Applications of Magnetic Circular Dichroism for Studies of Organic Molecules

Magnetic circular dichroism (MCD) spectroscopy provides extended information about molecular electronic structure and transitions. Because the chirality is induced by static magnetic field it can also be used for non-chiral compounds. However, interpretation of the spectra is difficult, and precise computations using quantum mechanical approaches were implemented in publicly available software packages only recently. We present introduction to the theory of MCD, explaining physical origins of the phenomenon. The ADF and Dalton software packages available for first principle calculations of MCD are compared. A short overview of the instrumentation is given. Finally we give an example of our ongoing study on 5,10,15,20-tetraphenyl-21H,23H-porphyrintetrasulfonic acid (TPPS) and its protonated, where the MCD spectra were interpreted by TDDFT (time dependent density functional theory). Introduction Magnetic circular dichroism (MCD) spectroscopy is a method which uses circularly polarized light to study transitions between molecular states. In contrast to other chiroptical spectroscopies like ECD (Electronic Circular Dichroism), VCD (Vibrational Circular Dichroism) or ROA (Raman Optical Activity) it can study nonchiral molecules. The effect of MCD can be observed as different absorption of rightand leftcircularly polarized light by the system in a static magnetic field oriented along the direction of propagation of the measuring light beam. MCD can be observed for number of molecular transitions, and in a wide spectral range. X-ray MCD is used in studies of solid state materials where it can probe the magnetic properties of the matter [Funk, 2005]. UV-vis and near IR region is characteristic for valence shell electronic transitions. MCD in this spectral range is usually used in studies of molecules in diluted solutions, much like absorption or ECD measurements. Finally, one can measure MCD even in mid IR region, and observe the effect of magnetic field on the vibrational transitions [Bour, 1996]. In our laboratory we are focusing on the most usual UV-vis MCD measurements. History of MCD is dated back to the times of Michael Faraday when he in 1845 observed that linearly polarized light was rotated by application of a longitudinal magnetic field. This effect, referred to as the magneto-optical rotation (MOR), was the first experimental evidence of a relation between electromagnetism and light. Dependence of MOR on the wavelength—so called MORD (Magnetic Optical Rotatory Dispersion)— is related to the MCD spectrum by Kramers-Kronig relations, similarly as the natural circular dichroism is related to the optical rotatory dispersion [Buckingham, 1966]. First theoretical explanations of MCD were attempted in 1930s [Serber, 1932]. Before the quantum mechanics fully developed to be able to describe molecules exactly, semiempirical approaches to interpretation of MCD were proposed [Michl, 1978a; Michl, 1978b]. In spite of the complicated MCD expressions, the complete theory was recently implemented in common quantum chemical software [Dalton; ADF; Seth, 2008a–c]. In recent years a lot of work has been done on calculating of MCD of metaloporphyrins [Solheim, 2008], [Peralta, 2007], but much less on free base species. In our laboratory, we are currently studying effects of protonation of free-base 5,10,15,20-tetraphenyl-21H, 23H-porphine-p,p′,p′′,p′′′-tetrasulfonic acid (TPPS) (Fig. 1). Because the phenyl-sulfogroups are relatively strongly acidic, they are not protonated until very low pH. The protonation occurs on the nitrogen atoms of the central ring [Farjtabar, 2010]. While there have been some studies of protonation of 5,10,15,20-tetraphenyl-21H, 23H-porphine (TPP) [Mack, 2005] to the best of our knowledge there are no MCD studies with TPPS. Theory The theory of MCD is complicated by many terms that have to be included, and for detailed derivation we 177 WDS'11 Proceedings of Contributed Papers, Part III, 177–181, 2011. ISBN 978-80-7378-186-6 © MATFYZPRESS