Calculation of two-dimensional infrared spectra of ultrafast chemical exchange with numerical Langevin simulations

We combine numerical Langevin simulations with numerical integration of the Schrodinger equation to calculate two-dimensional infrared spectra of ultrafast chemical exchange. This provides a tool to model and interpret such spectra of molecules undergoing chemical processes, such as isomerization and solvent exchange reactions. Two-dimensional infrared spectroscopy has already been used to extract reaction rates for ultrafast chemical reactions. We demonstrate that these spectra are not only sensitive to the rates, but also to the finite duration of the exchange. This is emphasised by comparing with the popular Kubo two-state jump models, which do not account for finite exchange times