Full geometric optimization of tetracyclo[18.104.22.168(4,11).0(6,10)] dodeca-2,7-diene (TCDD) has been done by DFT/B3LYP methods and the structure of the molecule was investigated. The double bonds of TCDD molecule are endo-pyramidalized. The cationic intermediates and products obtained as a result of the addition reaction have been studied using B3LYP/6-311G(d,p) and B3LYP/6-311+G(d,p) methods. The bridged bromonium cation isomerised into the more stable N- and U-type cations and the difference between the stability of these cations is small. The N- and U-type reaction products are obtained as a result of the reaction which takes place over the cations in question. Although the U-type cation was 0.68 kcal mol(-1) more stable than the N-type cation, the U-type product was 0.71 kcal mol(-1) more stable than the N-type product. (C) 2010 Elsevier B.V. All rights reserved.