A new computational tool for interpreting infrared spectra of molecular complexes

The popularity of the Infrared (IR) spectroscopy is due to its high interpretive power. This study presents a new computational tool for analyzing the IR spectra of molecular complexes in terms of intermolecular interaction energy components. In particular, the proposed scheme enables to associate the changes in IR spectra occurring upon the complex formation with individual types of intermolecular interactions (electrostatic, exchange, induction, dispersion), thus providing a completely new insight into the relations between the spectral features and the nature of interactions in molecular complexes. To demonstrate its interpretive power, we analyze for selected vibrational modes which interaction types rule the IR intensity changes upon the formation of two different types of complexes, namely π...π stacked (benzene...1,3,5-trifluorobenzene) and hydrogen-bonded (HCN...HNC) systems. The exemplary applications of the new scheme to these two molecular complexes revealed that the interplay of interaction energy components governing their stability might be very different from that behind the IR intensity changes. For example, in the case of the dispersion-bound π...π-type complex, dispersion contributions to the interaction induced IR intensity of the selected modes are notably smaller than their first-order (electrostatic and exchange) counterparts.