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

Calculations at the B3LYP/6-31 +G(d,p)//MP2(FC)/6-31+G(d,p) level of the at oms-in-molecules interbasin pair numbers were carried out for a number of s ubstituted cyclopentadienyl five-membered rings and compared to the electro n localization function (ELF) bond basin populations. A smooth correlation is found for the formally single C-C bond pair number with the correspondin g homomolecular-homodesmotic resonance energy, as was previously been shown for the ELF bond basin numbers. The two measures of bonding are essentiall y equal for the non-polar C-C bond, but this is shown to be an exception ra ther than a rule, the situation being more complex when bonds are polar and /or lone pairs are nearby. (C) 2000 Elsevier Science B.V. All rights reserv ed.