Using the density functional theory (DFT/B3LYP) method, the reaction of HO₂ radical with NO₂ was studied. The potential energy surfaces (PES) of the HO₂＋NO₂ reaction were calculated at B3LYP/6-311G^** and CCSD (T)/6-311G^** (single-point) levels of the theory. It is shown that the direct H-extrusion reaction mechanism is the main reaction channel on the singlet PES for the title reaction, namely, a direct H-shift from the HO₂ radical to the N atom of the NO₂ molecule to form the product P₁ (HNO₂＋³O₂). Another favorable reaction channel on the singlet PES involves that the end O atom of the HO₂ radical attacks on the N atom of the NO₂ molecule, forming the isomer 1 (HOONO₂), then H-shift follows, producing P₂ (trans-HONO＋³O₂). Both of the reaction channels mentioned above are exothermic with the energy of 90.14 and 132.52 kJ•mol^－1, re-spectively.

Key words: DFT, potential energy surface, HO₂＋NO₂, reaction channel