Rovibrational averaging shielding

There is a need for rovibrational averaging of theoretical shielding prior to comparison of calculated values with experiment. This becomes even more important as the theoretical methods begin to yield more accurate results. 

In order to make a rigorous comparison between calculated and experimental shielding tensors, it is necessary to correct the calculated shielding for rovibrational averaging (28-33). Typically, ab initio calculations are carried out on an isolated rigid... 

Shielding Surfaces and Rovibrational Averaging. - For shielding calculations to mimic exactly what is observed in experiment, not only does one require an accurate equilibrium geometry but also an incorporation of rovibrational averaging is necessary. As an example, Astrand and Ruud have demonstrated that the rovibrational corrections to shielding in the case of in 2-fluorobutane... 

Isotope Shifts. - Isotope shifts are intimately related to rovibrational corrections. Thus, with the same shielding surfaces used for rovibrational averaging, one can extract expected changes in chemical shift due to isotope substitution. This is once again illustrated in Dransfeld s work, which is cited near the beginning of the preceding section. Here too, some cautionary words are in... 

NMR isotope shifts arise from the same rovibrational averaging. Because a lighter atom undergoes greater amplitude motions than a heavier one, substitution of a heavy isotope alters the shielding of the nucleus replaced (primary isotope effect) and of other nuclei in the molecule (a secondary isotope effect). 

Table 1. F Absolute Isotropic Shielding of the Rovibrationally Averaged Free Molecule at 300 K Based on HF Molecule, Compared with Absolute Shielding Derived from...

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