Theoretical investigation of the role of intramolecular hydrogen bonding in beta-hydroxyethoxy and beta-hydroxyethylperoxy radicals in the tropospheric oxidation of ethene

In this work, we performed a quantum chemical B3LYP-DFT/6-31G"* characteriz ation of the geometries, vibrational frequencies, and relative energies of the various internal-rotation conformers of the title radicals and of the t ransition structures for the HOCH2CH2PO --> CH2OH + CH2O dissociation, ther eby obtaining the first evidence for intramolecular hydrogen bonding in the se molecules. For both the peroxy and oxy radicals, some of the equilibrium geometries were found to be stabilized by interactions between the hydroxy H and the (per)oxy O, lowering the energies by 1.5-2.5 kcal/mol, as confir med by single point CCSD(T) calculations; it is concluded that thermalized populations at ambient temperatures should consist predominantly of the H-b onded rotamers. Furthermore, the hydrogen bond was found to persist in the transition state fur dissociation of the H-bonded HOCH2CH2O rotamers, resul ting in an energy barrier calculated to be only 10 kcal/mol, in excellent a ccord with recent experimental results. Based on the DFT characterizations and using advanced statistical energy partitioning theories, a Master Equat ion analysis was performed predicting that at 298 K and 1 atm, 38% of the H OCH2CH2O radicals formed in the atmospheric HOCH2CH2OO + NO reaction dissoc iate "promptly" before collisional stabilization; also, the rate constant o f the thermal dissociation of HOCH2CH2O was theoretically evaluated at 2.1 x 10(5) s(-1) at 298 K and 1 atm. These values, which are crucially depende nt on the persistence of the H-bond during HOCH2CH2O dissociation, likewise compare favorably with recent experimental data. Isomerization of HOCH2CH2 OO to OCH2CH2OOH was found to be of only minor importance to the oxidation of ethene. Theoretical results are also presented on the thermal dissociati on of ethoxy and isopropoxy radicals.