The pair density description of aromaticity in some substituted cyclopentadienyl systems: a comparison of AIM and ELF bonding descriptors Original Research Article

Calculations at the B3LYP/6-31+G(d,p)//MP2(FC)/6-31+G(d,p) level of the atoms-in-molecules interbasin pair numbers were carried out for a number of substituted cyclopentadienyl five-membered rings and compared to the electron localization function (ELF) bond basin populations. A smooth correlation is found for the formally single C–C bond pair number with the corresponding homomolecular–homodesmotic resonance energy, as was previously been shown for the ELF bond basin numbers. The two measures of bonding are essentially equal for the non-polar C–C bond, but this is shown to be an exception rather than a rule, the situation being more complex when bonds are polar and/or lone pairs are nearby.