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Ce travail vise à comprendre l’effet de la chiralité sur la structure et la photoréactivité de biomolécules, isolées en phase gazeuse. Pour cela, nous combinons des méthodes de spectroscopie laser couplées à la spectrométrie de masse avec des calculs de chimie quantique. La comparaison entre spectres expérimentaux et simulés permet de déterminer la structure des espèces étudiées et de comprendre leur photoréactivité. La première partie étudie la différence de structure entre les deux diastéréoisomères d’un dipeptide cyclique (cyclo Tyr-Tyr). Une seule forme est observée quand les deux résidus tyrosine n’ont pas la même chiralité. Dans cette structure, un des cycles aromatiques est replié sur le cycle peptidique et l’autre est étendu à l’extérieur. Dans le stéréoisomère où les deux résidus ont la même chiralité, cette forme coexiste avec une autre, dans laquelle les deux cycles aromatiques interagissent par une liaison hydrogène. La deuxième partie concerne la structure et la nature des états excités de complexes d’alcaloïdes dérivés de la quinine avec l’acide sulfurique. Les complexes sont formés par l’ajout de l’ion bisulfate sur l’alcaloïde doublement protoné. L’ion bisulfate protège l’alcaloïde de la photodissociation UV grâce à un effet de cage, qui est identique pour la quinine et son pseudoénantiomère la quinidine. / This thesis aims at understanding chirality effects on the structure and photoreactivity of biomolecules in the gas phase by combining laser spectroscopy coupled to mass spectrometry and quantum chemical calculations. Comparison between experimental and simulated spectra allows determining the structure of the species under study and understanding their photoreactivity. The first part of this work focuses on the structural differences between the two diastereoisomers of a cyclic dipeptide, cyclo Tyr-Tyr. Only one form is observed when the two residues have different chirality. In this form, one of the aromatic rings is folded over the dipeptide ring and the other one is extended outwards. For the diastereomer with residues of identical chirality, this form coexists with a structure in which the two aromatic rings are in a stacked position, stabilized by hydrogen bond formation. The second part of this thesis deals with the structure and the nature of the excited states of complexes of cinchona alkaloids with sulfuric acid. An important finding is that the complexes are formed by adding the bisulfate ions to doubly-protonated alkaloid. The presence of the bisulfate ion has a protective effect towards photodissociation thanks to the cage effect, which is identical for quinine and its pseudo enantiomer quinidine.

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This dissertation describes the implementation of the molecular electronic structure calculations with an implicit solvent model using coupled-cluster theory. The theory for and the implementation of the solvent reaction field method (SCRF) and the reference interaction site model (RISM) at the coupled-cluster singles and doubles (CCSD) are presented. In the SCRF model a solute molecule is placed in a spherical cavity, and the outer solvent is represented by a dielectric continuum, which is characterized by the dielectric constant of the solvent. The reaction field is introduced to the system by using the multipole moment expansion of the electronic structure of the solute molecule and the dielectric constant. The SCRF method has been used to calculate the conformational equilibrium and the rotational barriers of 1,2-dichloroethane in vacuum and in different solvents. The calculated results are compared with experimental values. In addition, the solvent effects on the energetics of the mechanism of nitration of benzene are reported using the implemented CCSD-SCRF model. The idea of RISM is to replace the reaction field in continuum models by a microscopic expression in terms of the site-site radial distribution functions between solute and solvent, which can be calculated from the RISM integral equations. The statistical solvent distribution around the solute is determined based on the electronic structure of the solute, while the electronic structure of solute is influenced by the surrounding solvent distribution. Therefore, the wave function and the RISM equations are solved self-consistently with CCSD. Pair correlation functions, partial atomic charges, and solvation free energies of water and N-methylacetamide are calculated in liquid water using proposed theory. Both the CC-SCRF and CC-RISM methods have been implemented in a developmental version of the Q-Chem 3.2 quantum chemistry package.

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In this thesis, a method to calculate eigenpairs is implemented for the Multipsi library. While the standard implemtentations use the Davidson method with Rayleigh-Ritz extraction to calculate the eigenpairs with the lowest eigenvalues, the new method uses the harmonic Davidson method with the harmonic Rayleigh-Ritz extraction to calculate eigenpairs with eigenvalues near a chosen target. This is done for Configuration Interaction calculations and for Multiconfigurational methods. From calculations, it seems the new addition to the Multipsi library is worth investigating further as convergence for difficult systems with a lot of near-degeneracy was improved. / I denna avhandling implementeras en metod för att beräkna egenpar för Multipsi-biblioteket. Medan standardimplementeringarna använder Davidson-metoden med Rayleigh-Ritz-extraktion för att beräkna egenparen med de lägsta egenvärdena, använder den nya metoden den harmoniska Davidson-metoden med den harmoniska Rayleigh-Ritz-extraktionen för att beräkna egenparen med egenvärden nära ett valt mål. Detta görs för konfigurationsinteraktionsberäkningar och för multikonfigurationsmetoder. Utifrån beräkningarna verkar det nya tillskottet till Multipsi-biblioteket vara värt att undersöka vidare eftersom konvergensen för svåra system med mycket nära degenerering förbättrades.

