Abstract: Modeling gas-phase chemical kinetics relevant to combustion and atmospheric chemistry requires a complete description of elementary reactions involving ephemeral species such as hydroperoxyalkyl radicals, Q̇OOH, which undergo competing sets of unimolecular reactions and bimolecular reactions with O2. The balance of flux from the competition affects rates of chain-branching and inherently depends on temperature, pressure, and oxygen concentration. Accordingly, the influence of [O2] on species formed via reactions of O2 with carbon-centered radicals (Ṙ), and the subsequent fate of Q̇OOH and related products, is central to developing accurate chemical kinetics mechanisms. However, reactions consuming intermediates from Ṙ + O2 are often simplified to such a degree that mechanism truncation error (uncertainty derived from incomplete reaction networks) becomes significant and precludes high-fidelity simulations of chemical systems for sustainable transportation energy.
Intermediates produced from Ṙ + O2 reactions of hydrocarbons and biofuels include cyclic ethers and alkene isomers, which are shown to undergo two unique types of reactions that are neglected in current gas-phase combustion models: (1) non-Boltzmann reactions, wherein rovibrationally excited radicals produced during H-abstraction undergo prompt ring-opening prior to collisional stabilization, and (2) stereochemical-dependent reaction pathways originating in closed-shell cyclic ethers that follow from the preceding ring-closing transition state [Q̇OOH]≠ and from subsequent cyclic ether peroxy radicals, both of which can facilitate new reaction channels including chain-branching pathways.
To ameliorate predictive deficiencies, results from a coupled experimental-computational workflow are outlined wherein sub-mechanisms, informed by speciation experiments, are developed and utilized as input into AutoMech, an open-source code for quantum chemical mechanism development. AutoMech is employed to calculate ab initio thermochemical and rate coefficeints for all species and reaction pathways in an initial mechanism. Elementary reactions are translated by AutoMech from 2D descriptions into stereochemically-enumerated representations. Potential energy surfaces are calculated using explicitly-correlated coupled-cluster energies with dispersion-corrected double-hybrid density functional theory geometries and frequencies. Master equation theory is used to calculate pressure- and temperature-dependent rate coefficients and partition functions for each reaction and species including for non-Boltzmann reactions. Results discussed include ongoing projects on species derived from cyclopentyl radicals and alkyl-substituted cyclic ethers produced from pentyl radical isomers.