Chiroptical Spectroscopy of Small Elastic Organic Molecules

The presented PhD thesis is focused on applications of quantum chemical calcula tions of vibrational and electronic chiroptical spectra to structural studies. Some of the calculations are accompanied by experimental measurements. In the literature section of this work, detailed historical account of discoveries con cerning optical activity is presented, from the first observation of optical rotation, to the relatively recent advancements in instrumentation. Theory of the underlying phys ical phenomena and the basic methods of computational chemistry used for evaluation of relevant physical quantities are also presented. In the part of the thesis focused on electronic chiroptical spectra, recently imple mented computer code for evaluation of the residues of quadratic response functions within the framework of density functional theory was used for calculations of rota tional strengths of spin-forbidden electronic transitions. The physical quantities related to spin-forbidden circular dichroism and circularly polarized phosphorescence spectra were calculated for a set of benchmark molecules: hydrogen peroxide and selection of 7 bicyclic ketones, 3 of them with conjugated double bonds. Basis set size depen dence and gauge invariance of the length and velocity formulations were investigated. Performance of two approximate spin-orbit coupling operators: atomic mean field, and effective charge operator was also assessed in comparison to the full, two electron Breit-Pauli spin-orbit treatment. In the part of the thesis focused on vibrational chiroptical spectra, two vibrational optical activity techniques: Raman optical activity and vibrational circular dichroism were used to study the structure of cysteine in aqueous and heavy water solutions of various acidity. The spectra calculated within the framework of harmonic approxima tion and linear response density functional theory with different models of aqueous environment were compared with the results of conducted instrumental measurements. Conformational space of various protonation forms of cysteine was probed with the help of a systematic conformational search, by permutation of the molecular dihedral angles, and also by performing classical molecular dynamics simulations using AMBER force field definition. Water solvation effects were modelled with the help of polariz able continuum model, and by explicit incorporation of water molecules in the studied molecular structures. The performance of the models with regards to reproduction of the experimental spectra was assessed.