Proton shielding anisotropy

Because of the proton s chemical importance and its favorable characteristics for NMR detection, the overwhelming bulk of experimental investigation and correlation of chemical shifts has been centered on the proton. However, the situation regarding the determination of proton shielding anisotropies has been unsatisfactory in many respects. Similarly, the field of ab initio theoretical calculations of proton shielding tensors has, until very recently, enjoyed little success. Both these factors are related to the fact that the proton chemical shifts are comparatively small and hence influenced strongly by secondary factors such as neighboring-atom electron distribution and medium effects. 

Likewise, a systematic use of such anisotropy effects has been used to establish the conformation of small rings from the proton shielding in the corresponding allenic phosphine oxides,<1967,153tol56 1968,46 1969 164 to 166) for example in 170. 

The neighbor anisotropy term <7 of eq. (3.2) plays an important role in proton shielding, permitting, for example, a distinct differentiation between aromatic and olefinic protons due to the ring current effect. However, this contribution is small in 13C NMR (<2 ppm). A comparison of the methyl carbon shieldings in methylcyclohexene and toluene shows that the ring current effect often cannot be clearly separated from other shielding contributions ... 

Consequently, two of the four protons of each ring (HA and HB) come to lie in the shielding anisotropy region of the opposite ring, thus absorbing as a high-field AB-pattern, which is further split by meta coupling (21 5 = 5.72 ppm 22 ... 

The experimental result, Acr p = -5.3 ppm, which the theoretical community (see, e.g., Jameson, 1993) likes to cite for comparison with and confirmation of its calculations, is based on a single m.p. powder spectrum (Ryan et al., 1977). The asymmetry of that spectrum is desparately small and the value for Ao-g p that can be inferred is hardly more than an estimate of the upper limit of Act. Moreover, the experiment was done at a temperature of 77 K, which is too high to freeze out the well-known reorientational jumps of the benzene molecules about their sixfold axes. As Ryan et al. (1977) state explicitly, these jumps lead to a motional averaging of the in-plane shielding components. We feel, therefore, that the comparison of experimental and (converging) theoretical results for Aproton shielding in benzene is by no means fully specified by the... 

The most widely used method for obtaining experimental proton anisotropies has been that of liquid crystal NMR measurements. This method has several inherent sources of experimental error, which, in the case of proton shielding, often overshadow the resulting anisotropy. Many of the analyses of liquid crystal proton spectra have been based on the observed shift between the nematic and isotropic phases. Such a procedure relies on the assumption that the solvent effects are the same in both the ordered and isotropic phases.3... 

In summary, high resolution solid-state NMR of in rigid polymers is achievable using averaging techniques to remove shielding anisotropies and proton-proton dipolar couplings. But the resolution is at least two orders of... 

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