Room temperature ionic liquids (RTILs) have properties that are distinct from other ionic liquids and from common organic liquids.  RTILs are liquid around room temperature in contrast to, e.g., liquid NaCl, which melts at 1074 K.  The low melting point of RTILs is achieved by using molecular ions, often organic, that are asymmetric, large, have diffuse or delocalized charge, or some combination of these properties.  The presence of these properties in the cation, anion, or both, prevents crystallization by reducing the enthalpic incentive and/or increasing the entropic cost.  The vast number of cation and anion combinations result in a wide range of liquids with distinct properties that can have a variety technological applications.  The dynamics and interactions in RTILs were studied with a combination ultrafast two dimensional infrared (2D IR) experiments and polarization selective pump-probe (PSPP) experiments. The IR experiments are conducted on the vibrational transition of a probe solute molecule.  The vibrational transition frequency can couple to its environment through a directional vector interaction, i.e., electric fields via the Stark effect.  In such cases, reorientation of the vibrational transition dipole (molecular orientational relaxation measured with PSPP) and its frequency fluctuations can be strongly coupled.  The 2D IR experiments on probe molecules give differing frequency-frequency correlation function (FFCF) decays, due to reorientation-induced spectral diffusion (RISD).  In experiments in which different FFCFs are measured for different 2D IR pulse polarizations, 〈xxxx〉 vs. 〈xxyy〉 the observed spectral diffusion is a combination of structural spectral diffusion (SSD) and RISD.  Theoretical developments enable the SSD (evolution of the liquid structure) to be separated from the RISD (rotation of the probe molecules) for both first and second order Stark effects.

Dynamics of room temperature ionic liquids measured with two dimensional infrared spectroscopy: The influence of electric fields on observables

253rd National Meeting (2017)

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