Fluxional interconversion

Variable temperature NMR studies of the parent titanium-tartrate system (3,4, 5) revealed that isopropoxide exchange, fluxional interconversion (in which the coordinated ester and uncoordinated ester carbonyls, as well as tartrate alkoxides, exchange with respect to the titanium) and epoxidation rate are closely coupled. Thus, the faster the isopropoxide exchange, the more a vacant site becomes available for coordination of allyl alkoxide and the faster the latter anion associates with the titanium. Likewise, faster fluxional interconversion indicates more rapid dissociation of the ester carbonyls in 3 and thence more rapid coordination of the alkyl hydroperoxide to give 4. 

Figure 9. New pathway for fluxional interconversions of substituted icosahedra via tetracapped cuneane and tetracapped cubane intermediates (or transition states). This proposed scrambling process, which maximizes connectivity and obeys electron-counting rules, is illustrated for the interconversion of a 1,2-disubstituted ort/io-icosahedron into a 1,7-disubstituted meta-icosahedron.

Structural dynamical fluxionality Interconversion between different structural isomers at finite temperature. 

In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. ... 

The iminophosphoranes (181) with imide bromides gave the amidino-phosphonium bromides (183) which, it appears from their temperature-variable n.m.r. spectra, are interesting fluxional molecules. The free energy of activation for the interconversion of (183a) and (183b) (R R = Pr R3 = Ph) is 17.2 0.9kcalmol-i. 

The spiro dimer of a-tocopherol (9, see also Fig. 6.4) is formed as mixture of two diastereomers by dimerization of the o-QM 3 in a hetero-Diels-Alder reaction with inverse electron demand. Both isomers are linked by a fluxion process (Fig. 6.22), which was proven by NMR spectroscopy.53 The detailed mechanism of the interconversion, which is catalyzed by acids, was proposed to be either stepwise or concerted.53-55... 

Equilibration of fluxional molecules must not be confused with resonance. In each electrocyclic reaction, the nuclei alter their positions as bond lengths and angles change. Interconversion of fluxional molecules also must not be... 

For the equilibration processes to take place on the nmr time scale, the energetic barrier for interconversion of CO bonding modes cannot be great (usually <22 kcal/mol). While the fluxional properties of metal carbonyls will not be of concern to us here, they illustrate the facility for interconverting CO bonding modes, and suggest that if CO activation can be achieved beginning with a particular mode of coordination, carbonyls bound in other ways to metal atoms may also be activatable. ... 

The observant reader will undoubtedly have noted that no evidence of any stereochemical nonrigidity (on the NMR time scale) has been obtained for any of the mononuclear complexes derived from OFCOT. However, the dinuclear complex 88 does exhibit fluxional behavior (151). If the solid-state structure of 88 were rigidly maintained in solution, the 19F-NMR spectrum should show eight resonances due to symmetry inequivalent fluorines. However, only four broad resonances are present in the 19F NMR spectrum at room temperature, indicative of a dynamic process that interconverts the two enantiomorphs of 88. Two mechanisms have been proposed for such interconversions in COT analogues a glide and a twitch (Scheme 3). The... 

The ground-state interconversion via the distorted tetrahedron is prevented by a large barrier (see in the following). Similarly, although the electronic ground state of low-spin d6 Cr(CO)5 is square pyramidal (in the low-temperature matrix, any thermal fluxional behavior is frozen out), the geometry of the first excited state (intermediate-spin... 

There are two basic types of chemical processes reversible (leading to, and maintaining, an equilibrium mixture) and irreversible (proceeding in one direction, i.e., to completion). NMR methods can be used to study both types of processes. A common example of an irreversible process is a chemical reaction with a free energy more negative than ca. 20 kJ/mol. Reversible processes include interconversions of conformations by rotations around single bonds, interconversions of fluxional molecules and valence isomers, and proton transfers, to name a few. 

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