q region are related to the intermolecular anion鈥揳nion correlation decrease in the r range of 10鈥?2 脜. Appearance of the peak in the low Q region is probably related to the exclusion of the correlations among ions of the same sign in this r range by the alkyl chain aggregation. From MD simulations, we found unique and rather distorted NH路路路O hydrogen bonding between C_nAm+ (n = 2, 3, and 4) and NO₃鈥?/sup> in these ionic liquids regardless of the alkyl chain length. Subsequent ab initio calculations for both a molecular complex C₂H₅NH₂路路路HONO₂ and an ion pair C₂H₅NH₃+路路路ONO₂鈥?/sup> revealed that such distorted hydrogen bonding is specific in a liquid state of this family of ionic liquids, though the linear orientation is preferred for both the N路路路HO hydrogen bonding in a molecular complex and the NH路路路O one in an ion pair. Finally, we propose our interpretation of structural heterogeneity in PILs and also in APILs.}