The structural properties of the transition states for XONO (X = Cl, Br) loss in the presence of NO2 are examined by the use of B3LYP density functional theory with various basis sets. The energetics were refined with CBS-QB3 calculations. The energy barriers for reaction channels leading to XNO2 + NO2 are estimated to be 4.1 and 3.1 kcal mol(-1) for the chlorine and bromine reaction, respectively, whereas for the XONO2 + NO products the energy barriers of 22.4 and 24.9 kcal mol(-1), respectively, were found. These results suggest that the isomerization of XONO -> XNO2 is significantly facilitated in the presence of NO2 compared to the corresponding unimolecular isomerizations. (c) 2006 Elsevier B.V. All rights reserved.