Theoretical and kinetic study of H-abstraction from diisopropyl ether by key radicals: implications for combustion chemistry

Abstract

H-abstraction by reactive radicals OH, HO2, H, and CH3 governs diisopropyl ether (DIPE) oxidation kinetics, with preferential attack at α-carbon sites adjacent to the ether oxygen. Current kinetic models exhibit significant uncertainties due to scarcity of high-level experimental and theoretical data, necessitating rate estimation via structural analogs. To resolve these gaps, we employed high-accuracy multi-structural variational transition state theory with small-curvature tunneling correction (MS-VTST/SCT) coupled with M06-2X/cc-pVTZ//M08-HX/def2-tzvp (M08-HX/def2-tzvp is the combination with the smallest MUD value based on DLPNO-CCSD(T)/CBS(T-Q)) calculations. This approach systematically investigates H-abstraction across all carbon sites in DIPE + OH/HO2/H/CH3 systems. Activation energies of –0.62 to 22.69 kcal·mol⁻¹ reveal hydrogen-bonded complexes RCαOH and RCβ1HO2 stabilizing transition states in OH/HO2 pathways. Detailed analysis of temperature-dependent rate constants (200–1700 K) and branching ratios uncovers dominant torsional/anharmonic effects on microcanonical pathways: In DIPE + OH, R1a dominates below 550 K owing to hydrogen-bond-induced barrier reduction while R1b prevails at higher temperatures due to enthalpy advantage; R3a and R4a consistently control DIPE + H/CH3 consumption across combustion-relevant conditions. The total rate for DIPE + OH, ktotal=0.1015×T4.514exp(-3457.125/T) cm³·mol⁻¹·s⁻¹, not only agrees excellently with experimental data but also reveals non-Arrhenius behavior above 450 K. Implementation of these first-principles rates in an updated combustion model substantially improves predictions of CH3COCH3. C3H6 and C2H6 species profiles in jet-stirred reactor experiments at φ=1.0, 1 atm. Reaction pathway analysis further quantifies H-abstraction as the primary DIPE consumption route, contributing >75% fuel depletion below 900 K.