Slow exchange spectrum

Figure B2.4.5. Simulated lineshapes for an intennolecular exchange reaction in which the bond joining two strongly coupled nuclei breaks and re-fomis at a series of rates, given beside tlie lineshape. In slow exchange, the typical spectrum of an AB spin system is shown. In the limit of fast exchange, the spectrum consists of two lines at tlie two chemical shifts and all the coupling has disappeared.

This is most readily studied with cyclohexane- /n in which 11 of the 12 protons are replaced with deuterium. The spectrum of cyclohexane- /n resembles the behavior shown in Fig. 4-8 at about — 100°C (the slow exchange regime) two sharp lines are seen these broaden as the temperature is increased, reaching coalescence at — 61.4°C, and becoming a single sharp line at higher temperatures. (The deuterium nuclei must be decoupled by rf irradiation.) Rate constants t for the conversion were measured over the temperature range — 116.7°C to — 24.0°C by Anet and Bourne. It is probable that the chair-chair inversion takes place via a boat intermediate. 

Note that deuterium exchange of the -OH leads to incorporation of deuterium alpha to the carbonyl in the ketone form. This may happen, even if there is no evidence of any enol signals in the spectrum initially, i.e., it can occur even when the equilibrium is heavily in favour of the ketone. Aromatic protons of rings which bear two or more -OH groups are also prone to undergo slow exchange, as are nitrovinyl protons. 

The H nmr spectrum of Zr(Hf)(acac)4 in CHCIF2 shows two Me proton resonances of equal intensity in the slow exchange region only at lowered temperature ( —170°Ci). This observation is consistent with a square antiprism geometry 11. At — 145°C coalescence of the signals occurs arising from complicated intramolecular rearrangements. 

To study water exchange on aqua metal ions with very slow exchange of water molecules, an isotopic labeling technique using oxygen-17 can be used. A necessary condition for the applicability of this technique is that the life time, tm, of a water molecule in the first coordination shell of the ion is much longer that the time needed to acquire the 0-NMR spectrum. With modern NMR spectrometers and using enrichments up to 40% in the acquisition time can be as short as 1 s. 

Fig. 12. NMR spectrum of FN2O4 ligand with mixture of mono- and divalent cations Li+, Na+, K+, Rb+, Mg +, Ca +, Sr +, and Ba +. Due to slow exchange all species are detected simultaneously (reprinted with permission from Plenio and Diodone, JACS 118, 356-367 [314], Copyright 1996 American Chemical Society).

The same transformation has recently been examined by nmr in solution.46 In zinc sulfate solution, with two zinc atoms per hexamer, a certain nmr spectrum is observed. Addition of excess zinc and of chloride, iodide, or particularly thiocyanate but not sulfate converts this to a different spectrum. Detailed study suggests that this is of the six-zinc insulin, which is in equilibrium with the two-zinc form, but in slow exchange. 

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