Structure and dynamics of the protic ionic liquid monomethylammoniumnitrate ([CH_3NH_3][NO_3]) from ab initio molecular dynamics simulations

摘要

The dynamics of the protic ionic liquid monomethylammonium nitrate is investigated by Car-Parrinello molecular dynamics simulations. On average, 1.8 of 3 possible hydrogen bond contactsare formed. Therefore, one hydrogen bond acceptor and one donor site in each ion pair ofmonomethylammonium nitrate remains free, which is similar to water. Furthermore, like water,monomethylammonium nitrate exhibits a fast fluctuating hydrogen bond network. The comparablehydrogen bond network and dynamics of both liquids might explain the similar impact on reactivityand selectivity found for chemical reactions. However, the hydrogen bond network ofmonomethylammonium nitrate and water show some structural differences. While the hydrogenbonds in water arrange in parallel fashion, the hydrogen bonds of monomethylammonium nitrateprefer angles of 0°, 90°, and 180°. The ion dynamics of monomethylammonium nitrate indicate thatat about 85% of the ion pairs are still connected after 14.5 ps. A closer inspection of the firstsolvation shell dynamics of one cation reveals that after 11 ps the current ion pair conformation isindependent of the initial ion pair conformation because the ion pairs lose their information of theinitial ion pair conformation much faster than the time needed to escape from their solvent cage. Theion dynamics of monomethylammonium nitrate can be described by the following model: There areions rattling in long living cages which are formed by long living ion pairs.