Approximate π-electron ‘ring current’ intensities in some sulphur-heterocyclic analogues of fluoranthene

Abstract

Approximate MO calculations are reported on the π-electron ‘ring current’ properties of two sulphur-heterocyclic isosteres of fluoranthene (I)[acenaphtho[1,2-b]thiophen (II) and acenaphtho[1,2-c]thiophen (III)]. On the basis of recently published ¹H n.m.r. experimental measurements, order-of-magnitude estimates have been made of the relative π-electron ‘ring current’ intensities in the various rings of these polycyclic molecules. Comparison is therefore made here between the calculated values of relative ‘ring current’ intensities being reported, and those derived from experiment. The experimentally deduced values are qualitatively confirmed by these approximate calculations in that the computed ‘ring current’ intensities in the ‘naphthalenic’ parts of (II) and (III)(rings A and B), and in the central five-membered rings (rings C), are very similar to the intensities previously evaluated for the analogous homocyclic rings in fluoranthene itself, whilst the heterocyclic rings (rings D) in (II) and (III) appear to bear a smaller π-electron ‘ring current’ intensity than does the corresponding benzenoid ring D, in (I). This reduction in the ‘ring current’ intensity associated with ring D, on going from the hydrocarbon (I), to the heterocycles (II) and (III), is attributed almost entirely to the assumed smaller area of the thiophenoid rings.