Fluxional timescale

In the above discussion it was assumed that the barriers are low for transitions between the different confonnations of the fluxional molecule, as depicted in figure A3.12.5 and therefore the transitions occur on a timescale much shorter than the RRKM lifetime. This is the rapid IVR assumption of RRKM theory discussed in section A3.12.2. Accordingly, an initial microcanonical ensemble over all the confonnations decays exponentially. However, for some fluxional molecules, transitions between the different confonnations may be slower than the RRKM rate, giving rise to bottlenecks in the unimolecular dissociation [4, ]. The ensuing lifetime distribution, equation (A3.12.7), will be non-exponential, as is the case for intrinsic non-RRKM dynamics, for an mitial microcanonical ensemble of molecular states. 

It has 6-coordination with a chelating acetate [106] and may be converted (reversibly) into Ru(OAc)2(PPh3)3, which has the/ac-configuration with one monodentate and one bidentate acetate. It is fluxional at room temperature but at —70°C the phosphines are non-equivalent on the NMR timescale [107],... 

For carbonyl complexes, a combination of IR (p. 11) and 13C NMR spectroscopy will often reveal the molecular symmetry and also provide further information about the nature of the metal centre to which the CO ligand(s) is bound. Flowever, the spectroscopic events involved occur within completely different time frames. Molecular vibrations (IR and Raman spectroscopy) are rapid relative to molecular fiuxional processes. NMR transitions, however, are slow, often comparable in rate to intramolecular fluxionality and even intermolecular ligand exchange processes. This can lead to time-averaged chemical environments being observed on the NMR timescale . So long as this is borne in mind, 13C NMR spectroscopy is a very valuable technique and can provide thermodynamic and kinetic data about such processes over a temperature range [variable temperature (VT) NMR]. 

The Zr and Hf tetrahydroborates M(BH4)4 are the most volatile compounds of these elements, with boiling points of 123 and 118°C they are also very sensitive to oxidation and hydrolysis. They can be obtained by the reaction of the chlorides with alkali metal borohydrides or of the complex fluorides NaMFs with aluminum borohydride (the fluorides MF4 do not react), followed by distillation from the reaction mixture. According to low-temperature X-ray data, all the borohydride groups are tridentate in both M(BH4)4 complexes, which have Td symmetry. The same is trae for their substituted M(BH3Me)4 derivatives, that is, the metal atom is 12-coordinate. Proton NMR indicates fast exchange of bridging and terminal protons on the NMR timescale in these fluxional molecules see Fluxional Molecule). 

Polyhydrides that contain both (H2) ligands and terminal hydrides are often highly fluxional on the NMR timescale. This is normally thought to occur via oxidative addition of the (H2) and rearrangement of the resulting intermediate. 

For the sulphur derivative, the most recent value reported is provided here [54]. At low temperature, all species appear rigid on the NMR timescale and display NMR spectra which are in agreement with the solid state structure. At higher temperatures, they appear fluxional. This is clearly shown for [RhioP(CO)22] - The P NMR spectrum in tetrahydrofuran reveals a triplet of nonets at — 80°C and an undecet at +76°C in sulpholane solution. 

All these species appear fluxional on the Pt NMR timescale down to — 85°C. The different triangles rotate around the pseudo three-fold axis passing through the centres of these triangles [22]. Large clusters, Pti2 and Pti5 are in equilibrium with each other, even at low temperature [23]. The Pt NMR parameters of such species are reported in Table 10, the values for n = 3... 

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