On the alleged correlations between simple LCAO-MO reactivity indices and proton chemical shifts in planar,condensed, benzenoid hydrocarbons

Previous investigations into the proposed correlations between proton chemical shifts in conjugated hydrocarbons and the various LCAO-MO ‘reactivity indices’ (at the corresponding carbon atoms to which the peripheral protons are bonded), are here reassessed in the light of more-recently acquired experimental data, for the case of the planar, alternant, condensed, benzenoid hydrocarbons. A correlation coefficient of 0·73, statistically ‘significant’ at less than the ½% level, is obtained. Nevertheless, there are several unsatisfactory features of such proposed correlations (which are discussed), and, in the final analysis, no causative relation is expected between proton chemical shifts and reactivity indices in these molecules. Furthermore, the relative chemical shifts of the sterically unhindered protons in planar, alternant, benzenoid hydrocarbons can be accounted for by the ‘ring current’ effect alone, without the need to postulate a dependence of the proton chemical shifts on reactivity indices, which is in any case considered to be unlikely on physical grounds.