Raman noncoincidence effect: A spectroscopic manifestation of the
intermolecular vibrational coupling in dipolar molecular liquids
M. G. Giorgini
Dipartimento di Chimica Fisica ed Inorganica, Università
di Bologna, Viale del
Risorgimento 4, I-40136 Bologna, Italy
Abstract:
This lecture addresses the analysis of the noncoincidence effect (NCE),
a spectroscopic manifestation of the intermolecular coupling in molecular
liquids. The vibrational bandshapes of molecular groups like C=O (strongly
active in the IR spectrum) in dipolar liquids exhibit this phenomenon
at a rather large extent. It will be shown that the vibrational exciton
approach, developed under the assumption of the transition dipole coupling
(TDC) mechanism, predicts how the orientational structure of the molecular
liquid determines the magnitude and sign of the NCE. Specifically, it
predicts that in simple molecular liquids, solely structured by dipolar
forces, the 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 may give rise
to negative NCEs.
This lecture is intended to offer a general overview of NCEs observed
in dipolar (simple and structured) liquids in different thermodynamic
conditions and of the theoretical and simulation results that assisted
in their interpretation.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.
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