Solid polarization transfer

Pines and coworkers70 showed the feasibility of spin polarization transfer by SPINOE from laser-polarized 129Xe to surface 13C nuclei on low surface area materials in high-resolution solid-state NMR experiments. This technique provides the basis for novel surface 13C NMR investigations, e.g. of surface coatings, supported catalysts and electrode materials. 

Polarization transfer pathways for spectra (c) and (d) are depicted on a partial structure of 1 shown in the inset. Solid and dotted lines represent TOCSY and NOESY transfers, respectively. (Reprinted with adaptation with permission from ref. [39]. Copyright 1994 Academic... 

A suitable way to enhance NMR signals of nuclei with small magnetogyric ratios or low concentrations (rare spins 5) interacting with abundant spins I is the polarization transfer from the spin I to the spin S ensemble via a cross polarization (CP) experiment. This experiment is based on a double resonance technique, which can be applied in combination with MAS for the characterization of surface sites of solid catalysts and rigidly bound surface complexes. The pulse sequence used for CP experiments is demonstrated in Fig. 4. 

Recently, it was shown that ss-NMR spectroscopy can be used to determine the conformation of EpoB in the solid state [116]. The method relies on the measurement of intramolecular short H-H distances (in the range 1.8-3.0 A) from 2D CHHC correlations under MAS [117]. Regarding the sample preparation, a small amount of 13C labeled compound was diluted with EpoB with natural abundance of carbon isotopes. This reduces the signal to noise but, on the other hand, it excludes contributions from intermolecular H-H polarization transfer. Under these conditions, all CHHC cross-peaks result from intramolecular polarization transfer and reflect intramolecular interproton distances. 

Calculation of the frequency at Oppm for Hg from the measured frequency of 0 ppm for protons in the sample is a prudent alternative with modem spectrometers. For solid-state NMR, hexakis(dimethylsulphoxide)mercury(II) trifluo-romethanesulphonate, 5( Hg) = -2313 ppm, is a suitable secondary standard. Its high symmetry Octahedral coordination environment results in small chemical shift anisotropy and short H-Hg separations lead to high H Hg polarization transfer efficiency. 

As demonstrated by Hartmann and Hahn (1962), energy-matched conditions can be created with the help of rf irradiation that generates matched effective fields (see Section IV). Although Hartmann and Hahn focused on applications in the solid state in their seminal paper, they also reported the first heteronuclear polarization-transfer experiments in the liquid state that were based on matched rf fields. A detailed analysis of heteronuclear Hartmann-Hahn transfer between scalar coupled spins was given by Muller and Ernst (1979) and by Chingas et al. (1981). Homonuclear Hartmann-Hahn transfer in liquids was first demonstrated by Braunschweiler and Ernst (1983). However, Hartmann-Hahn-type polarization-transfer experiments only found widespread application when robust multiple-pulse sequences for homonuclear and heteronuclear Hartmann-Hahn experiments became available (Bax and Davis, 1985b Shaka et al., 1988 Glaser and Drobny, 1990 Brown and Sanctuary, 1991 Ernst et al., 1991 Kadkhodaei et al., 1991) also see Sections X and XI). 

Historically, Hartmann-Hahn polarization transfer was first applied to systems in the solid state (Slichter, 1978 Ernst et al., 1987), even though, in their seminal paper, Hartmann and Hahn (1962) reported applications to liquid samples. In general, Hartmann-Hahn experiments in the solid and liquid states differ with regard to the coupling mechanism (dipolar or indirect electron-mediated J coupling), the magnitude of the coupling... 

The first heteronuclear Hartmann-Hahn transfer in the liquid state preceded the homonuclear analogs of the experiment by about two decades. In their seminal paper on nuclear double resonance in the rotating frame, Hartmann and Hahn (1962) focused on heteronuclear polarization transfer in the solid state with the help of two matched CW rf fields with However, in the same paper, Hartmann and Hahn also discussed the coherent heteronuclear transfer of polarization for pairs of /-coupled heteronuclear spins in liquids and reported polarization-transfer experiments between H and P in hypophosphorous acid. Heteronuclear Hartmann-Hahn transfer in liquids with CW irradiation was applied by several groups (Maudsley et al., 1977 Muller and Ernst, 1979 Bertrand et al., 1978a, b Murphy et al., 1979 Chingas et al., 1979a, b, 1981). A detailed analysis of the experiment was presented by Muller and Ernst (1979) and by Chingas et al. (1981). Matched CW irradiation at the... 

Fig. 10. Plots of and against /pi in various electroless plating baths. Hollow symbols polarization resistance i p, solid symbols transfer resistance (I. Ohno, 1985 [5]).

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