We made B3LYP/6-311G** theoretical study on the bond dissociation energy (BDE) for the carbonyl compounds and heavy atom analogues (H(2)AB, A = C, Si, Ge; B = 0, S, Se), and their halogen substituted compounds. We demonstrate that the second BDE is much weaker than the first one due to the electronic relaxation effect on the AB fragment. The difference in BDE (DeltaBDE) appears to be a property of AB alone correlated well with its value of DeltaE(ST) (singlet-triplet energy gap), nearly independent of the substituents involved. The normal bond length of the second bond, the weak A-X bond in XAB radical can be interpreted as a result of potential surface avoided-crossing. (C) 2002 Elsevier Science B.V. All rights reserved.