USING ULTRAFAST VIBRATIONAL SPECTROSCOPY FOR A COMPREHENSIVE UNDERSTANDING OF STRUCTURAL AND ROTATIONAL MOTIONS FOR WATER TO PROTIC IONIC LIQUIDS

In this work, two-dimensional infrared (2D-IR) spectroscopy investigates the timescale of solvent fluctuations for proton and hydride transfers. To elucidate hydride transfer dynamics, the BH stretch of \ce{BH4-} is probed in various solvents from \ce{H2O} to ionic liquids (ILs). For proton transfer dynamics, a vibrational probe (\ce{SCN-}) explores the three-dimensional hydrogen bonding environment of a protic ionic liquid (PIL). \ce{BH4-} is first investigated in increasing NaOH concentrations to develop a molecular understanding of suppressing the hydrogen evolution reaction. As the concentration increases, the timescale of frequency fluctuations decrease. Born Oppenheimer molecular dynamics (BOMD) simulations suggest that a crowding effect of ions around \ce{BH4-} inhibits the rearrangement of dihydrogen bonds between \ce{BH4-} and \ce{H2O}. To completely suppress the hydrogen evolution reaction, ILs with \ce{BH4-} as the anion are investigated. The linear and 2D-IR spectra of the antisymmetric BH stretch of \ce{BH4-} are complicated due to Fermi resonances. The narrow linear and 2D-IR linewidths of \ce{BH4-} in an IL allow a comprehensive assignment of all diagonal peaks and crosspeaks. Confirmed with a model Hamiltonian, two anharmonicities for the antisymmetric BH stretch of \ce{BH4-} are characterized. Polarization- and temperature-dependent 2D-IR is employed to investigate the hydrogen bonding network of the PIL ethyl-ammonium nitrate (EAN). \ce{SCN-} experiences two hydrogen bonding subensembles in EAN as two separate vibrational relaxation times are resolved. Furthermore, the polarization-weighted frequency fluctuation correlation function can be separated into two components: structural spectral diffusion (SSD) and reorientation-induced spectral diffusion (RISD). For \ce{SCN-} in EAN, the timescales of frequency fluctuations are in the rotational limit as the SSD is unresolved. Temperature-dependent 2D-IR extracts the enthalpy and entropy of activation for frequency fluctuations. For \ce{SCN-} in EAN, the enthalpy of activation for rotational motions are similar as to \ce{SCN-} in \ce{H2O}, and this suggests that the breaking and forming of hydrogen bonds around \ce{SCN-} undergoes a similar mechanism in EAN as in \ce{H2O}