Unraveling interactions and catalytic mechanisms of ionic liquid [Bmim][BF4] and styrene oxide using sum frequency spectroscopy

The utilization of ionic liquids as versatile reaction media and catalysts has garnered significant attention in the field of green and sustainable chemistry. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) is employed to investigate the interactions between l-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and styrene oxide (SO) at the air/liquid interface. Spectral analysis in the range of 1000 cm(-1) to 3700 cm(-1) reveals that only vibrational modes of C-H bonds are observed. Notably, the orientation of the epoxy ring of SO is found to be towards the bulk phase, while the three C-H groups on the imidazolium ring of the cation are parallel to the surface. Consequently, there are no observed interactions between the cation and SO. However, in the vibrational spectra of C-H bonds, interactions between the anion BF4- and the CH2 group of the epoxy ring result in blue shifts in the vibrations of CH (SO) and CH3 (cation). These findings support a proposed reaction mechanism where the ionic liquid catalyzes SO first and subsequently reacts with CO2 providing valuable insights into intermolecular interactions and molecular orientations at reaction interfaces.