Ab initio molecular orbital study of the O + C₆H₅O reaction

An ab initio molecular orbital study of the potential energy surface of the C₆H₅O + O reaction was performed at the (PUMP3/6‐31G*//UHF/6‐31G*) level of theory. Various reaction channels were considered. The most favorable mechanism, la and Ib, start from the attachment of the oxygen atom to the carbon atom of the C₆ ring in the ortho‐ or para position with respect to CO, taking place without activation energy. Then, either hydrogen elimination by mechanism Ia or 1,2‐H shift from the C(H)(O) group takes place; the latter process leads to the formation of the very stable C₆H₄(O)(OH) radical, which can also eliminate H by mechanism Ib. Thus, the main products of the C₆H₅O (²B₁) + O(³P) reaction are o/p‐benzoquinones and the hydrogen atom. At low temperatures, however, the system may be trapped in the potential well of the C₆H₄(O)(OH) intermediate. At high temperatures, the reaction may proceed by the formation and decomposition of o/p‐benzoquinones. Because of their higher activation energies, the reaction mechanisms giving rise to other products–the attachment of the oxygen atom to the bridging position to form an epoxy intermediate, followed by insertion of O into the CC bond and dissociation to give C₅H₅ and CO₂ (channel IIc), in addition to the attachment of oxygen to the terminal O atom of C₆H₅O followed by elimination of O₂ (channel III) – cannot compete with channel Ia or Ib. RRKM calculation was carried out for the total and individual rate constants for channels Ia and Ib. The three‐parameter expression for the total rate constant, fitted by the least‐squares method for the temperature range of 300–3000 K, is given as k_tot = 5·52×10⁻¹⁷ T^1·38 e^+148/T cm³ mol⁻¹ s⁻¹.