Oxywater (water oxide): new evidence for the existence of a structural isomer of hydrogen peroxide

Ab initio molecular electronic structure theory has been applied in an investigation of the oxywater-hydrogen peroxide isomerization. Oxywater, hydrogen peroxide, and the transition state connecting them have been located using the self-consistent-field (SCF), configuration interaction including all single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD) methods with several basis sets, the largest being of triple-zeta plus double-polarization (including f functions on the oxygen atoms) quality (TZ2P+f). Harmonic vibrational frequencies have been evaluated at the SCF, CISD, and CCSD levels of theory and the stationary points characterized as minima or transition states. The CCSD method with connected triple excitations [CCSD(T)] also has been used to obtain oxywater`s equilibrium geometry and frequencies as well as to compute single-point energies of all CCSD optimized structures. A classical barrier to isomerization of 5.7 kcal mol{sup {minus}1} has been predicted at the highest level of theory. After correction for zero-point vibrational energies, the comparable ground-state activation energy is 3.2 kcal mol{sup {minus}1}. Although these ab initio predictions could be decreased by 1 or 2 kcal mol{sup {minus}1} at yet higher levels of theory, there can be little doubt that oxywater is a genuine minimum of the H{sub 2}O{sub 2} potential energy hypersurface.