The cyclic, two-hydrogen bond form of the HO2 dimer

Abstract

Among possible forms of the hydroperoxyl radical dimer H₂O₄, one of the most appealing and plausible is the six‐membered ring 4. This cyclic isomer of H₂O₄ has been investigated using nonempirical molecular electronic structure theory. For the lowest triplet state of H₂O₄, the single configuration self‐consistent field (SCF) method was used in conjunction with double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets. At both levels of theory, the six‐membered ring is predicted to be a minimum on the H₂O₄ potential energy hypersurface. The DZ SCF and DZ+P SCF dimerization energies are 4.5 and 4.9 kcal, respectively. Vibrational frequencies are predicted within the harmonic approximation and compared with the analogous monomer predictions and with the recent experimental findings of Diem, Tso, and Lee. It is concluded that the cyclic HO₂ dimer is composed of two weak hydrogen bonds, each about one‐half the strength of that observed for the water dimer.