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 1...