Chapter 10 - Vibrational optical activity

Abstract

The extension of optical activity measurements into the vibrational spectrum can be a source of valuable new information about chiral molecules. Conventional optical rotation and circular dichroism are typically two orders of magnitude smaller in the IR than in the near UV. The Raman-scattering approach culminated in the first genuine observation of vibrational optical activity in typical chiral molecules. Raman optical activity (ROA) is more favorable than vibrational circular dichroism (VCD) for detecting vibrational optical activity, because visible exciting light is used. However, the ROA measurement is beset with difficulties, not the least of which is the fact that the effect is less than 10–3 times that of the conventional Raman effect. In addition, the measurements are susceptible to large artifacts that can swamp any genuine signals. The topic of vibrational optical activity also includes the vibrational analogue of the Faraday effect in which optical activity is induced in vibrational transitions of achiral molecules by an external static magnetic field parallel to the propagation direction of the incident radiation.