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

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. (C) 2019 Elsevier B.V. All rights reserved.