Ricerc@Sapienza

In this study we have explored, by means of ab initio molecular dynamics, a subset of three different
protic ionic liquids (ILs). We present both structural and dynamical information of the liquid state of
these compounds as revealed by accurate ab initio computations of the interactions. Our analysis
figures out the presence of a strong hydrogen bond network in the bulk state, that is more stable in
those ILs characterised by a longer alkyl side chain. Indeed it becomes more long-lasting passing from
ethyl ammonium to butyl ammonium, owing to the hydrophobic effects stemming from alkyl chain
contacts. Furthermore, the relative free energy landscape of the cation–anion interaction exhibits a
progressively deeper well as the side chain of the cation gets longer. The hydrogen bond interaction,
as already mentioned in previous works, leads to loss of degeneracy of the asymmetric stretching
vibrations of the nitrate anions. The resulting frequency splitting between the two normal modes is
about 90 cm?1.