Bond Alternation and Bond Equalization in Linear and Cyclic Conjugated Hydrocarbons: Compared Roles of Sigma and pi Binding using Orbital Forces

The phenomena of bond alternation and bond equalisation in conjugated hydrocarbons are studied using dynamic orbital forces (DOF) which provide an index of intrinsic CC binding, with its sigma and pi components. Some linear polyenes, polyynes and cumulenes have been analysed. Dealing with linear polyenes and polyynes, it is shown that sigma bonds can be considered as weak "inverted" ones in formally multiple bonds and strong "superdirect" ones in formally single ones. This alternance in sigma bonding partly compensates and, in some cases, overcome the alternance in pi binding. Moreover, it was shown from a panel of seven aromatic annulenes and allyl compounds that the bond equalization is favoured by sigma bonding. It can be unfavoured or favoured by the pi binding according to the system and the nature of its deformation into a bond alternant structure.