Oxygen-18 Isotopic Studies of HOOO and DOOO

Owing to questions that still persist regarding the length of the O-H and central O-O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly bound complex between OH and O₂, new ¹⁸O isotopic measurements of HOOO and DOOO were undertaken using Fourier transform microwave and microwave-millimeter-wave double resonance techniques. Rotational lines from three new ¹⁸O species of DOOO (D¹⁸OOO, DO¹⁸O¹⁸O, and D¹⁸O¹⁸O¹⁸O) were detected, along with the two singly substituted ¹⁸O isotopic species of HOOO (HO¹⁸OO and HOO¹⁸O) that were not measured in the previous isotopic investigation. From a least-squares fit, spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its ¹⁸O species is uniformly negative: of order -0.04 amu Ų, regardless of the number or location of the ¹⁸O atoms, in contrast to that found for HOOO or its ¹⁸O isotopic species. A reanalysis of the molecular structure was performed using either normal HOOO and its four singly substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r₀) structures are generally quite small, of order ±0.01 Å for the bond lengths and ±1° for the bond angle. The length of the O-H bond remains unrealistically short compared to free OH, and the central O-O bond length is consistently very close to 1.68 Å. On the basis of the effective O-H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼22°.