Spectroscopic timescale

Tl(PhTpfBu) shows an unprecedented structure in which one of tert-butylpyrazolyl groups is rotated by ca. 90° and the T1 interacts with the nitrogen attached directly to the boron via a tt-orbital component of the aromatic Jt-system of the pyrazolyl nucleus. [Tl(PhTptBu)] is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature.187... 

A number of hydrated cations in aqueous solution undergo exchange with the solvent at rates slow enough to be observed on the NMR spectroscopic timescale by using isotopic labelling has / = f, while both and are NMR inactive. Different chemical shifts are observed for the nuclei in bulk and coordinated water, and from the signal intensity ratios, hydration numbers can be obtained. For example, Al + has been shown to be present as [A1(H20)6] +. 

In solution, many covalent complexes containing the [BH4] ligand exhibit dynamic behaviour which may be observed on the NMR spectroscopic timescale. For example, the room temperature H NMR spectrum of [A1(BH4)3] shows only one signal. 

This means that the molecule is fluxional, and each quaternary C nucleus sees four equivalent Li nuclei on the NMR spectroscopic timescale. We can conclude that at 185 K, the molecule possesses a static structure but as the temperature is raised to 299 K, sufficient energy becomes available to allow a fluxional process to occur which exchanges the Bu groups. 

In (Ti -Cp)Rh(Ti -C2H4)2, two alkene proton signals are observed at 233 K (the dilferent H environments are red and black respectively in 24.55). At 373 K, the proton environments become equivalent on the NMR spectroscopic timescale as each alkene ligand rotates about the metal-ligand coordinate bond. 

The NMR spectroscopic examples discussed so far have assumed that, with the exception of free rotation about single bonds, the molecule or ion is static in solution. For the majority of organic and inorganic species, this assumption is valid, but the possibUily of stereochemical non-rigidity (fluxionality) on the NMR spectroscopic timescale must be considered. Five-coordinate species such as Fe(CO)5, 4.10, PF5,4.11, and BrF5,4.12, constitute one group of compounds for which the activation barrier for dynamic behaviour in solution is relatively low, and exchange of substituent positions is facile. 

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