In the context of the search and tuning for novel magnetic materials, transition metal compounds exhibit remarkable features where the spin-orbit interaction is crucial. The collective interactions between various effects, like spins and charges, create different classes of unique magnetic systems. For heavy transition-metal compounds, the strength of spin-orbital coupling is enhanced. The jeff. = 1/2 Mott insulating state emerges from the combination of the spin-orbit interaction and the electronic correlations. The quantum-chemistry methods are employed in this thesis to investigate single- and two-site magnetic interactions of the selected transition-metal compounds. We also provide different estimations for the single- and two-site magnetic interactions based on the level of calculation accuracy. In this thesis, we apply ab initio quantum-chemistry methods to explore the electronic and magnetic properties of several d/f compounds. The thesis structure is as follows: In Chapter 1, the introduction of the thesis provides a short discussion of the electronic correlations and magnetism in transition metal compounds. In Chapter 2, the fundamentals of the quantum chemistry wavefunction-based approach are covered. This chapter gives an overview of the applied methods in this thesis. In Chapter 3, we discuss the quantum chemistry approach to investigate the material candidates to host Kitaev physics. The technique to obtain the strength of two-site magnetic couplings, including the Kitaev coupling, is discussed in-depth. In Chapter 4, we apply the technique, which is described in Chapter 3, to investigate the two-site magnetic interactions in the H3LiIr2O6, and Cu2IrO2 compounds as Kitaev candidates. The two-site magnetic couplings are reported in these compounds. In Chapter 5, we use quantum chemistry methods to investigate the on-site electronic and magnetic properties in the KCeO2 compound where 4f1 Ce3+ ions form a triangular two-dimensional lattice with sites of effective spin-1/2. Similar ytterbiumbased delafossites had been investigated as candidates for quantum spin liquid ground states. The absence of ordinary magnetic order is characteristic of quantum spinliquid states where quantum entanglements and fractionalized excitations are enriched. In Chapter 6, the magnetic properties of Co 3d8 ions doped in the Li3N crystalline solid are discussed. The results of the quantum chemistry investigation are been set side by side along with the experiment’s results. The Co ion in such a rare environment gives rise to single-site magnetism of an easy-plane anisotropy.:Table of Contents . . . . . . . . . . . . . . . . . . . . . . iv List of Figures . . . . . . . . . . . . . . . . . . . . . . . . .vi Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i Acknowledgements . . . . . . . . . . . . . . . . . . . . . .iii 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 1.1 Electronic correlations and magnetism in transition metal compounds ...........1 1.2 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Quantum chemistry methodology . . . . . . . . . . . . . . . . .6 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Many-electron Hartree-Fock approximation . . . . . . . . . . . . . . . 9 2.3 Multi-configurational self-consistent field and multi-reference configuration methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4 Spin-orbit interaction and g-factors calculation . . . . . . . . . . . . . 15 2.5 Embedded cluster approach . . . . . . . . . . . . . . . . . . . . . . . 18 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3 Quantum chemistry investigation of Kitaev material candidates . . . . . . . . . . .21 3.1 Introduction to the Kitaev model . . . . . . . . . . . . . . . . . . . . 23 3.2 Kitaev materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.3 Two-site quantum chemistry calculations . . . . . . . . . . . . . . . . 36 3.4 Effective Model of Two Spin-1/2 . . . . . . . . . . . . . . . . . . . . . 38 3.5 Non-canonical correspondence between two-site QC results and the effective Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.6 Pseudospin coordinate system and canonical correspondence between two-site QC results and the effective Hamiltonian . . . . . . . . . . . 51 3.7 Signs of the g-tensor in the Kitaev limit . . . . . . . . . . . . . . . . 53 3.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4 Kitaev material candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2 Details of QC calculations . . . . . . . . . . . . . . . . . . . . . . . . 64 4.3 QC investigation of H3LiIr2O6 . . . . . . . . . . . . . . . . . . . . . . 66 4.4 QC investigation of Cu2IrO3 . . . . . . . . . . . . . . . . . . . . . . . 75 4.5 Impact of local symmetries on the obtained sets of magnetic couplings ......... 82 4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 5 Ce ions in two-dimensional triangular spin-1/2 lattices . . . . . . . . . . . . . . . . . . . . 89 5.1 Spin-1/2 frustrated triangular lattice . . . . . . . . . . . . . . . . . . 90 5.2 Correlated 4f -compounds as frustrated triangular lattices . . . . . . 94 5.3 Crystal structure of KCeO2 . . . . . . . . . . . . . . . . . . . . . . . 95 5.4 QC results for the electronic structure of Ce3+ ions in KCeO2 . . . . 100 5.5 The competition of SOC and crystal field splittings in KCeO2 . . . . 102 5.6 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 6 Co-ion substitutes with linear coordination in Li3N . . . . . . . . . . . . . . . . . . . . . . 109 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 6.2 Crystal structure of Li2(Li(1−x)Cox)N and spectroscopic measurements .......112 6.3 QC computational details . . . . . . . . . . . . . . . . . . . . . . . . 115 6.4 Ab initio QC investigation of the Co+ 3d8 electronic structure doped into Li3N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.5 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135