Theory of Optical Spectroscopy

Abstract

The most important aspects of the theory of electronic absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopy are described. The bands observed in each case arise from the same set of electronic transitions. The ellipticity and differential absorbance units which are used in CD spectroscopy are explained and the interaction of the electric and magnetic dipole moments required to create a helical redistribution of electric charge in chiral molecules is described based on the Rosenfeld equation. Various intensity mechanisms, which can result in significant CD intensity in the UV–visible region even in the absence of π → π* transitions with intrinsic chirality, such as excition coupling, and induced circular dichroism (ICD) based on Kuh–Kirkwood coupled oscillator and “CD stealing” mechanisms are introduced. When a magnetic field is applied, there is a lifting of state degeneracies into 2J+1 microstates based on the MJ quantum number for orientation relative to the applied field. This forms the basis of MCD spectroscopy. The intensity equation and the use of the three Faraday terms, 1, 0 and 0, to analyse MCD spectra, is described. Issues related to the signs of the Faraday terms and the sign observed in the MCD spectrum are outlined. The use of 1/0 ratios, where 0 refers to the dipole strength of the corresponding absorption band, to derive excited state magnetic dipole moment values is described. The use of MCD spectra recorded at room and cryogenic temperatures to study Faraday 0 terms associated with degenerate groundstates is also introduced.