CCL: structural isomerism and fluxional isomerism



This is a very wide question. Fluxional molecules: molecules that exhibit rapid intramolecular rearrangements among their component atoms [E. L. Eliel, S. H. Wilen and L. N. Mander; Stereochemistry of Organic Compounds, JW 1994]. As in structural isomerism and tautomerism, fluxional compounds maintain the same number of component atoms. For example; keto-enol tautomerism (CH3COCH2CO2Et ¡ê CH3C(OH)=CHCO2Et) when warmed in the presence of base they become rapidly equilibrating and their NMR spectra may coalesce. Nevertheless, two molecular species still coexist, as can be readily verified by IR spectroscopy.
Bullvalene represents another example of tautomerism called "valence tautomerism". This molecule exists 1,209,600 structures because there is a possibility of bond migration.
Cyclohexane, the 1H-NMR spectrum at room temperature shows all protons are equivalent.
The crystal structure of the complex [Co(abap)Cl2]Cl shows the presence of eight chemically inequivalent carbon atoms. However, the 13C NMR of this species in solution shows only 5 peaks.Why?
The molecule is undergoing a fluxional process, which is rapid on the NMR timescale and results in the observation of the time-averaged signals from all the carbon atoms. This makes the two sets of propyl carbon atoms ¡°equivalent¡± and hence we only see 5 distinct signals.
Finally, as the process gets faster still, A and B can no longer be distinguished and we only see one narrow peak at the average frequency of A and B. the peak appears narrow because, on NMR timescale, the molecule appears stationary and hence has infinite lifetime. Fluxional processes are temperature dependent. Their rate constant can be determined from the line-width of the peak and the activation parameters, Ea, ∆G¢Ô, ∆H¢Ô and ∆S¢Ô, at different temperature.
You can see the following article: Quantum-mechanical calculations of the stabilities of fluxional isomers of C4H7+ in solution, Joseph Casanova, David R. Kent IV, William A. Goddard III, and John D. Roberts.
I hope this information can help you.
 
Best Regards
Akef
 
 
 
 


"may abdelghani may01dz#%#yahoo.fr" <owner-chemistry:-:ccl.net> wrote:
dear CCLers,

First, what is the different between the structural isomerism and fluxional isomerism?
How we can distinct between them?
 How we can expect, or study theoretically, the fluxional behaviour of some molecules (from the shape of the orbital and its interaction with each other, for exemple)?

thanks


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