Geometry determination of complexes in a molecular liquid mixture using electron-vibration-vibration two-dimensional infrared spectroscopy with a vibrational transition density cube method.

We demonstrate the use of a new vibrational transition density cube (VTDC) method for determining the geometry of complexes in a molecular liquid mixture from electron-vibration-vibration two-dimensional infrared (EVV 2DIR) spectra. The VTDC method was used to calculate the electrically-mediated intermolecular vibrational coupling and thereby the EVV 2DIR spectra. Using the 1:1 benzonitrile-phenylacetylene (BN-PA) liquid mixture as a test case, the new method leads to a distance of 3.60 Å between the interacting BN-PA pair, a much more accurate value than the distance previously obtained using a dipolar approximation for the electrical coupling. We also show that molecular dynamics simulations of the liquid mixture predict a modal geometry of complexation which agrees well with the geometry determined from the 2DIR data via VTDC analysis. We therefore conclude the combination of VTDC and EVV 2DIR data is a useful approach for the determination of the geometry of molecular complexes in the condensed phase.