| In recent years, ionic liquids(ILs) have attracted more and more attention in chemical research because of its unique and controllable physical and chemical properties. Generally speaking, it can be divided into the protic ionic liquids and the aprotic ionic liquids. Among them, since the formation of three dimensional hydrogen bonding network structure which is similar to that of water, the protic ionic liquids have a broad application prospect in the field of biology and medicine. Despite the current experimental study for the research and application of the protic ionic liquids has made certain progress, the comparatively unsophisticated research of basis theory seriously hinders the further experimental research. What’s more, it is difficult to establish relationship between the microscopic structure and macroscopic properties of the protic ionic liquids just relying on experimental studies. The computational chemistry has become one of the main methods in current chemical research after years of development, which can effectively make up for the inadequacy of experimental research for micro information and closely connect the microstructure and macro properties from the molecular level.In this article, we selected methyl ammonium nitrate(methyl ammonium nitrate, MAN) and ethyl ammonium nitrate(ethyl ammonium nitrate, EAN) as our research objects. Both of them are typical protic ionic liquids and have the simplest structure in ionic liquids, which is helpful for us to focus on the nature of the ionic liquids. This paper adopted the combination of quantum chemistry and molecular dynamics simulation method and systematically studied:(1) the change rule of MAN ionic liquid cluster microstructure with cluster size;(2) the structure, dynamics and hydrogen bonding properties of the bulk phase of EAN ionic liquid.On the one hand, in the theory study of the MAN clusters, we used the secondary sieving method through molecular mechanics and quantum chemistry to get the low energy structures of MAN clusters from one to nineteen ion pairs respectively and summarized the structure evolution rule of the MAN clusters. With the increase of cluster size, we observed the cage structure whose center was "single anion", "two anion", "three anion", "three anions and single cation" and "four anion and single cation" in turn. By comparing, we found that the NO3– was more likely to enter the bulk phase of clusters, the CH3NH3+ tended to be on the surface of clusters with-CH3 outwards due to the solvophobic effect of-CH3, which is consistent with the recent gas-liquid interface experimental results for the type of quaternary ammonium ionic liquids. In addition, we found that with the number of ion pair increasing, the average binding energy of each ion pair increased gradually and finally tended to be a constant value, which was about 48 kcal/mol. This indicated there was an obvious synergistic effect between ion pairs, which is conducive to the stability of clusters.On the other hand, we systematically investigated the structure, dynamics and hydrogen bonding properties of the bulk phase of EAN ionic liquid by molecular dynamics simulation method under different temperature. Simulation results manifested that the heterogeneous ions gathered easily as a result of the attraction between them. Compared to the bulk phase of water, the anion and cation of EAN ionic liquid showed extremely low translation and rotation rate, which confirmed the higher viscosity of ionic liquid from micro perspectives. In addition, the dissociative autocorrelation function of EAN ion pair displayed that the formation of ion pair between the anion and cation could keep dozens of nanoseconds, which was an order of magnitude longer than that of aprotic ionic liquids. This was mainly because of the longer relaxation time of hydrogen bonds in protic ionic liquids. Besides, the change of temperature would not affect the structure of ionic liquids obviously, but would cause a significant change in the dynamics and hydrogen bonding properties. |
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