Phosphorus-31 chemical shifts temperature dependence

The chemical shifts of the phosphorus atoms in germaphosphenes is temperature-dependent. The value shifts to higher fields at low temperature and to lower fields at high temperature.20... 

The occurrence of the singlet at room temperature is caused by the different temperature dependence of the chemical shifts of the two phosphorus nuclei contained in the molecule. In the case of R = Et... 

In contrast to the tricarbonyl halide complexes, it is the exo conformation of () -allyl)FeCp(CO) complexes (163) which is preferred, and at equilibrium only traces of the endo isomer can be observed. Existing crystal structures of members of this class are of the exo conformation.Reasonable percentages of the endo conformer are realized upon low-temperature photolyses of their () -allyl)Fp precursors, however, and the activation barrier to endolexo conversion has been determined to be AG = 100.8kJmol for the 2-methallyl complex. In addition, the anti isomers, which can be prepared stereospecifically from the (j7 -allyl)Fp complexes, isomerize to the syn isomers with brief heating. The Fe NMR spectra are known for several of the complexes where CO has been replaced by a phosphine or phosphite (145) the chemical shift is highly dependent on the nature of the phosphorus ligand. ... 

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