Evans-Polanyi-like models for rapidly predicting barriers of hemicellulose motifs under pyrolysis conditions based on the reactant reactivity parameters

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 leverage computationally cheap local and global electron-density-based chemical reactivity descriptors, such as Fukui’s functions (f), electron population of CO bonds (N), and the gross intrinsic strength bond index (Δgpair), 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 the relatively simple multilinear regression analysis, four EPL 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), namely, ΔHDFT = –168.82f ̅– 66.28N ̅ + 328.10Δ ̅gpair – 18.80, ΔHCBS = –189.01(f ) ̅– 65.11N ̅ + 266.44Δ ̅gpair + 13.96, ΔHG4 = –184.99f ̅ – 64.85N ̅ + 275.10Δ ̅gpair + 8.52, and ΔHDLPNO = –187.82f ̅ – 72.45N ̅ + 296.14Δ ̅gpair + 7.72, respectively. An adjusted coefficient of determination of 0.80 characterizes all these parametric polynomials. Moreover, MAE and RMSE equal to ≈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: ΔHCBS = 0.96ΔHDFT + 1.67, ΔHG4 = 0.96ΔHDFT + 1.72, ΔHDLPNO = 1.02ΔHDFT – 1.57, ΔHG4 = 0.96ΔHCBS + 2.86, ΔHDLPNO = 1.01ΔHCBS – 0.14, and ΔHDLPNO = 1.05ΔHG4 – 2.77. R2 ranges from 0.94 to 0.98 across these polynomials. 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, thereby contributing to the faster deployment of bioproducts at a commercial level.