On the molecular structure of HOOO

The structure of transient oxygen-containing radicals remains poorly understood despite their relevance in combustion and interstellar chemistry. Prior research has shown that hydridotrioxygen (HOOO) exists in different isomeric forms, but its exact geometry has remained elusive. This gap motivated researchers to investigate HOOO using advanced spectroscopic methods. Earlier studies suggested that HOOO might form via reactions involving OH and O₂, but the precise structural details were unclear. No prior work had resolved the vibrational effects on HOOO's geometry. The need for a detailed molecular structure of HOOO arises from its potential role in atmospheric and astrophysical processes. This paper contributes by combining experimental and computational approaches to determine HOOO's structure. The study also explores the production mechanisms and spectral properties of HOOO and its isotopologues.

The aim of this research is to determine the molecular structure of trans, planar HOOO and its isotopic variants. The specific problem addressed is the lack of experimental data on HOOO's bond lengths and angles. The motivation stems from the molecule's potential in atmospheric and interstellar chemistry. Researchers sought to confirm the dominant formation pathway of HOOO in electrical discharges. They also aimed to detect new isotopic species to support the proposed reaction mechanism. The study's purpose is to combine spectroscopic data with quantum-chemical calculations to derive structural parameters. Another goal is to assess the role of vibrational effects in HOOO's geometry. The work also aims to identify other oxygen-bearing species in the discharge environment.

The researchers used isotopic spectroscopy with Fourier transform microwave and microwave-millimeter-wave double resonance techniques. They employed high-level coupled cluster quantum-chemical calculations to model HOOO's structure. Electrical discharges of H₂O and O₂ in an inert buffer gas were used to generate HOOO. The team also tested H₂ and O₂ as precursor gases to increase HOOO abundance. They introduced isotopically labeled O₂ to detect new isotopic species of HOOO. Rotational constants were derived from experimental spectra and compared with theoretical predictions. The team calculated vibrational corrections using coupled-cluster methods. A wide spectral survey was conducted between 6 and 25 GHz to identify other oxygen-bearing species.

The study detected three new isotopic species of HOOO: H¹⁸OOO, HO¹⁸O¹⁸O, and H¹⁸O¹⁸O¹⁸O. These findings support the reaction OH + O₂ → HOOO as the dominant formation pathway. The fully experimental structure of HOOO revealed a longer central O–O bond (1.684 Å) compared to theoretical predictions. The O–H bond distance was shorter (0.913 Å) than expected from equilibrium calculations. The HOO angle was found to be 92.4°, more acute than theoretical models suggest. Vibrational corrections from coupled-cluster calculations were applied to the experimental data. An empirical equilibrium structure (rₑ(emp)) was derived, showing partial agreement with theoretical models. The spectral survey identified only about 50% of observed lines as known oxygen-bearing species.

The authors propose that the observed structural discrepancies in HOOO arise from vibrational effects not fully captured by equilibrium calculations. They suggest that higher-order vibrational corrections are needed to model large-amplitude motions in HOOO. The detection of new isotopic species supports the OH + O₂ → HOOO formation mechanism. The empirical equilibrium structure provides partial agreement with theoretical predictions but leaves residual inertial defects unexplained. The spectral survey indicates a rich, unexplored oxygen chemistry in the discharge environment. The absence of cis HOOO or vibrationally excited trans HOOO suggests limitations in current detection methods. The study highlights the need for further vibrational analysis to refine HOOO's structure. The findings contribute to understanding the molecular behavior of oxygen-chain radicals in atmospheric and astrophysical contexts.