Signal averaging solid-state

Conventional utilization of solution-phase NMR data acquisition techniques on solid samples yields broad, featureless spectra (Fig. 1A). The broad nature of the signal is due primarily to dipolar interactions, which do not average out to zero in the solid state, and chemical shift anisotropy (CSA), which again occurs because our compound of interest is in the solid state. Before one describes the two principal reasons for the broad, featureless spectra, it is important to understand the main interactions that a nucleus with a magnetic moment experiences when situated within a magnetic field in the solid state. In addition, manifestations of these interactions in the solid state NMR spectrum need to be discussed. 

Low temperatures studies, for instance in THF at 175K, can elucidate annular tautomerism (92JCS(P2)1737) a large collection of 15N chemical shifts of NH-pyrazoles with tautomerism frozen has been published (94MRC699). The tautomerism is also frozen in the solid state. Thus two 15N shifts at 150 ppm (NH) and 222 ppm ( = N-) are observed for solid imidazole the average is close to the single signal at 186 ppm found in solution. 

The two signal 15N NMR spectrum of solid imidazole indicates that tautomerism is slow in the solid state. The average (186 ppm) is the same as the shift observed in solution. As the solution pH decreases the 15N signal moves progressively upheld until both nitrogens are protonated (82JA1192). 

In the solution state, there is a free rotation around the C(l)—N (fast on the NMR time scale) and, thus, average chemical shifts are observed for C2 and C6 and likewise for C3 and C5 in azobenzene as well as in 4,4 -disubstituted trans-azobenzenes. In the solid state, locked planar trans configurations are detected, resulting in nonequivalence of C2/C6 and C3/C5 signal pairs. Chippendale et al.86 reported in 1981 the I3C solid-state NMR spectra of azobenzene and symmetrically 4,4 -disubstituted rranj-azobenzenes (26). 13C chemical shifts are given in Table 12. 

One further feature of the structure of the cyanide adduct is of interest. The molecule is fluxional, showing a singlet Cp resonance down to -52° which splits into two signals at -60° and below. This fluxionality is ascribed to a "wind-shield wiper" motion of the bridging cyanide (eq. 20) which averages the Cp environments. The solid state structure of 2 shows disorder for the cyanide position which was interpreted as the overlapping of the two orientations, 2a and 2b. The solid state structure is thus a "stop-action" photograph of the fluxional process (15). 

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