The Gas Phase HO-Initiated Oxidation of Furan: A Theoretical Investigation on the Reaction Mechanism

The reaction mechanism of the gas-phase HO-initiated oxidation of furan has been investigated by means of high level theoretical methods. The reaction is a complex process that begins with the formation of a pre-reactive hydrogen bonded complex, previous to the addition of the HO radical to furan, forming the 2 and 3-HO-adducts. In the prereactive complex, the hydrogen bond is formed by interaction between the hydrogen of the hydroxyl radical and the system of furan and its stability is computed to be 1.6 kcalmol -1 (including the BSSE corrections). The 2 and 3-HO-adducts are computed to be 30.5 and 12.5 kcalmol -1 respectively, more stable than the reactants. The transition state leading to the formation of the 2-HO-adduct lies below the energy of the separate reactants (0.5 kcalmol -1 ) and the transition state producing the 3-HO-adduct that is computed to lie 3.4 kcalmol -1 above the sum of the energies of furan and hydroxyl radical. There are four reaction paths for the ring-opening of the 2-HO-adduct, leading to the formation of different conformers of 4-hydroxybutenaldehyde radical. The most stable of these conformers presents a cyclic symmetric (C 2V ) structure and can be characterized as a low-barrier hydrogen bond. The geometry optimizations and characterizations done in this work were carried out at MP2/6-311G(d,p), MP2/6-311+G(2df,2p) and QCISD/6-311G(d,p) levels of theory, whereas the relative energies are obtained at CCSD(T)/cc-pVTZ level of theory.