Theoretical studies of natural and magnetic circular dichroism

Abstract

This dissertation presents theoretical studies mainly of natural and magnetic circular dichroism spectra. For magnetic circular dichroism, the importance of electron correlation effects, here included at the density-functional functional level of theory, and solvent effects are discussed, both being shown to have significant impact on the final spectra. In addition, a unified approach for calculating the temperate-independent contribution to MCD spectra are presented based on damped response theory. This approach is applicable also in energy regions with a high density of electronic states, where the calculation of MCD B terms based on residues of quadratic response functions may give unphysical results. It is argued for an abandonment of the conventional separation of the temperature-independent contribution into A and B terms since this separation may lead to incorrect analysis of the excited states.

For natural circular dichroism spectra, calculations have been performed both at the electronic level, as part of a study on Vitamin B12 including calculations of absorption and magnetic circular dichroism spectra, and at the vibronic level, in both cases employing density-functional theory. The study of vibrationally resolved circular dichroism was performed for a system where the observed spectrum is solely due to isotopic substitution. The model system, 2(R)-deutoriocyclopentanone, exists in two distinct, but near isoenergetic conformations with circular dichroism signals that nearly cancel each other, emphasizing a high requirement for computational accuracy.

The dissertation is concluded with a discussion on the construction of accurate model Hamiltonians for the simulation of vibronic spectra beyond the adiabatic approximation, using pyrazine as an example. Here, multireference wave-function based methods were employed in order to accurately describe potential energy surfaces over a large region of nuclear configuration space.