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 π system
of furan and its stability is computed to be 1.6 kcal·mol-1 (including the BSSE corrections). The 2 and 3-HO-adducts
are computed to be 30.5 and 12.5 kcal·mol-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 kcal·mol-1) and the transition state producing
the 3-HO-adduct that is computed to lie 3.4 kcal·mol-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 (C2V) 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.