Anisotropic solids, powders

In conclusion, we mention three areas where further work could be quite profitable. The first is the extension of these measurements to other large organic radicals such as PAC, whose structure is shown in Figure 1. In particular, studies in solid powders should be attempted, since the fast relaxation processes are expected to lead to averaging of the anisotropic dipolar and quadrupolar interactions. To our knowledge, such experiments have not yet been reported. 

An exception to this rule arises in the ESR spectra of radicals with small hyperfine parameters in solids. In that case the interplay between the Zeeman and anisotropic hyperfine interaction may give rise to satellite peaks for some radical orientations (S. M. Blinder, J. Chem. Phys., 1960, 33, 748 H. Sternlicht,./. Chem. Phys., 1960, 33, 1128). Such effects have been observed in organic free radicals (H. M. McConnell, C. Heller, T. Cole and R. W. Fessenden, J. Am. Chem. Soc., 1959, 82, 766) but are assumed to be negligible for the analysis of powder spectra (see Chapter 4) where A is often large or the resolution is insufficient to reveal subtle spectral features. The nuclear Zeeman interaction does, however, play a central role in electron-nuclear double resonance experiments and related methods [Appendix 2 and Section 2.6 (Chapter 2)]. 

Molecules in the solid state are in fixed orientations with respect to the magnetic field. This produces chemical shift anisotropic powder patterns for each carbon atom since all orientations are possible (Fig. 2). It was shown as early as 1958 that rapid sample rotation of solids narrowed dipolar-broadened signals [18]. Several years later, it was recognized that spinning could remove broadening caused by CSA yet retain the isotropic chemical shift [19]. 

Polv(ether) Synthesis. The poly(ethers) were made by a two-phase reaction with dibromoalkanes. In a typical reaction, 0.75 g (2.4 mmol) of (II, R=H)) was mixed with 70 ml of 2N NaOH in a 3-necked round-bottom flask equipped with a mechanical stirrer. To this was added an equimolar amount of 1,9-dibromononane in 20 mL nitrobenzene and approximately 10 mg of tetrabutylammonium iodide and the mixture was stirred overnight at 50°C. The resulting solid mass was washed with methanol and then with 2N NaOH. After washing with 0.1 N HC1, the product was Soxhlet-extracted with methanol and dried to yield 0.68g (64.7%) of a light green powder which melted to an anisotropic liquid at 290°C. The other poly(ethers) were prepared in the same manner, using spacer lengths of 7, 9,11, 7/9 mixture, and 9/11 mixture. The yields and IR spectra of the poly(ethers) is shown in Table I. 

Samples. For the purposes of demonstrating NMR imaging of anisotropic chemical reactions (37,38), the reactions of ammonia with solid organic acids were used. It has been shown that single crystals of acids such as 4-chlorobenzoic acid react anisotropically with ammonia. Most faces of the crystal react rapidly however, the (100) face reacts slowly as this face has an array of C-Cl units on the surface that protect the carboxylic acid function from ammonia (39). Powders of organic acids react with ammonia in 1 1 ratio to form the simple ammonium salts. 

In solution samples or in a solid matrix where the radicals can tumble rapidly, only the average g-value can be measured. For powder or polycrystalline samples the spectrum is the envelope of spectra arising from all possible orientations of the radical. It is often possible (95) to determine directly from the shape of the first derivative ESR line the values of either the axially symmetric (g, g, g. ) or the fully anisotropic (g, g2 g3 9 tensor. The major disadvantage is that the orientation of the radical in the medium cannot be determined, but this is less important in most photochemical studies. 

We report solid State C NMR measurements on powder samples of C o and of a mixture of and C70. The NMR results show that, at 296 K, Cm molecules rotate rapidly and isotropically in the solid state, while C70 molecules rotate somewhat more anisotropically. These results are consistent with the proposed spherical geometry of Cm and prolate spheroidal geometry of C70. The rotational correlation time of Cm molecules in the solid state becomes greater than 50 /ts at about 100 K. 

Interest in the structures and properties of fullerenes has received new impetus from the recent discovery that the molecules Cm and C70 can be prepared in large quantities by comparatively simple procedures. The ready availability of solid samples of Cm and C70 now permits their characterization by a variety of physical methods. In this paper, we report the results of solid-state C nuclear magnetic resonance (NMR) measurements on powder samples of Cm and of a mixture of Cm and C70. Our NMR results indicate that Cm rotates rapidly and nearly isotropically in the solid state at 296 K and that C70 also rotates at 296 K, although somewhat anisotropically. The rotation of Cm molecules becomes slow on the time scale of our measurements at about 100 K. 

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