Relativistic Density Functional Calculations of ¹²⁵Te NMR Chemical shifts

The theoretical determination of ¹²⁵Te NMR chemical shifts is a very challenging task since both electron correlation and relativistic effects will play a major role. Further, the size of these systems can be considerable, see the figures below for some examples. Hence, ¹²⁵Te NMR is a powerful example to illustrate the capabilities of modern first principle density functional theory (DFT) as applied to magnetic properties. The three pictures show results from our recent relativistic DFT-GIAO ("gauge including atomic orbitals") calculations [J. Phys. Chem. A 1997, 101, 4121-4127].

Some of the remaining differences between theory and experiment have been attributed to solvent effects and ro-vibrational corrections, the latter being due to finite temperatures in the experimental setting. (The theoretical model is based on the usual Born-Oppenheimer approximation, i.e., a single, ridgid molecule. This situation is closest described by the zero-temperature, zero-pressure limit of a gas phase NMR experiment.)

Chemical shifts are given in ppm relative to dimethyltellurium, (CH₃)₂Te, the experimentally accepted standard.

Figure 1.
Calculated and experimental ¹²⁵Te chemical shifts of organic tellurium compounds. The average deviation between theory and experiment is 235 ppm. The calculated chemical shifts span a range of about 3000 ppm which covers almost the entire known chemical shift range for the ¹²⁵Te nucleus. Note the size of some of the molecules.

Figure 2.
Calculated and experimental ¹²⁵Te chemical shifts of inorganic tellurium compounds. The average deviation between theory and experiment is 160 ppm. The calculated chemical shifts span a range of about 3400 ppm which covers almost the entire known chemical shift range for the ¹²⁵Te nucleus.

Figure 3.
Calculated and experimental ¹²⁵Te chemical shifts of organometallic tellurium compounds. The average deviation between theory and experiment is 155 ppm. The calculated chemical shifts span a range of about 2400 ppm. Note the size these complexes. While relativistic effects are already important for a correct description of ¹²⁵Te NMR in general, they are crucial for complexes of 5d elements like tungsten.

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