Quantum chemical calculations of structures, bonding, and spectroscopic properties of some sulfur and selenium iodine cations

Abstract

The characterization and understanding of electronic structures of sulfur and selenium iodine species have presented many difficulties, some of which have puzzled scientists for decades. The present thesis provides answers to some of the open questions with the aid of modern quantum chemical tools. The research presented in the first half of this thesis describes the bonding in several sulfur and selenium iodine cations, which is of interest because many of the structures display unconventional bonding arrangements and because sulfur and selenium iodine cations are an exception to the normal trend that elements within the same group exhibit similar bonding with other elements. So far the only unequivocally characterized sulfur and selenium iodine cations that even have the same formula are S2I42+ and Se2I42+, and even their structures are fundamentally different. The investigation of the bonding in S2I42+ and Se2I42+ reveals that the different structures arise from the intricate balance between the strengths of homoatomic and heteroatomic π and o bonds. The non-existence of SI3+ compared to experimentally observed SeI3+ is shown to arise from their different stabilities with respect to the formation of E2I42+ (E = S, Se) dications in solution. These results provide a basis for the study of the stabilities of larger sulfur and selenium iodine cations in different phases and for the exploration of more general reasons why the cations appear to adopt different structures. The theoretical bonding analyses also confirm the validity of simple π*- π* bonding and charge delocalization models that have been used to explain the unconventional bonding observed in cationic sulfur and selenium species. The validation of the π*- π* bonding description gives theoretical justification for its use in describing various multi-center bonding situations presented in recent literature. The second half of the thesis summarizes the requirements for producing accurate and computationally feasible theoretical predictions of 77Se NMR chemical shifts for selenium iodine species in solution. The consideration of relativistic and solvent effects together with the use of efficient DFT methods is shown to facilitate the calculation of 77Se chemical shifts with sufficient accuracy for them to be helpful in the identification of unknown species. The theoretical calculations are utilized together with NMR spectroscopic measurements to identify selenium iodine cations present in the equilibrium solution of the reversible dissociation of (Se6I2)[AsF6]2 in SO2. These new cations provide a significant addition to the group of selenium iodine species. They can be used in further studies to elucidate the understanding of the bonding and factors that lead to the different structures observed in different phases. The computational methods and requirements reviewed and employed in this thesis can be readily applied to the prediction of the chemical shifts of other NMR active heavy elements.