Effect of the external electric field on the electronic structure, spectroscopic features, NLO properties, and interionic interactions in ionic liquids: A DFT approach

Abstract

Ionic liquid (IL)s are a unique group of chemicals with extraordinary physical and chemical properties. Due to the advantage of the tunability of their features by varying the ion compositions and either IL/IL or IL/conventional solvent mixing ratios, uncountable functions of them have been discovered. In the field of electrochemistry and energy applications, the function of ILs under external field should also be understood well. The most fundamental way of gaining insight into the function of a substance is to study the structure. Therefore, this work focuses on the investigation of structural variations of an exemplary IL, 1-hexyl 3-methylimidazolium chloride (HmimCl), under external electric field (EEF) via ab initio quantum chemistry calculations.Field dependent geometry analysis show that the interionic bond and the molecular conformation can be modulated by the strength and the direction of the applied field, and the disassociation occurs at ~0.7 V/Å level. Mulliken, atomic polar tensor, and natural bond orbital charges were obtained within field dependency and their correlation was examined. Charge displacement curves were constructed to visualize the field inducement on the charge transfer. Interionic interactions were profiled as hydrogen, van der Walls, and steric effect by using reduced density gradient analysis and the breaking of the single hydrogen bond was determined to occur at ~0.6 V/Å level. The vibrational spectra (IR and Raman) were simulated and the critical peaks that are affected by the field inducement were interpreted. The analysis of the field dependent Nonlinear Optical (NLO) properties shows that the only important component is in the field direction for the field intensity >0.2 V/Å. Electronic transition properties determined via TD-DFT calculations with B3LYP/6-311G** level of theory indicate that the UV–Vis spectrum and underlying transitions are depended on the direction of the field, and experience a bathochromic shift with increasing inducement showing that the IL becomes more susceptible to excitation.