Theoretical studies of proton magnetic shielding anisotropies

Proton magnetic shielding tensors are calculated using a gauge-invariant molecular orbital approach. Calculated chemical shifts determined from the isotropic values of such tensors are in excellent agreement with the observed results for some simple polyatomic molecules. Theoretical values of proton anisotropies are presented in a principal axis system and with respect to art X-H bond axis framework. Such anisotiopies for the protons in simple, acyclic polyatomic molecules show poor agreement with most of the available experimental data. Reasons for these discrepancies are discussed. This theoretical study suggests that, in general, proton magnetic shielding tensors are not axially symmetric when referred to an X-H bond axis coordinate system.