Vibrational spectroscopy of imidazolium-based ionic liquids: A combined MD/DFT study

Abstract

Vibrational spectroscopy is a powerful tool to probe liquid structure and characterize intermolecular interactions, including hydrogen-bonded interactions. Herein, C H stretching vibrations of ionic liquids based on 1-butyl-3-methylimidazolium (BMI+) cations paired with Cl−, Br− or BF4− anions are studied via molecular dynamics (MD) simulations and ab initio methods and compared with experiments. MD results with the CHARMM and the OPLS-AA force fields show that the vibrations of the acidic hydrogen of the BMI+ ring shift to higher frequencies as the anion basicity increases. This is at variance with recent experimental results. The density functional theory (DFT) combined with MD in the QM/MM (quantum mechanics/molecular mechanics) framework, on the other hand, predicts a red shift for the same C H stretching vibrations. The red shift tends to increase with the anion basicity and thus with the interionic hydrogen-bond strength, in concert with experiments. In addition, MD/DFT yields a narrowing of C H vibrational bands of the BMI+ ring in BMI+BF4−, compared to those in BMI+Cl− or BMI+Br−. This is also in good qualitative agreement with the deconvoluted IR spectra of these ionic liquids.