From Electron Density to Kinetics: Circumventing Transition-State Calculations in Hemicellulose Pyrolysis

This work elucidates Evans-Polanyi-like (EPL) relations to rapidly estimate the standard activation enthalpy of three reaction classes central to hemicellulose pyrolysis: ring-opening, ring contraction, and elimination. EPLs leverage computationally 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 are used under the 20-80 % partition scheme for validating-deriving purposes. Four EPLs 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 these polynomials. MAE and RMSE equal to ≈3.3 and ≈4.1 kcal mol-1 describe the best-fitting models at DFT and G4. 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 R2 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.