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 S-Qpeak(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-anion correlation decrease in the r range of 10-12 angstrom. 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 center dot center dot center dot 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 center dot center dot center dot HONO2 and an ion pair C2H5NH3+center dot center dot center dot NO2- 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 center dot center dot center dot HO hydrogen bonding in a molecular complex and the NH center dot center dot center dot O one in an ion pair. Finally, we propose our interpretation of structural heterogeneity in PILs and also in APILs.