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

Ionic liquids (ILs) are usually prepared via the Menshutkin SN2 reaction. Herein, the effects of the alkyl chain length of the substrate on the reaction kinetics and microdynamics between 1-methylimidazole (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.