Mechanism, thermochemistry, and kinetics of formation of PhC(O)H and HOPhC(O)H during unimolecular decomposition of P-PhC(O₂^•)HPhOH

Bisphenols are one of the main components of bio-oil, produced during the pyrolysis of lignin-containing biomass. Synthetic bisphenols are used in polycarbonate plastics, epoxy resins, and thermal papers. Their mechanism of oxidation is important for the determination of the fire safety of these materials and the possibility of using them as additives in fuels for the decrease and description of ignition delays, as well as for the determination of their health risk assessment in medicine.

One representative of bisphenols is p-benzylphenol (p-PhCH₂PhOH), which is formed during the fast pyrolysis of lignine-containing biomass. Its thermochemistry of oxidation has been partially studied previously. It is shown that the reaction of chain oxidation of p-PhCH₂PhOH is thermochemically favorable at low temperatures. However, these studies consider only two pathways of this reaction: (1) the chain oxidation of RH by RO₂^• and (2) the tautomerization of R’HO₂^• to R’O₂H with following production of R’O^• and OH radicals. At the same time, the reactions of intramolecular rearrangement of RO₂^•, produced PhC(O)H and ^•PhOH or HOPhC(O)H and ^•Ph, are not reported but can be an important part of its oxidation mechanism.

The five DFT (M06-2X (i = 1), B3LYP (i = 2), wB97XD (i = 3), M08HX (i = 4), MN15 (i = 5)) approaches with 6–311 +  + G(d,p) basis set are used for the determination of standard enthalpies of atomization (Δ_raH°(X_i)) of considered compounds (molecules, radicals, and transition states). These values of Δ_raH°(X_i) are corrected using the empirical linear calibration dependencies, reported previously. The different calibration dependencies are used for the hydrocarbons (including the aromatics and simple oxygenated derivatives) and for the peroxides. The corrected values of Δ_raH°(X_i, CORR) are used according to Hess’s law for the determination of Δ_fH°(X_i, CORR). The most consistent values of Δ_fH°(X, MEAN) are derived from the coordination of the values of Δ_fH°(X_i,CORR) using the intersection of their values of standard deviations (3SD_i). These values of Δ_fH°(X, MEAN), as well as the B3LYP values of S°(X), which are accounting the frequencies correction and internal rotations, as well as their temperature dependencies, are used for the determination of thermochemistry of considered reactions and of the calculation, within transition state theory (TST), of the values of high pressure limits of the rate constant. The values of H°(X_i), S°(X_i), and G°(X_i) are calculated using the Gaussian 16w program. The considered mechanism is prepared using ISIS/Draw package. The temperature dependencies of thermochemical properties and the values of rate constants are determined using the ChemRate program (v.1.5). The optimized structures are visualized using the Chemcraft package.