Steric strain and electron‐impact. The behaviour of some n,n'‐dimethyl‐1,1‐binaphthyls, some n,n'‐dimethylbiphenyls and model compounds

For the title compounds I(M), A[M  H]+ and A[M  Me]+ have been determined. In the biaryl compounds, unlike the xylene and the two dimethylnaphthalenes studied, A[M  Me]+ is noticeably dependent on the position of the methyl group. Deuterium labelling of the methyl groups in the biaryl compounds reveals that more than one process is involved in the formation of the [M  Me]+ ion. In contrast, only in the case of 2,2′‐dimethylbiphenyl is the appearance potential of the [M  H]+ ion position dependent. The labelling results suggest complete scrambling of the hydrogen atoms before formation of the [M  H]+ ions. Some comments are made on the problem of trying to relate differences in strain energies and ionisation and appearance potentials in alkyl substituted aromatics.