Evans–Polanyi-like Formulations for Rapidly Predicting Key Depolymerization Barriers in Xylopyranoses: Toward the Faster Development of Kinetic Models for Hemicellulose Pyrolysis

This work elucidates Evans-Polanyi-like (EPL) relations to rapidly estimate the standard activation enthalpy of three ubiquitous reaction classes playing a central role in hemicellulose pyrolysis: ring-opening, ring contraction, and elimination. These models bypass computing the reaction enthalpy by leveraging computationally cheap local and global electron-density-based chemical reactivity descriptors, such as Fukui's functions (<i>f</i>), electron population of C-O bonds (<i>N</i>), and the gross intrinsic strength bond index (Δ<i>g</i>^pair), evaluated for reactants solely. More than 270 reactions observed in twenty-eight functionalized β-d-xylopyranoses, the hemicellulose building block, are used under the 20-80% partition scheme for validating-deriving purposes. By using multilinear regression analysis, four EPL equations are proposed for informing barriers at the M06-2X/6-311++G(d,p), CBS-QB3, G4, and DLPNO-CCSD(T)-F12/cc-pVTZ-F12//M06-2X/6-311++G(d,p) levels. An adjusted coefficient of determination of 0.80 characterizes these parametric polynomials. Moreover, MAE and RMSE of ≈3.3 and ≈4.1 kcal mol^-1 describe the performance of the best-fitting models at DFT and G4. Conversely, the highest values, MAE = 3.6 and RMSE = 4.7 kcal mol^-1, are associated with the CBS-QB3 level. The benchmarking of the computed activation enthalpies at 298 K yields simple functions for high-level estimations from low levels of theory with <i>R</i>² ranging from 0.94 to 0.98. Extrapolating the DPLNO barriers to the complete basis set limit tends to lower them by 0.63 kcal mol^-1. EPL expressions are tailored to facilitate the development of chemical kinetic models for hemicellulose pyrolysis, as the reactant structure is the only input required.