Peak Broadening Due to Exchange

do not forget that when the spectrum of a pure acid or alcohol is determined in an inert solvent (e.g., CDCI3 or CCI4), the NMR absorption position is concentration dependent. You will recall that this is due to hydrogen-bonding differences. If you have not, now is a good time to reread Sections 3.11C and 3.19F. 

VERY RAPID NMR sees average -environment only 

338 Nuclear Magnetic Resonance Spectroscopy Part Four 

The effect of the rate of exchange on the NMR spectrum of a hydroxyhc compound 

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When solutions of alcohols in fluorosulphonic acid or fluorosulphonic acid-sulphur dioxide were prepared, tertiary carbonium ions could be generally observed, but peak broadening (due to exchange) and side products are observed. Secondary and primary alcohols form generally only the monosulphates. 

NMR studies of alcoholysis reactions show the metal alkoxide sites in [(R0)M M50ig] to be comparatively inert. Alkoxide peaks in NMR spectra show no broadening due to exchange up... 

Fig. 34. Magnetic structure function for CeAI calculated in the dispersionless model. Broadening of (anti-) boimd state peaks is due to an exchange interaction with conduction electrons of strength / JV,(0) = 0.06.

In ethanol, the hydroxyl peak should be split by spin-spin interaction with the methylene protons. However, the high-resolution NMR spectrum shows only a single hydroxyl peak. The lack of apparent spin-spin coupling in this peak is due to the fact that this spectrum was taken on a sample that contained a small amount of water. In the NMR spectrum of pure ethanol, the hydroxyl peak will be a triplet as we expect. However, if a small amount of water is present, a rapid proton-exchange reaction between the —OH proton in ethanol and H30 in the water will occur that has the effect of broadening the three peaks in the —OH triplet so that they coalesce into a single observed peak. 

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

Although effective, residual polyphenols in crude samples resulted in less separation than possible with this method. Such binding often resulted in peaks containing several different activities (9). And increased sample loading often broadened and reduced the number of peaks (9,74). Due to these interferences, two different scales of anion exchange chromatography were used. Analytical separations were used to gather information about the enzymes present and preparative separations were used to purify enzyme quantities sufficient for characterization. 

Finally a reminder is made that the flows of the mobile phase generally are rather modest, about 0.6-0.7 ml/min, and they rarely go over 1 ml/min. In fact, peak broadening may be observed on certain stationary ion-exchange phases, due to the slowness of the mass transfer. This means that very low flows must be maintained. Despite this, most separations may be completed in times that can vary between about 20 min and 40 min. A useful paper to consult, both because of the number of analytes considered, 63, 15 of which are alcohols, and for the choice of operative conditions applied to a cation exchange column is Ref. 13. Table 1 enumerates the alcohols considered Fig. 1 shows the separation of a standard solution. 

Milone and co-workers (8) examined the 13C-NMR spectra of FeCo3H(CO)12 (32) and some of its substituted derivatives. At -89°C, two resonances in a 1 2 ratio were observed in the spectrum of FeCo3H(CO)12. As the temperature was raised, the farthest upfield, and more intense, peak broadened significantly. It was assumed that the cobalt carbonyls rapidly exchange at -85°C, and this peak was attributed to an average cobalt carbonyl resonance that broadens at room temperature due to coupling to Co. The second resonance was attributed to rapidly exchanging Fe carbonyls. The observed disparity of intensities (1 2 observed, 1 3 expected) is similar to that found for the isoelectronic Co4(CO),2. 

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