Theoretical kinetics of HO2 + buten-4-ol-3 radicals and improved predictions for 1,3-butadiene combustion

Abstract

This work investigates the kinetics and thermodynamics of HO₂ radical addition to the resonantly stabilized buten-4-ol-3 radical, a key reaction in low-temperature 1,3-butadiene oxidation. High-level theoretical methods, including ROCCSD(T)/CBS energy calculations and variable reaction coordinate transition state theory (VRC-TST), were employed to characterize the potential energy surfaces (PESs) and compute pressure- and temperature-dependent rate constants for the addition and subsequent decomposition pathways. The central addition of HO₂ to buten-4-ol-3 was identified as the most favorable pathway, leading primarily to the formation of 3-oxyl-1-buten-4-ol radical at higher temperatures. Thermochemical properties for key species are computed using composite methods and show good agreement with available literature data. The newly calculated kinetic and thermodynamic parameters were incorporated into the NUIGMech1.3 mechanism. Model validation against ignition delay time (IDT) data from a rapid compression machine (RCM) and species profiles from a jet-stirred reactor (JSR) shows that the updated mechanism improves predictions, particularly for IDTs between 770 and 900 K and for acrolein (C₂H₃CHO) formation in JSR experiments. Sensitivity and flux analyses reveal that the enhanced model performance stems from the competition between HO₂ and O₂ addition to buten-4-ol-3, which reduces system reactivity by decreasing the flux through a highly promoting O₂ addition pathway. The original mechanism's underestimation of peroxide decomposition rate constants by over five orders of magnitude artificially enhanced this promoting pathway, leading to overpredicted reactivity. This study provides crucial fundamental data, significantly improving the predictive capability of combustion models for 1,3-butadiene—a key soot precursor—and demonstrating the necessity of high-level theoretical kinetics for accurate combustion chemistry modeling.

Novelty and significance statement

This work presents the first high-level theoretical kinetics investigation of HO₂ addition to the buten-4-ol-3 radical, a critical sub-mechanism in 1,3-butadiene combustion. Using ROCCSD(T)/CBS methods and variable reaction coordinate transition state theory (VRC-TST), we provide accurate pressure-dependent rate constants for barrierless addition and decomposition pathways, moving beyond prior estimations. Central addition is identified as the dominant pathway, with its competition against O₂ addition representing a completely new mechanistic insight. The updated kinetics systematically correct model over-prediction of reactivity at 770–900 K and successfully reproduce experimental C₂H₃CHO profiles. Implemented in NUIGMech1.3, these parameters enhance predictive capability for soot formation studies and larger hydrocarbon mechanisms, establishing a refined hierarchical framework for developing cleaner combustion models.