Per iodoannulenes: A Generalized Annulene-within-an-Annulene Paradigm for Combined σ and π Ring Currents.

Per iodoannulene molecules and ions C _x I _x ^q in planar geometry offer examples of systems with the potential for outer σ and inner π ring-current double aromaticity, given a sufficient overlap of tangential p _σ -orbital manifolds on the large atoms of the outer cycle. Previous theoretical work indicated concentric diatropic currents in the dication C ₆ I ₆ ²⁺ . Ab initio ipsocentric calculations support an account in terms of frontier-orbital selection rules for current contributions in C ₆ I ₆ ²⁺ (and radical C ₆ I ₆ ⁺ , implicated in recent experimental work on the oxidation of per iodobenzene). A σ/π analogue of the annulene-within-an-annulene model is applied here to per iodo systems based on cyclooctatetraene. Model species C ₈ I ₈ ^q with charges q = 0, +1, +2, +4, -2 and structures constrained to a planar D _{4 h} symmetry exhibit maps with all combinations of σ/π con- and counter-rotation, comprising global σ ring currents on the iodine perimeter and central π ring currents on the carbocycle. All can be rationalized by the separate application of the tropicity selection rules to the two subsystems, whether in singlet or triplet states.