Theoretical and kinetic studies 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 RC_αOH and RC_β1HO₂ 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, k_total = 0.1015 × T^4.514 exp(−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.