Exchange processes, intramolecular

In classical kinetics, intemiolecular exchange processes are quite different from the uniniolecular, first-order kinetics associated with intramolecular exchange. However, the NMR of chemical exchange can still be treated as pseudo-first-order kinetics, and all the previous results apply. One way of rationalizing this is as... 

C. Pseudorotation.—An alternative intramolecular exchange process to Berry pseudorotation has been suggested, which also occurs with conservation of angular momentum. It has been called a turnstile-rotation process because it involves the rotation of an apical-radial pair of ligands... 

In spite of bearing nonequivalent fluorine substituents, both PF5 and all compounds of the type R-PF4 exhibit only a single signal in their fluorine NMR spectra. The observed magnetic equivalence of the fluorines in such compounds is believed to derive from a rapid intramolecular, pseudorotational exchange process that is too rapid, even at -80 °C, to allow distinction of the axial and equatorial fluorine atoms (Scheme 7.7). 

Upon further contact with a redox reagent or at higher redox potentials, additional electrons can be transferred. After a two-electron transfer, each redox unit can accept one charge with formation of a singlet or triplet dianion, or (less favourably from an electrostatic point of view) both charges can enter one redox unit. Here again, an intramolecular electron-exchange process is possible. 

If Q-symmetric ligands are employed in asymmetric hydrogenation instead of the corresponding C2-symmetric ligands, there coexist principally four stereoiso-meric substrate complexes, namely two pairs of each diastereomeric substrate complex. Furthermore, it has been shown that, for particular catalytic systems, intramolecular exchange processes between the diastereomeric substrate complexes should in principle be taken into account [57]. Finally, the possibility of non-estab-hshed pre-equilibria must be considered [58]. The consideration of four intermediates, with possible intramolecular equilibria and disturbed pre-equihbria, results in the reaction sequence shown in Scheme 10.3. This is an example of the asymmetric hydrogenation of dimethyl itaconate with a Rh-complex, which contains a Q-symmetrical aminophosphine phosphinite as the chiral ligand. 

The added primary amine may facilitate the cleavage of the Ti-N bond of the key intermediate 107 through the coordination to the titanium center followed by u-bond metathesis. Such an intramolecular exchange process is expected to be facile. The amine exchange product is 106, which can then be rapidly converted to 105 and the corresponding silylated amine to complete the catalytic cycle. 

Fig. 5. Reaction scheme proposed for intramolecular exchange process in [V3Oi0]5. Local anionic charges are ignored. Subscripts t and c refer to terminal and central vanadium atoms. (Reprinted with permission from Andersson, I., Petterson, L., Hastings, J. J., Howarth, O. W., J. Chem. Soc. Dalton Trans. 1996, 3357.)...

According to these explanations, the N.M.R. spectra of proton addition complexes can unambiguously be observed only if the rapid intramolecular or intermolecular exchange processes have been eliminated. 

Apparent NMR equivalence of nuclei can also arise by a quantum mechanical intramolecular tunneling process. In principle, this process may be differentiated from intermolecular exchange processes because although the exchanging nuclei are rendered equivalent insofar as the NMR experiment is concerned, spin-spin splitting by other magnetic nuclei is not washed out. This type of intramolecular exchange is manifested in several boron hydride derivatives. It was first proposed by Ogg and Ray (98) to explain the NMR spectra of aluminum borohydride, whose structure is... 

These experiments while moving beyond simple structure determination, still depend primarily on structure determination as their primary tool. A number of experiments have moved beyond this to exploit other characteristics of NMR. For example, spin saturation transfer can be used to study internal rotation of molecules (59, 60), and line shape analyses can be used to study intramolecular exchange processes (61), conformational interchange (62) and... 

Dynamic NMR of Intramolecular Exchange Processes in EDTA Complexes of Sc3+, Y3 and La3+ 61... 

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