NMR timescale

The principal dilTerence from liquid-state NMR is that the interactions which are averaged by molecular motion on the NMR timescale in liquids lead, because of their anisotropic nature, to much wider lines in solids. Extra infonnation is, in principle, available but is often masked by the lower resolution. Thus, many of the teclmiques developed for liquid-state NMR are not currently feasible in the solid state. Furthemiore, the increased linewidth and the methods used to achieve high resolution put more demands on the spectrometer. Nevertheless, the field of solid-state NMR is advancing rapidly, with a steady stream of new experiments forthcoming. 

If the molecular motion is faster than the NMR timescale, the distance pre-... 

For the NH azoles (Table 3), the two tautomeric forms are usually rapidly equilibrating on the NMR timescale (except for triazole in HMPT). The iV-methyl azoles (Table 4) are fixed chemical shifts are shifted downfield by adjacent nitrogen atoms, but more by a pyridine-like nitrogen than by a pyrrole-like iV-methyl group. 

It has 6-coordination with a chelating acetate [106] and may be converted (reversibly) into Ru(OAc)2(PPh3)3, which has the/ac-configuration with one monodentate and one bidentate acetate. It is fluxional at room temperature but at —70°C the phosphines are non-equivalent on the NMR timescale [107],... 

An insight into the equilibria present in solutions of several complexes of the type (R3P)2HgX2 (X = C104, CF3COO) is provided by electrospray mass spectrometry (ES MS) (cf. Section 6.9.2.1.8). 39 In all cases the principal ions [(R3P)2HgX]+ are observed, even if these ions and other ionic constituents of the equilibria are known to be labile on the NMR timescale. In the presence of excess R3P the principal ions [(R3P)3HgX]+ appear. Also fragments of collisionally activated decomposition (CAD, can be influenced and controlled to some extent) are detected, e.g., [(R3P)HgX]+. 

If you take a pure sample of ethanol, and run its NMR spectrum in dry CDCI3, the hydroxyl proton will appear as a well-defined triplet, which couples to the adjacent -CH2-, rendering it a multiplet. This is because the hydroxyl proton remains on the oxygen for relatively long periods of time, as there is nothing in the solution to entice it off, i.e., exchange (if any) is said to be very slow on the NMR timescale (less than about 1 s). 

The presence of a trace of acid and water however, causes collapse of the hydroxyl-OH to a singlet (at lower held), the proton can now protonate, and de-protonate the oxygen very rapidly, as the process is catalysed by the acid, i.e., exchange is said to be fast on the NMR timescale (less than about 10-6 s). 

In open chain compounds that lack any chiral centre of course, rotation about all single bonds can be assumed to be both relatively free and fast on the NMR timescale and the 7-9 Hz range quoted is the result of averaging of this angle. The same is of course not true in cyclic systems where structures are rigid and bond angles constrained. We will deal with this topic thoroughly in Section 6.6.5. 

It is worth noting that whilst we have restricted discussion in this section to conformational interconversion based on the slow rotation of bonds, the concept of the NMR timescale is equally applicable to other types of interconversion, such as can sometimes be seen in cyclic systems which may exist in two different conformational forms. 

A misconception that we commonly encounter is that a spectrum can be a mixture of the salt and the free base. This is an excuse that is often used by chemists to explain an inconveniently messy looking spectrum Don t be tempted by this idea - proton transfer is fast on the NMR timescale (or at least, it is when you use a polar solvent ) and because of this, if you have a sample of a compound that contains only half a mole-equivalent of an acid, you will observe chemical shifts which reflect partial protonation and not two sets of signals for protonated and free-base forms. It doesn t happen - ever ... 

Whilst dealing with protonation issues, it is well worth considering the time dependence of the process in the context of the NMR timescale. A compound of the type shown in Structure 6.22 provides an interesting example. 

As a free base, the Ar-Cfb-N protons would present themselves as a simple singlet. The lone pair of electrons on the nitrogen invert very rapidly on the NMR timescale and so the environment of the two protons is averaged and is therefore identical. However, on forming a salt, the whole process of stereochemical inversion at the nitrogen is slowed down dramatically because the sequence of events... 

We have discussed the significance of the NMR timescale in earlier sections and it is worth knowing that the NOE timescale is somewhat longer and that this can have consequences for NOE experiments in molecules that have dynamic processes taking place within them. To give a more specific example, consider the isomers shown in Structure 8.3. 

As mentioned above the 1,1-organoboration reaction is reversible, and exchange is slow on the NMR timescale. This statement is in agreement with the chemical behavior of equilibrated mixtures of products. Thus, treatment of silicon borahomoadamantane derivative 92 with bis(trimethylstannyl)ethyne leads to the tin-containing compound 93 and liberation of bis(trimethylsilyl)ethyne (Scheme 44). With pyridine, the equilibrium is moved toward 1-boraadamantane completely due to the complexation <2001JOM(620)51>. 

Pt and 119Sn NMR data show Pt(SnCl3)5 to be non-rigid (on the NMR timescale) down to 183 K, owing to an intramolecular process, possibly a Berry twist mechanism [132],... 

NaB (C6H5 )4/l 8-crown-6/dioxolane systems the exchange was found to be slow on the nmr timescale two 23Na resonances were observed for solutions containing an excess of the sodium salt. 

If the equilibrium in (36) is established rapidly on the nmr timescale, the observed nmr spectrum will correspond to an average of the spectra of the... 

Cyclic ligands like DOTA or DOTAM are known to exist in two isomeric forms in solution, usually termed M for the major and m for the minor isomer (Fig. 11). They interconvert slowly on the NMR timescale (251-253). It was demonstrated by NMR that the m-isomer exchanges its 1st shell water much faster than the M-isomer, most probably due to steric effects (238,244). 

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