Proton magnetic resonance of non-planar condensed benzenoid hydrocarbons

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Abstract
The Hückel-London-McWeeny theory of ‘ring current’ effects on proton chemical shifts has been extended to non-planar aromatic systems. As a special case the effect of a planar ring on a generally situated proton is also treated. The predictions are compared with the experimental results on helicenes reported in Part I of this series. A satisfactory account is given of the shifts of protons primarily in the lateral deshielding regions of rings; however, shielding effects tend to be underestimated for protons situated directly above rings. For overcrowded protons an additional downfield steric effect is postulated. The theory's limitations appear to arise from the London approximation to integrals. Comparison is also made with the results of semi-classical calculations.