Chapter 9: Analysis of MCD Spectra

Representative examples of the analysis of the Faraday ₁, ₀ and ₀ terms are described. Many of the examples revolve around the study of porphyrin complexes, since the technique has been particularly widely utilised in this context. The magnetic circular dichroism (MCD) spectra of two high symmetry organic ions, the cyclononatetraenide anion (C₉H₉⁺) and the tropylium cation (C₇H₇⁺), and that of fullerene (C₆₀) provide examples of the analysis of Faraday ₁ terms. The MCD spectrum of zinc tetraphenyltetraacenapthoporphyrin provides an example of how spectral band deconvolution can be used to identify electronic bands, which cannot be identified in the electronic absorption spectrum owing to the presence of intense overlapping vibrational bands, based on the distinctive band morphology of the Faraday ₁ term. The spectra of azaporphyrins and metal phthalocyanine anion radicals provide examples of the similar use of coupled pairs of oppositely signed Faraday ₀ terms to identify and assign the main electronic bands. In the latter case a temperature dependence study was used to identify a non-degenerate groundstate, since Faraday ₀ terms would be anticipated in the absence of Jahn-Teller effects. The qualitative and quantitative analyses of Faraday ₀ terms based on the “fingerprint method” and the use of magnetisation curves, derived from spectra recorded using different field strengths at cryogenic temperatures, are described in the context of the MCD spectra of haem proteins. The recent application of time dependent density funcitional theory (TD-DFT) to MCD spectroscopy is briefly outlined and the prospects for future advances in this regard are examined.