Molecular structure, vibrational frequencies, energetics, and excited states of the HOONO+ ions

Abstract

Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for neutral HOONO and its cation employing quadratic configuration interaction methods and correlation-consistent basis sets. The lowest energy structure of HOONO+ found is a planar conformer on the A'2 potential-energy surface with a cis–trans arrangement [τ(OONO)=0° and τ(HOON)=180°]. This is quite the opposite from neutral HOONO, which prefers a cis–cis arrangement because of hydrogen bonding. The adiabatic ionization potential for HOONO is predicted to be 260.0±1 kcal mol−1 (11.3 eV) at the complete basis set limit. A planar ion–molecule complex formed from HO2 and NO+ was located on the A"2 surface. The binding energy of this complex relative to HO2 and NO+ is estimated to be 18.1±1 kcal mol−1 at the complete basis set limit. Excited states of the HOONO+ ion were also calculated with quasi-restricted Hartree–Fock and unrestricted Hartree–Fock coupled-cluster methods including connected triple excitations.