The Valence-Bond Description of Conjugated Molecules. 4. Theoretical Study of the Mills-Nixon Effect, a Phenomenon of π-Bond Localization in Small Ring Annelated Aromatics

The so-called Mills-Nixon effect, a phenomenon of 7r-bond localization in aromatic rings annelated by strained saturated rings, is studied by means of ab initio calculations of valence-bond type on benzocyclopropene and benzocyclobutene, in their equilibrium geometries calculated by using a gradient technique. In apparent discrepancy with an experimental work, recently published in this journal,¹ dealing with cyclobutane-annelated dihydropyrenes, our results evidence a significant Mills-Nixon effect. To understand its origin, additional calculations have been performed on benzocyclopropene and benzocyclobutene in standard geometries and in distorted benzenes, showing that the influences of cyclopropene and cyclobutene on aromatic rings are of different natures. Tendencies arising from these calculations and additinal ones on naphtho[b]cyclopropene and naphtho[b] cyclobutene suggest a possible explanation for the absence of sizable Mills-Nixon effect in cyclobutane-annelated dihydropyrene.