Time dependence, of NMR

Stereochemical Nonequivalence of Protons 591 13-11 Time Dependence of NMR Spectroscopy 594 Problem-Solving Strategy Interpreting Proton NMR Sectra 597... 

If this were all NMR had to offer, it would not be considered particularly useful in chemical investigations, since all one achieves is a costly and inconvenient estimate of the total hydrogen, fluorine, etc., content in a sample. In practice, all applications of NMR to chemistry are from three secondary phenomena the chemical shift the time-dependence of NMR phenomena and spin-spin coupling. These effects will now be discussed. 

If the resonance from proton A (which is coupled to proton B) is observed while simultaneously proton B is strongly irradiated at its resonant frequency, the normal doublet expected of (half of an AB quartet) will collapse to a singlet. This results from the fact that the second irradiating field causes rapid transitions of Hb between its two spin states, so that experiences only the averaged spin state of Hb and hence no splitting in its energy levels results. Clearly, this phenomenon is related to the time dependence of NMR. 

Fig. 3.1.4 Anisotropic self-diffusion of water in and filled symbols, respectively). The horizon-MCM-41 as studied by PFG NMR. (a) Depen- tal lines indicate the limiting values for the axial dence of the parallel (filled rectangles) and (full lines) and radial (dotted lines) compo-perpendicular (circles) components of the axi- nents of the mean square displacements for symmetrical self-diffusion tensor on the inverse restricted diffusion in cylindrical rods of length temperature at an observation time of 10 ms. / and diameter d. The oblique lines, which are The dotted lines can be used as a visual guide, plotted for short observation times only, repre-The full line represents the self-diffusion sent the calculated time dependences of the...

Fig. 3.3.7 Time dependence of the axial dispersion coefficients D for water flow determined by NMR horizontal lines indicate the asymptotic values obtained from classical tracer measurements. (a) Water flow in packings of 2 mm glass beads at different flow rates and (b) water flow in catalyst.

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

In an NMR/MRI flow experiment, we would like to measure parameters such as velocity without regard to the starting position of the particle. Thus, mo is always set to zero. The moments m, are under the control of the experimenter in that they are manipulated by the choice of the time dependence of the gradient G. Thus, it is easy to see that m0 can be set to zero by simply making sure that the time integral of the gradient is zero. The easiest way to accomplish this is to have a bipolar gradient of equal absolute amplitude and duration. 

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. 

LIG. 22 A schematic illustration of the dependence of NMR relaxation times T and T2 on the molecular correlation time, xc, characterizing molecular mobility in a singlecomponent system. Both slow and fast motions are effective for T2 relaxation, but only fast motions near w0 are effective in Tx relaxation. 

For PIB the apparent activation energy found for the structural relaxation time in the NSE window is almost twice that determined by NMR [136] (see Fig. 4.9 [125]). For aPP, the temperature dependence of NMR results [138] seems, however, to be quite compatible with that of the NSE data nevertheless, 2D exchange NMR studies on this polymer [139] reveal a steeper dependence. This can be seen in Fig. 4.11 [ 126]. 

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

Refluxing benzene solutions of Cjq in the presence of a 20-fold excess of BujSnH leads to hydrostannylation (Scheme 6.15) [73]. Multiple additions can also take place. To maximize the yield of the monoadduct CgoHSnBuj (24), the time dependence of the reaction was followed quantitatively by HPLC. After about 4 h, the concentration of the monoadduct 24 reaches its maximum. Compound 24 can be isolated by preparative HPLC on a Cjg-reversed-phase stationary phase with CHCI3-CH3CN (60 40, v/v) as eluent. The structure of C5oHSnBu3 (24) was determined by NMR spectroscopy and other methods, showing that a 1,2-addition takes place regio-selectively (Scheme 6.15) [73]. 

Thus, although the nmr methods are important for comparison of the structure of proteins in the solid state and in solution, they are also of importance in areas where X-ray crystallography can provide little information. One of these areas concerns the time dependence of protein structure, for molecular motion over a wide range of time scales can be detected. Table IV indicates the methods and references to these studies. The range of the nmr technique is from about 10 10 s to slower than 10 s. Thus, although more restricted than X-ray crystallography in direct structure determination, the nmr studies can check the solution structure and complement the diffraction studies once the overall similarity between the solid and solution structure is proved. In this way nmr relates the static picture of a protein structure to the kinetic data of solution chemistry. 

In the preceding decade, solid-state NMR spectroscopy has provided important and novel information about the nature and properties of surface sites on working solid catalysts and the mechanisms of these surface reactions. This spectroscopic method offers the advantages of operation close to the conditions of industrial catalysis. A number of new techniques have been introduced and applied that allow investigations of surface reactions by solid-state NMR spectroscopy under both batch and flow conditions. Depending on the problems to be solved, both of these experimental approaches are useful for the investigation of calcined solid catalysts and surface compounds formed on these materials under reaction conditions. Problems with the time scale of NMR spectroscopy in comparison with the time scale of the catalytic reactions can be overcome by sophisticated experimental... 

Figure 2. Time dependence of 129Xe NMR spectrum (left) and intensity (inset) as HP Xe diffuses into a cylinder of porous Vycor glass (right). The stronger line close to 0 ppm is due to Xe gas.

A mutarotational equilibration is demonstrated in Fig. 5.5 by the time dependent 13C NMR spectrum of a solution of D-glucose. Initially, the solution contains almost pure a-D-glucose, but the signals at lower field corresponding to the /i-anomer soon appear. At equilibirium, the relative signal intensities indicate that the /i-anomer is favored (60% P, 40% a Fig. 5.5) [685],... 

Tanner [49] measured diffusion coefficients of water in three different types of frog muscle cells. He used a variety of magnetic field gradient techniques so as to cover a wide range of diffusion times A= 1 ms to 1 s. The time dependence of the diffusion coefficient was analyzed to obtain the intracellular diffusion coefficients and estimates of the permeability of the cell membranes. In restricted diffusion studies three 90 degree r.f. pulse sequences (stimulated echo) are often used which provides PG-NMR experiments with long diffusion times to explore the dependence of diffusion time on the echo attenuation [49]. 

Contact time dependences of signal intensities in 29 Si and 13 C CP MAS NMR spectrum (Figure 54) were measured for kaolinite and kaolinite-DMSO and kaolinite-DMSO-dg... 

The time-dependent H NMR spectrum of 22 provided an insight into the thermodynamic features of the ring/ open-chain ratio of ca. 1 4 after a week. The energy barriers of the ring/open-chain and the ring inversion processes of 22 were estimated to be higher than those for 19-21. 

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