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

Liquid structure and the closest ion–ion interactions in a series of primary alkylammonium nitrate ionic liquids [C<sub><i>n</i></sub>Am<sup>+</sup>]­[NO<sub>3</sub><sup>–</sup>] (<i>n</i> = 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 <i>Q</i> (<i>Q</i>: a scattering vector) region of X-ray structure factors <i>S</i>(<i>Q</i>)'s for all ionic liquids studied here, and they increased in intensity with a peak position shift toward the lower <i>Q</i> side by increasing the alkyl chain length. Experimentally evaluated <i>S</i><sup><i>Q</i><sub>peak</sub></sup>(<i>r</i><sub>max</sub>) functions, which represent the <i>S</i>(<i>Q</i>) intensity at a peak position of maximum intensity <i>Q</i><sub>peak</sub> as a function of distance (actually a integration range <i>r</i><sub>max</sub>), revealed that characteristic peaks in the low <i>Q</i> region are related to the intermolecular anion–anion correlation decrease in the <i>r</i> range of 10–12 Å. Appearance of the peak in the low <i>Q</i> region is probably related to the exclusion of the correlations among ions of the same sign in this <i>r</i> range by the alkyl chain aggregation. From MD simulations, we found unique and rather distorted NH···O hydrogen bonding between C<sub><i>n</i></sub>Am<sup>+</sup> (<i>n</i> = 2, 3, and 4) and NO<sub>3</sub><sup>–</sup> in these ionic liquids regardless of the alkyl chain length. Subsequent ab initio calculations for both a molecular complex C<sub>2</sub>H<sub>5</sub>NH<sub>2</sub>···HONO<sub>2</sub> and an ion pair C<sub>2</sub>H<sub>5</sub>NH<sub>3</sub><sup>+</sup>···ONO<sub>2</sub><sup>–</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···HO hydrogen bonding in a molecular complex and the NH···O one in an ion pair. Finally, we propose our interpretation of structural heterogeneity in PILs and also in APILs.