Tellurium: Understanding the NMR spectroscopic features of an exotic element

Significance 

Tellurium is a main-group element (group 16, period 5) with a broad range of accessible oxidation states (from -II to +VI). Organotellurium compounds, containing tellurium in combination with organic substituents, were first reported in the mid-19th century and have ever since been the subject of fundamental studies regarding their structure and bonding. These compounds have found applications e.g. in the functionalization of carbon–carbon multiple bonds or as glutathione mimics. In the context of Nuclear Magnetic Resonance (NMR) spectroscopy, tellurium has two spin 1/2 isotopes, 123Te and 125Te, the latter being the isotope of choice due to its higher natural abundance (7%). In fact, 125Te displays a particularly large chemical shift window of approximately 5000 ppm, indicative of the high sensitivity of Te chemical shift to electronic structure. To date, despite the fact that calculations of 125Te chemical shifts have been reported, little is known on the relation between 125Te chemical shift and the structural motif of organotellurium compounds.

Generally, comprehending the electronic origin of NMR chemical shifts is a subject of on-going research in physical, inorganic, and organic chemistry. In this view, researchers from the Swiss Federal Institute of Technology (ETH Zurich): Dr. Ewa Pietrasiak, Christopher Gordon, Professor Christophe Copéret and Professor Antonio Togni investigated the electronic origin of 125Te NMR chemical shifts witnessed in compounds previously prepared in their laboratory. Their goal was to demonstrate that a natural chemical shift analysis of the chemical shielding tensors of a series of chosen compounds was an important approach to understand the electronic structure of organotellurium compounds. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

In order to link the 125Te chemical shift of a series of perfluoroalkyl aryl tellurides to their electronic structure, the researchers calculated the chemical shielding tensors of the 125Te nuclei by density functional theory (DFT) and further analyzed by a decomposition into contributions of natural localized molecular orbitals (NLMOs).

Their analysis revealed that the variation in 125Te chemical shifts in their selected molecules was mainly due to the magnetic coupling of the tellurium p-character lone pair with antibonding orbitals perpendicular to it {σ*(Te–X) and σ*(Te–C(Ar))} upon action of an external magnetic field. In addition, they noted that the strength of the coupling was affected by electronic properties of the X-substituents, polarization of the antibonding orbitals and presence of secondary interactions perturbing the energy of these orbitals.

In summary, the Swiss scientists reported that 125Te chemical shielding in their selected molecules was to a major extent determined by magnetically-induced coupling of a p-character tellurium lone pair and Te-ligand antibonding orbitals. Additionally, they noted that the strength of the coupling was particularly sensitive to the energy of the antibonding orbitals and hence the electronic properties of the ligands on tellurium and the presence of secondary interactions affect the chemical shift. In a statement to Advances in Engineering, Professor Antonio Togni highlighted that their research work established guidelines to understand better the variation of 125Te chemical shifts based on a simple localized molecular orbital picture and would help in improving our understanding of chemical shifts in general.

Reference

Ewa Pietrasiak, Christopher P. Gordon, Christophe Copéret, Antonio Togni. Understanding 125Te NMR chemical shifts in disymmetric organo-telluride compounds from natural chemical shift analysis. Physical Chemistry Chemical Physics 2020, volume 22, 2319.

Go To Physical Chemistry Chemical Physics

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