Timescale of the NMR experiment

On the timescale of the NMR experiment, isochrony (chemical shift equivalence) arises from symmetry equivalence of homotopic and enantiotopic nuclei5 17, while anisochrony (chemical shift nonequivalence, JK s <5) arises from symmetry nonequivalence of diastereotopic nuclei. 

Most spectroscopic techniques (e.g. infrared and Raman spectroscopy) provide a snapshot view of the structure of a liquid because the timescale of the techniques is of the order of lattice vibration. However, NMR can probe much lower frequency motions, motions which are important in the glass transition and the viscosity of a silicate liquid. In addition, the timescale of the NMR experiment may be varied (by changing the magnetic field, or the type of experiment, T or T fJ, or observing quadrupolar effects) from a few hertz to several hundred megahertz. 

The H NMR spectrum of 1,4-oxaselenane at 155 K consists of ABCD multiplets, but these coalesce as the temperature of the experiment is raised, eventually leading to an AA BB system, when ring inversion is faster than the timescale of the NMR experiment AG = 42.6 kJ mol ). However, even at 155 K the spectrum of 1,4-oxatellurane appears as AA BB multiplets, slightly broadened. In this molecule the greater ring flexibility is considered to be a reflection of the increased bond lengths (C—Te vs. C—Se) <75JCS(P2)1354>. 

Most importantly, however, photoexcitation methods allow the detection of transient species. While an entity with a lifetime of 50 ns or less can be comfortably analyzed by transient absoiption or fluorescence spectrometry, other physical methods of steady-state character are likely to fail. For example, the timescale of most NMR experiments is longer than ca. 100 ms. which renders this method unsuitable for studies of short-lived species. 

A critical point in NMR determinations of thermodynamic data is to have a proton whose chemical environment changes sufficiently upon binding to lead to a change in chemical shift, that is, 5//free 7 SHbound- As has been discussed above, binding determinations by NMR can be divided into two cases those that are slow on the NMR timeframe and those that are fast. The observed com-plexation and decomplexation rates (25) are those of uni-molecular processes (units s ), and it is the slower of these two processes that we must compare with the rate of NMR data acquisition. Our previous discussion emphasized how the timeframe of the NMR experiment was dependent on the external field strength of the instrument, but it is also important to note that whether a complexation is observed to be fast or slow on the NMR timescale depends on the chemical shift difference between the free and the bound state. The key equation is (37), which defines the boundary between fast and slow timescales, that is, the observed rate constant for coalescence ( coai) of the signals for the proton in question in the free and the bound state ... 

NMR spectroscopy of enols has been a key tool in determining the relative proportions of keto and enol forms in highly enolized species. For example, in pentane-2,4-dione, the keto and enol forms (Figure 17.7) can be readily identified in the NMR spectrum of the material the keto-enol tautomerism is slow compared with the timescale of the NMR spectroscopy experiment. This experiment has been extended by changing the temperature, which allows the calculation of thermodynamic parameters for the equilibrium, and the use of a range of solvents to study the effect of polarity. The process has also been studied in the gas phase, where enols predominate for almost all p-dicarbonyl compounds. In solution, the keto forms are generally some 8-9 kj moH more stable than in the gas phase. 

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. 

The cyanide-bound and cyanide-free forms of SOD are in slow exchange on the NMR timescale. On addition of increasing amounts of anion, a new set of resonances appears to be due to the CN-bound form. The assignment of the NMR spectrum was achieved through saturation transfer experiments (Paci et at., 1988), which also show that binding of this anion produces structural rearrangements similar to those observed for the other anions. 

In spin-diffusion studies it is possible to detect not only two but three domain sizes. The third domain can be considered the interface (i) between the other two domains, which can be different chemical species in a polymer blend or rigid crystalline (r) and mobile amorphous (m) material in a semicrystalline polymer. To illustrate this point, a mobility timescale is depicted in Fig. 7.2.25(a) and the simplified ID domain structure of PE underneath in (b). Rigid crystalline and mobile amorphous materials exhibit motion of chain segments with different correlation times Tc. The chains at the interface between both domains exhibit intermediate mobility. The exact ranges of correlation times in the individual domains depend on the particular choice of filters. Therefore, the values of domain sizes derived through spin-diffusion NMR also depend on the type of filters used. In particular, the interface is defined solely by the NMR experiment and can only be detected if the filters are properly chosen. 

The exchange of nuclei between chemically different sites in a molecule often occurs at a frequency comparable with the difference between the resonance frequencies of these different sites, i.e. the timescales of NMR spectroscopy and fluxionality are similar. For this reason NMR spectroscopy is the technique par excellence for investigating dynamic processes. Nevertheless, as we shall see, dynamic i.e. variable-temperature) X-ray crystallography can provide information which is not available from the NMR experiment. If the exchange rates become sufficiently fast, exchange broadening on the much shorter IR timescale can occur,though this is a much rarer phenomenon. 

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