Ab Initio Calculation of Proton Shielding Tensors Comparison with Experiments

line-narrowing technique has added an impressively long list of proton shielding tensors a to our knowledge of material properties. Complementary access to such tensors is to calculate them by quantum chemical methods. An important advantage of this approach is that, in addition to the symmetric constituent of a, it also yields the components of the antisymmetric constituent o that are virtually inaccessible to measurements by spectroscopic means. So far only relaxation studies have provided experimental information about (Kuhn, 1983 Anet and O Leary, 1992). Note that these experiments were done on carbons. 

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 A r lacks a safe basis even for benzene. We point out that the anisotropy of the proton shielding in benzene is by no means fully specified by the 

Bernd Tesche (1994) has verified in the meantime that approximately the same values are valid for H4. These values agree remarkably well with those from the (old) multiple-pulse line-narrowing experiment and one might be tempted to conclude that this is it. It would be an illusion to expect still better agreement because the calculation was done for an isolated molecule, whereas the measurement includes intermolecular shielding contributions. These contributions can be estimated (but not more than that) on the basis of a point dipole model (Post, 1978 Avara-mudhan and Haeberlen, 1979). For malonic acid we expect that these contributions do not exceed 0.5 ppm for the various elements of the a tensor. 

Thus the comparison of experiment and theory looks very satisfactory. However, caution and a measure of skepticism are still appropriate. Namely, if Tesche does the shielding tensor calculation with larger basis sets and with FULL LORG, which should give better results, the values for (Txx and increase to -4.11 and -1-4.40 ppm (average for H3 and 

H4 the differences are marginal), whereas o-yy = -0.29 ppm remains near zero. Tesche (1994) also employed the IGLO method to calculate ctCHj) and ct(H4) and the results hardly differed from those of FULL LORG (Tesche, 1994). 

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V. Ab Initio Calculation of Proton Shielding Tensors Comparison with Experiments References... 

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