بررسي و مطالعه ي خواص ساختاري، ديناميكي و طيفي مايعات يوني دوكاتيوني با استفاده از شبيه سازي ديناميك مولكولي از اساس (AIMD)

In this study, we want to gain a deeper understanding of the various molecular properties of imidazolium-based dicationic ionic liquids (DILs) at the molecular level. So, we conducted ab initio molecular dynamics (AIMD) simulations on two DILs, namely [C1(mim)2][PF6]2 and [C3(mim)2][NTF2]2, to calculate their structural, dynamical, and vibrational properties. The initial part of our research focused on investigating the behavior of nanodroplets of [C1(mim)2][PF6]2 using AIMD simulations. The vibrational features as well as the structural, interfacial, and dynamical properties of different sized droplets were analyzed and compared to the bulk phase system. Structural properties of the droplets, such as π−π stacking, radial distribution functions (RDFs), structure factors, combined distribution functions (CDFs), and angular distribution functions (ADFs) were analyzed to understand the interactions and orientations of their ions. The vibrational features and hydrogen bonding strength of droplets were studied by calculating their infrared (IR) and power spectra, determining the contribution of different types of hydrogen bonding to each vibrational mode. The calculated spectra showed good overall agreement with the experimental results. The interfacial properties of the droplets and the orientation of their ions were analyzed using density profiles and the exposed surface. The results showed that, in all systems studied, cations and anions were equally likely to exist in both inner and outer layers, and the cations tended to be oriented toward the center of droplets with obtuse angles. Additionally, the droplet densities were extrapolated to predict the bulk phase density with less than 2% deviation. The dynamical properties of hydrogen bonds, mean square displacement, and van Hove correlations of cations and anions were also analyzed. The results indicated that there was no regular trend in the dynamic properties of droplets with an increasing system size. In the second part of this research, the OPLS-VSIL force field (FF) was extended to DILs and eva‎luated the ability of different OPLS-based FFs (i.e., OPLS-2009IL, 0.8*OPLS-2009IL, and OPLS-VSIL) in predicting different properties of the studied DIL by comparing their results with AIMD simulation and experimental results. To achieve this purpose, MD simulations with three different OPLS-based FFs as well as AIMD simulation were performed for [C3(mim)2][NTF2]2 DIL and its structural, dynamical, vibrational, and volumetric properties were analyzed. Structural properties of the studied DIL i.e., RDFs, structure factor, and hydrogen-bond network showed that compared to 0.8*OPLS-2009IL FF, there is a much better agreement between the results of both OPLS-2009IL and OPLS-VSIL FFs with the AIMD simulation. On the other hand, the results of dynamical properties such as mean square displacements, van Hove correlation functions as well as hydrogen bond, ion pair, and ion cage dynamics depicted that in both 0.8*OPLS-2009IL and OPLS-VSIL FFs, the dynamics of the system is almost similar, and compared to OPLS-2009IL FF, they have better agreements with experimental results where they exist. So, it can be seen that although reducing the total charge of studied DIL by 20% leads to an increase in the dynamics of the system, the type of distribution of partial charges on each atom does not significantly affect the system’s dynamics. The calculated IR and power spectra showed that the vibrational features of studied DIL in three OPLS-based FFs are mostly the same and reducing total charge and different type distribution of partial charges have no significant effect on the studied system.

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