Dipole moment of the HOOO radical: resolution of a structural enigma

Abstract

A global six-dimensional potential surface for the hydridotrioxygen radical (HOOO) is needed for an accurate assessment of its atmospheric abundance. We report inertial dipole moment components obtained from Stark spectra of the trans-HOOO system solvated in superfluid helium, and these are shown to be stringent benchmarks for theoretical computations of the potential surface. Computed dipole moment components at the CCSD(T)/CBS equilibrium geometry disagree qualitatively with the experimental values. The role of large-amplitude motion and vibrational averaging is assessed by computing the ground-state wave function on a relaxed, two-dimensional potential surface for the HO1O2O3 torsional and O1O2 bond-stretching coordinates. The experimental and computed vibrationally averaged dipole moments agree only after shifting the potential along the O1O2 bond coordinate, indicating that single-reference CCSD(T)/CBS computations underestimate re(O1O2) by ∼0.08 Å. An optimized trans-HOOO geometry at the composite all-electron CCSDT(Q)/CBS level reveals that the inclusion of full triples and a perturbative treatment of quadruple excitations leads to an increase in re(O1O2) by 0.07 Å.