Studies in bonding electron density. Part I

Abstract

The electron-density sections in the planes of the benzene rings of penta-m-phenylene (PMP), hexa-m-phenylene (HMP), and 1:2,5:6,9:10-tribenzocyclododeca-1,5,9-triene-3,7,11-triyne (TTT) have been computed and averaged as well as the sections parallel to the ring planes at a distance of 0·4 Å. The results demonstrate σ-density in the centres of the (aromatic) C–C bonds and π-density above the ring planes. Sections passing through the aromatic and exocyclic bonds of the three compounds have been averaged and show ‘π’ density maxima at approximately 0·5 Å from their bond centres. The averaged electron-density distribution in the –CC– triple bond of TTT contains a trough of –0·2 e/ų in the centre as well as ‘π’ peak density of 0·1 e/ų at 0·6 Å distance from the centre of the bond. The anhydrous form of acetylenedicarboxylic acid (ADA) also exhibits a trough in the centre of its –CC– triple bond.