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 [CnAm+]­[NO3] (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 SQpeak(rmax) functions, which represent the S(Q) intensity at a peak position of maximum intensity Qpeak as a function of distance (actually a integration range rmax), revealed that characteristic peaks in the low Q region are related to the intermolecular anion–anion correlation decrease in the r range of 10–12 Å. 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···O hydrogen bonding between CnAm+ (n = 2, 3, and 4) and NO3 in these ionic liquids regardless of the alkyl chain length. Subsequent ab initio calculations for both a molecular complex C2H5NH2···HONO2 and an ion pair C2H5NH3+···ONO2 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.