Use of isotope effects in studies of intermolecular interactions by Raman spectroscopy

Abstract

A very brief introduction to vibrational spectroscopy is given with a particular emphasis on the effect of isotopic substitution. A few key references to studies of both the gaseous and the crystalline states are cited. Intermolecular interactions in liquids are studied by (i) Resonance Energy Transfer giving rise to a non-coincidence effect between the anisotropic and the isotropic Raman components, (ii) Coalescence of Bands in Mixtures of Isotopomers, giving rise to an observation of fewer bands than expected from a simple addition of bands from the individual isotopomers in the mixture, and finally (iii) low-frequency Raman spectroscopy, for which the R( v ̄ ) representation is used in order to get rid of the intense Rayleigh line appearing in the Raman spectra of liquids. Experimental data include results for isotopically substituted liquid formamides and a solution of HCOND 2 in D 2O. The results show that intermolecular interactions via transfer of vibrational energy occur for the carbonyl stretching vibration at around 1650 cm −1 (amide-I band). A band in the low-frequency spectrum at around 100 cm −1 is assigned to a mode involving displacements of atoms in the intermolecular hydrogen bonds. Similar interactions might be of importance for the fast dynamics of biological macromolecules like proteins and nucleic acids.