OH-initiated oxidation of toluene. 3. Low-energy routes to cresol and oxoheptadienal

The toluene-OH-O2 system implicated in the atmospheric degradation of toluene is studied further using quantum chemistry methods. Two new reaction mechanisms are explored as alternatives to the previously proposed mechanism. While the previous mechanism involves surmounting a 170 kJ/mol barrier, the new equivalent cresol formation route has a barrier above the asymptotic state calculated to be 12 kJ/mol at the B3LYP/6-311G(2df,2pd) level. The new oxoheptadienal formation route occurs via two successive reactions with OH, with the highest barrier lying 200 kJ/mol below the energy of the reactants. Neither of the newly proposed reaction mechanisms involves forming a toluene oxide intermediate.