Understanding the Reaction Kinetics and Microdynamics between Methylimidazole and Alkyl Thiocyanate for Ionic Liquid Synthesis through Experiments and Theoretical Calculation

Abstract

Ionic liquids (ILs) are usually prepared via the Menshutkin S N 2 reaction. Herein, the effects of the alkyl chain length of the substrate on the reaction kinetics and microdynamics between 1methylimidazole (MIm) and alkyl thiocyanate (RSCN, R = methyl, ethyl, and butyl groups) for the preparation of thiocyanate-based ILs were analyzed by time-dependent Fourier transform infrared (FTIR) spectroscopy, classical molecular dynamics (MD) simulation, and ab initio molecular dynamics (AIMD) simulation. Experimental results show that the longer chain length of thiocyanate leads to a significantly slower rate constant and much higher activation energy. The rate constant of the MIm/BuSCN system is reduced by a factor of 21 fold compared to that of the MIm/MeSCN system at 330 K. The activation energy of the MIm/BuSCN system is increased by 27.5 kJ/mol in the MIm/MeSCN system. Metadynamics calculations in the explicit solvent offer a consistent trend for transition state energy barrier with experiments. MD simulations indicate that the long alkyl chain of thiocyanate results in a weaker spatial distribution of reaction sites as well as slower diffusion and reorientation, which can reduce the intermolecular collision probability. Hopefully, the insights from this work would guide the industrial synthesis of ILs.

Reaction Kinetics and Microenvironments between Alkylimidazole and Methylthiocyanate for the Synthesis of Ionic Liquids

“…The activation energies of the MIm/MeSCN, EIm/MeSCN, and BIm/MeSCN systems are 66.3, 68.4, and 74.6 kJ/mol, respectively, which indicates the increased activation energy caused by alkylimidazole with a longer alkyl chain. For comparison, the activation energies of the MIm/MeSCN, MIm/EtSCN, and MIm/BuSCN systems are 65.8, 85.4, and 93.3 kJ/mol, respectively, as reported in our previous work . Thus, compared to the change of activation energy with a prolonged alkyl chain of imidazole, it is much smaller than that of the alkylthiocyanate substrate with the same substituent.…”

“…Thus, it can be said that the longer alkyl chain of imidazole can result in a slower reaction rate, following the same trend as the alkylthiocyanate. 20 The normalized peak area of SCN − as a function of time within 4 h is plotted in Figure 3. Table 1 lists the rate constants (k) obtained from the slope of the fitting line based on pseudofirst order.…”

“…The interactions among MIm, EIm, BIm, and MeSCN were represented by the Lennard-Jones (L-J) potential in the form of the OPLS-AA force field. The force field parameters of MeSCN were taken directly from our previous work . The optimized structures of MIm, EIm, BIm, and MeSCN were gained at the B3LYP/6-311G­(d,p) level, and the ChelpG charges were refitted at the same level using the Gaussian 09 program .…”

“…MD simulations indicate that the long alkyl chain of thiocyanate results in a weaker spatial distribution of reaction sites as well as slower diffusion and reorientation, which can reduce the intermolecular collision probability. 20 The imidazolium-based ILs with the same alkyl substituents can also be synthesized by the low-cost alkylimidazole nucleophile and MeSCN substrate, which motivates us to further probe the reaction kinetics and microenvironments between alkylimidazole and MeSCN with emphasis on the imidazole nucleophile with different alkyl chain lengths. MD simulation can act as a promising and effective tool to probe the complex microscopical structure and dynamics behaviors, which has been extensively used to reveal the physicochemical properties of alkylimidazolium-based ILrelated systems at the molecular level.…”

“…For example, the activation energies of the MIm/BuSCN system are increased by 27.5 kJ/mol compared to that of the MIm/MeSCN system. MD simulations indicate that the long alkyl chain of thiocyanate results in a weaker spatial distribution of reaction sites as well as slower diffusion and reorientation, which can reduce the intermolecular collision probability . The imidazolium-based ILs with the same alkyl substituents can also be synthesized by the low-cost alkylimidazole nucleophile and MeSCN substrate, which motivates us to further probe the reaction kinetics and microenvironments between alkylimidazole and MeSCN with emphasis on the imidazole nucleophile with different alkyl chain lengths.…”