Fluxionality rates

Pyrrole Complex Ri Ri Pdt k(s lATl)xl(r3 Fluxionality Rate Constant k (s i)... 

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. 

In another fluxional process involving ruthenium instead of rhodium, it has been shown that the rate-controlling step is the complex dissociation and that the ligand exchanges between the two annular nitrogen atoms by an intermolecular process. 

The spectral line widths are related to the rate of the rotational motions, which average anisotropies in the g- and hyperfine matrices (Chapter 5), and to the rates of fluxional processes, which average nuclear positions in a radical. 

Table 5. Fluxionality and Reactivity Rate Constants for r)2-Pyrrole...

Ans. The molecules with small R are fluxional. At low temperature the rate of interchange between bridging and free C02R groups is slower than the NMR time scale. 

The rate constant for the lithium-lithium exchange within the mixed complexes of chiral lithium amides and lithioacetonitrile also differ, depending on the structure. The C-lithiated structures are significantly less fluxional than the /V-lithiated mixed dimers. The activation energy, AG, has been determined for two C-lithiated nitrile complexes in Et20... 

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]. 

Among 4-coordinate transition metal complexes fluxional behavior based on planar/tetrahedral interconversions is of considerable importance. This is especially true of nickel(II) complexes, where planar complexes of the type Ni(R3P)2X2 have been shown to undergo planar tetrahedral rearrangements with activation energies of about 45 kJ mol 1 and rates of —105 s 1 at about room temperature. 

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