On the molecular structure of HOOO

Abstract

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H(2)O and O(2) heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H(2) and O(2) as precursor gases. Using equal mixtures of normal and (18)O(2), it has been possible to detect three new isotopic species, H(18)OOO, HO(18)O(18)O, and H(18)O(18)O(18)O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O(2) → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r(0)) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 Å), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 Å), and a more acute [angle]HOO angle (92.4°) when compared to equilibrium (r(e)) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (r(e) (emp)) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.

Gas‐Phase Structure Determination of Dihydroxycarbene, One of the Smallest Stable Singlet Carbenes

“…[24,25] More recently, Schreiner et al reported the generation of trans,trans-and trans,cis-HO À C À OH by the thermal extrusion of CO 2 from oxalic acid and its subsequent isolation in an argon matrix at 10 K. [23] A similar experiment showed that hydroxycarbene (HÀC ÀOH) rapidly isomerizes to formaldehyde (CH 2 O) at 10 K, a phenomenon attributed to quantum tunneling through the 125 kJ mol À1 ÀOH, with the CCSD(T)/cc-pVTZ relative energies calculated by Schreiner et al [23] barrier. The 5-43 GHz Balle-Flygare-type FTMW spectrometer used in these experiments has been described elsewhere, [39][40][41][42] so only a brief description is provided here; a more extensive discussion can be found in the Supporting Information. Subsequent work with several substituted oxycarbenes indicates that the propensity for tunneling is greatly influenced by the identity of the substituent.…”

“…[6,26] In contrast, HOÀC ÀOH displays no tunneling behavior and is stable under the low-temperature matrix conditions. [42][43][44][45] Sample FTMW and DR spectra are shown in Figure 2. [7,27,37,38] We present here the first direct gas-phase evidence for trans,cis-HOÀC ÀOH, and a precise determination of its rotational spectrum and geometric structure, using a combination of Fourier transform microwave spectroscopy (FTMW), microwave-millimeter-wave double resonance (DR) spectroscopy, and high-level theoretical calculations.…”

Peroxide intermediates of oxidation processes: Organic trioxides

“…The revealed multiconfiguration nature of the wave function for HOOO • and its unusual structure suggest that both the static and the dynamic electronic correlations should be considered in order to obtain an adequate result. In a recent paper, the molecular structure of HOOO • was studied again using isotopic FTMW spectroscopy and double-resonance technique 66 . The hydrotrioxy radical was generated by an electric discharge in an H 2 O/O 2 or H 2 /O 2 mixture in an inert gas mixture.…”

Theoretical characterization of hydrogen pentoxide, H2O5

“…The most extreme case is the HOOO radical whose enthalpy of formation was not established until 2010 . The infrared spectra of cis and trans HOOO was extremely difficult to observe, and the determination of equilibrium structure is even more challenging . However, the importance of HOOO in solution and atmospheric chemistry elicited important experimental and theoretical advances that improved our understanding of the chemistry of HOOO.…”