MAS techniques

The chemical shifts are for the solid state spectra obtained by the CP-MAS technique and referred to hexamethyl benzene (132.3 ppm compared to TMS). 

Fig. 1. 13C-NMR fast exchange-slow exchange transition for the conformational interconversion of cyclododecane in solution of propane-d, (left side) and in the solid by CP-MAS techniques (right side) at 75.47 MHz. The temperature decreases from top to bottom as indicated at the spectra. Chemical shifts are given at the signals and refer to TMS = 0 ppm. (Ref.7))...

With an understanding of magnetic interactions in the solid state, inherent relaxation processes, and experimental techniques to overcome these difficulties, Schaefer et al. acquired the first liquid-like NMR spectrum of a solid by CP/MAS techniques [27]. Before a review of recent applications of CP/MAS NMR to the pharmaceutical sciences, the brief Experimental section details some important information regarding practical solid state NMR. 

Figure 8.3 Illustration of HR-MAS techniques applied to a resin-bound trisaccharide (a) static XH spectrum of the solvent swollen sample (b) XH spectrum with magic-angle spinning at 3.5 kHz (c) H spectrum with MAS and spin echo pulse sequence.

In addition to well-resolved one-dimensional (ID) 1H and 13C spectra, which are usually sufficient for monitoring synthetic steps, HR-MAS techniques can be applied to two-dimensional (2D) homonuclear and heteronuclear experiments, which allow a wealth of structural information to be obtained. H,13C HMQC (heteronuclear multiple quantum coherence) spectra are particularly useful in the analysis of solid support-bound oligosaccharides, since the anomeric protons exhibit characteristic resonances. Such a spectrum of a polymer-bound trisaccharide glycal is shown in Figure 8.4. 

We have characterized a resin-bound pentasaccharide by HR-MAS techniques. A comparison of the solution spectrum of the resin-cleaved pentasaccharide with the HR-MAS spectrum of the resin-bound pentasaccharide is shown in Figure 8.5. It is immediately obvious that the HR-MAS technique provides data of a quality similar to that of the solution technique, but in both cases, only four of the five anomeric protons are visible. However, a 2D homonuclear total correlation spectroscopy (TOCSY) spectrum (Fig. 8.6) of the resin-bound pentasaccharide allowed us to clearly observe the overlapped anomeric protons (demonstrating a resolution of 4.4 Hz). 

RP-HPLC methods have been frequently applied for the investigation of various chemical, biochemical and biophysical processes in in vitro model systems. Thus, the separation of new compounds achieved by enzymatic oxidation of phloridzin was carried out by semi-preparative RP-HPLC. Phloridzin was incubated with a polyphenol oxidase prepared from apple pulp for 6h at 30°C under air agitation. After incubation the suspension was filtered, stabilized by NaF and injected into the RP-HPLC column using diluted acetic acid-ACN gradient. The new compounds were isolated and identified by NMR and MA techniques. The proposed mechanism of the formation of new phloridzin derivatives 3 and 4 is shown in Fig. 2.159. The results illustrate that RP-HPLC can be successfully used for the study of enzymatic processes in model systems [331],... 

The C chemical shifts of benzotriazole in the solid state are obtained using the CP-MAS technique. The chemical shifts are slightly different from those observed in DMSO-dg or CDCI3 <83H(20)1713>. CP-MAS C NMR spectroscopy is a powerful tool which enables determination of the tautomeric structure of benzotriazole in the solid state. Thus, the solid-state C NMR spectra of some A-(dialkylaminomethyl)benzotriazoles support the existence of only the N(l) isomer <87JCS(P1)2673>. 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

Hence, application of the MAS technique for the investigation of quadrupole nuclei leads to a decrease of the linewidth of the CT by a factor of about 3.6. A further improvement of the resolution of the NMR spectra of quadrupole nuclei requires the application of more sophisticated techniques, such as the DOR (27) or the more recently developed MQMAS NMR spectroscopy (26,28). Until now, however, neither of these techniques has been applied for investigations of working solid catalysts, and they are therefore not considered in this review. 

As a consequence of the line broadening effects of internal magnetic interactions on solid-state NMR spectra (Section ILA), experiments that characterize working solid catalysis require the application of the MAS technique. Because of the salient feature of MAS NMR spectroscopy (rapid sample spinning during the measurement), specific techniques had to be developed to allow characterization of solids in sealed vessels under batch reaction conditions and in fixed-bed reactors under flow conditions. 

SiMouO ]4-.134 The structure of the heteropoly blue species (four-electron-reduced PM012O40]3-) formed in the molybdenum blue determination of phosphorus has been reported135 and a review of molybdate heteropoly blues has appeared.130 A stability index of polyanion structures has been discussed.137 The presence of two PMo, anions in aqueous solutions of molybdate and phosphate has been demonstrated by 31P NMR.138 Solid state NMR (nonspinning and MAS techniques) has been used for characterization of heteropolyanions.139 Molybdenum-95 NMR spectra of some polymolybdates have been reported.140... 

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