Molecular level interpretation of excess infrared spectroscopy

Maciej Śmiechowski

doi:10.1016/j.molliq.2021.117544

Molecular level interpretation of excess infrared spectroscopy

Abstract

Infrared (IR) spectroscopy is an invaluable tool in studying intermolecular interactions in solvent mixtures. The deviation of the IR spectrum of a mixture from the spectra of its pure components is a sensitive measure of the non-ideality of solutions and the modulation of intermolecular interactions introduced by mutual influence of the components. Excess IR spectroscopy, based on the established notion of excess thermodynamic functions, provides a well-defined picture of such deviation. On the other hand, the difference spectra method strives to obtain so-called affected spectra by numerically removing the bulk component contribution until the IR spectrum of the spectrally affected component is isolated. Although tremendously useful, excess IR spectroscopy remains poorly studied from a computational point of view. Based on ab initio molecular dynamics (AIMD) simulations, IR spectra of a dimethyl sulfoxide (DMSO)–γ-butyrolactone (GBL) liquid mixture are obtained here from first principles, without resorting to experimental input. Using dipolar decomposition techniques, the excess IR spectrum is further analyzed by rigorously separating the contributions from the modulation of the intra- and intermolecular parts of the spectra of both components and the mutual interaction spectrum. It is found that while the intramolecular part of excess IR spectrum is a good predictor of its overall shape, the intermolecular parts are crucial for elucidation of the band shifts and fine details of the IR spectrum of the mixture. Furthermore, the affected IR spectrum is obtained for both components by applying the spectral similarity method. The exact relationship between the affected and excess IR spectrum is derived and the latter is discovered as the predictor of the band shifts in the affected spectrum. Owing to a reduced cancellation effect of intermolecular contributions, the affected spectrum is a physically justified counterpart of the excess IR spectrum, providing a complementary view of the interactions in the mixture.