Ab Initio Kinetics for the Unimolecular Reaction C₆H₅OH → CO + C₅H₆^†

The unimolecular decomposition of C₆H₅OH on its singlet-state potential energy surface has been studied at the G2M//B3LYP/6-311G(d,p) level of theory. The result shows that the most favorable reaction channel involves the isomerization and decomposition of phenol via 2,4-cyclohexadienone and other low-lying isomers prior to the fragmentation process, producing cyclo-C₅H₆ + CO as major products, supporting the earlier assumption of the important role of the 2,4-cyclohexadienone intermediate. The rate constant predicted by the microcanonical RRKM theory in the temperature range 800−2000 K at 1 Torr − 100 atm of Ar pressure for CO production agrees very well with available experimental data in the temperature range studied. The rate constants for the production of CO and the H atom by O−H dissociation at atmospheric Ar pressure can be represented by k_CO = 8.62 × 10¹⁵ T^-0.61 exp(−37 300/T) s^-1 and k_H = 1.01 × 10⁷¹ T^-15.92 exp(−62 800/T) s^-1. The latter process is strongly P-dependent above 1000 K; its high- and low-pressure limits are given.