Reorientation-induced spectral diffusion of non-isotropic orientation distributions.

When reorientation of a vibrational chromophore is faster than the relaxation of its local environment, the frequency fluctuation correlation function (FFCF) measured by two-dimensional infrared spectroscopy (2D-IR) spectroscopy is an interplay of scalar structural spectral diffusion and vectorial reorientation-induced spectral diffusion (RISD). Theory has been established to calculate the RISD component of different polarization configurations with the assumption that the molecule orients randomly in a local electric field. We show here that in the [BMIM][SCN]/AOT/chlorobenzene system, where the local electric field is strong, this assumption is incapable of reproducing the experimental results. We modify the current theory by assuming a Boltzmann distribution of transition dipoles of the chromophores in a local electric field and use a Markov chain model to calculate the RISD component of 2D-IR with different polarization configurations. The result reproduces key features of the experiment and suggests a potential ion-exchange in the [BMIM][SCN]/AOT/chlorobenzene system.