The Raman non-coincidence effect: a spectroscopic manifestation of the intermolecular vibrational coupling in dipolar molecular liquids

Abstract: This lecture is addressed to the analysis of the non-coincidence effect (NCE), a spectroscopic manifestation of the intermolecular coupling in molecular liquids. Molecular groups like C=O (strongly active in the ir spectrum ) in dipolar liquids such as ketones, aldheides, immides, esters.., exhibit this phenomenon at a very high degree. It will be shown that the vibrational exciton approach developed by the assumption of the Transition Dipole Coupling (TDC) mechanism, predicts the how the orientational structure of the molecular liquid determine the magnitude and sign of NCE. Specifically, it predicts that in simple molecular liquids, solely structured by dipolar forces, NCE is large and positive whereas when liquid structures are dominated by non-dipolar forces (as those present in H-bonded liquids), this scenario dramatically changes and ir active modes (such as, for instance, the C-O stretching in methanol) may give rise to negative NCE's. This lecture is intended to offer a general overview of non-coincidence effect observed in dipolar liquids in different thermodynamic conditions and the theoretical and simulation results that assisted their interpretation.