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Date of Award

Spring 2012

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Supervisor

Ignacio Vargas-Baca

Language

English

Abstract

This thesis presents the results of investigations on the application of modern spectroscopic techniques to the characterisation of secondary bonding interactions (SBIs) in selected organo-chalcogen compounds. Although the research was mostly focused on two families of compounds, ditellurides and chalcogen-substituted azodyes, the observations and conclusions are applicable to a wide variety of compounds. Because the effects of the SBIs are subtle, great care was taken to carefully distinguish contributions from other factors. DFT computational studies showed that the molecular conformation influences the electronic excitations observable in the UV-vis spectrum and the frequency of resonance in 125Te NMR of organoditellurides. In the absence of protection to shield the chalcogen from intermolecular interactions, the 125Te NMR chemical shift is dependent on the concentration of ditellurides; intramolecular SBIs attenuate the concentration dependency and steric protection cancels it entirely. The nature of the solvent impacts the results of both spectroscopic techniques through several mechanisms, including the solvatochromic effect, conformational changes due to the polarity of the medium and solvation. Solution 125Te NMR spectroscopy is sensitive to all those contributions but, for the same reason, it cannot be applied in an unambiguous way to identify the presence of SBIs.

Conversely, the use of intramolecular SBIs to modify the spectroscopic properties of a conjugated chromophore was investigated. Push-pull azobenzenes were derivatised with functional groups containing divalent chalcogen atoms through metathesis with a mercurated derivative of the azodye. The regiochemistry of formation of the intermediate was shown to be under kinetic control. In the chalcogen-substituted molecules, the efficiency of electron delocalisation through the SBIs was assessed by calculations of the nucleus independent chemical shift (NICS). The linear (UV-vis absorption) and nonlinear (second harmonic-generation) optical responses of the modified chromophore were investigated and interpreted in the context of the SBIs. Substitution with the chalcogen groups, and the consequent perturbation of the π-system, cause a blue shift in the first absorption maxima but little change was observed in the hyperpolarisability of the chromophore ortho-functionalised with the phenylselenenyl group. The sensitivity of the electronic spectrum to the SBI was applied to monitor the process of halide exchange in the halo-chalcogenyl derivatives of the push-pull azobenzene.

Very strong SBIs can result in interatomic distances so short that they are difficult to distinguish from hypervalent covalent bonds. This is the situation observed in the structure of the first 2,5,8,11,1,4,7,10,3,6,9,12-tetraoxatetratelluratetrazacyclododecane. DFT calculations showed that this unusual macrocycle is stable with respect to dissociation into isotellurazole oxide and VT NOESY experiments indicate it remains intact in solution.

McMaster University Library

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