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

Context: 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-PhCHPhOH), 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-PhCHPhOH is thermochemically favorable at low temperatures. However, these studies consider only two pathways of this reaction: (1) the chain oxidation of RH by RO and (2) the tautomerization of R'HO to R'OH with following production of R'O and OH radicals. At the same time, the reactions of intramolecular rearrangement of RO, 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.

Methods: 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 (ΔH°(X)) of considered compounds (molecules, radicals, and transition states). These values of ΔH°(X) 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 ΔH°(X, CORR) are used according to Hess's law for the determination of ΔH°(X, CORR). The most consistent values of ΔH°(X, MEAN) are derived from the coordination of the values of ΔH°(X,CORR) using the intersection of their values of standard deviations (3SD). These values of ΔH°(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), S°(X), and G°(X) 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.