The HOOO radical. A matrix isolation study

Spectroscopic characterization of the first excited state and photochemistry of the HO3 radical

We report the one-dimensional cuts of the six-dimensional potential energy surfaces (PESs) of the ground and lowest doublet and quartet electronic states of trans-HO₃ at the MRCI-F12/aug-cc-pVTZ level of theory. Theoretical calculations predict that the first excited state (A²A) presents a real minimum on its PES and possesses a nonplanar structure. The adiabatic excitation energy at the MRCI+Q and MRCI-F12 levels shows that the A²A state lies in the near-infrared region. Both the transition dipole moment and the oscillator strength were predicted to be weak, which suggests that photodissociation of HO₃ to produce OH and O₂ after UV-Vis absorption is not a plausible mechanism. The harmonic vibrational frequencies and rotational constants of the weakly bound complex OH-O₂ in the two electronic states were predicted to help in its detection. Our PES shows that the reactions of H + O₃ or HO₂ + O in their ground states do not lead to trans-HO₃ in its ground electronic state if one of the component fragments, i.e., HO₂(A²A') + O(³P) or H(²S) + O₃(³B₂), is excited.

Riddles of the structure and vibrational dynamics of HO3 resolved near the ab initio limit

The hydridotrioxygen (HO₃) radical has been investigated in many previous theoretical and experimental studies over several decades, originally because of its possible relevance to the tropospheric HO_x cycle but more recently because of its fascinating chemical bonding, geometric structure, and vibrational dynamics. We have executed new, comprehensive research on this vexing molecule via focal point analyses (FPA) to approach the ab initio limit of optimized geometric structures, relative energies, complete quartic force fields, and the entire reaction path for cis-trans isomerization. High-order coupled cluster theory was applied through the CCSDT(Q) and even CCSDTQ(P) levels, and CBS extrapolations were performed using cc-pVXZ (X = 2-6) basis sets. The cis isomer proves to be higher than trans by 0.52 kcal mol^-1, but this energetic ordering is achieved only after the CCSDT(Q) milestone is reached; the barrier for cis → trans isomerization is a minute 0.27 kcal mol^-1. The FPA central r_e(O-O) bond length of trans-HO₃ is astonishingly long (1.670 Å), consistent with the semiexperimental r_e distance we extracted from microwave rotational constants of 10 isotopologues using FPA vibration-rotation interaction constants (α_i). The D₀(HO-O₂) dissociation energy converges to a mere 2.80 ± 0.25 kcal mol^-1. Contrary to expectation for such a weakly bound system, vibrational perturbation theory performs remarkably well with the FPA anharmonic force fields, even for the torsional fundamental near 130 cm^-1. Exact numerical procedures are applied to the potential energy function for the torsional reaction path to obtain energy levels, tunneling rates, and radiative lifetimes. The cis → trans isomerization occurs via tunneling with an inherent half-life of 1.4 × 10^-11 s and 8.6 × 10^-10 s for HO₃ and DO₃, respectively, thus resolving the mystery of why the cis species has not been observed in previous experiments executed in dissipative environments that allow collisional cooling of the trans-HO₃ product. In contrast, the pure ground eigenstate of the cis species in a vacuum is predicted to have a spontaneous radiative lifetime of about 1 h and 5 days for HO₃ and DO₃, respectively.

Products and Mechanistic Investigations on the Reactions of Hydrazines with Ozone in Gas-Phase

The toxic transformation products of hydrazines are of great concern. These products’ properties combined with their formation mechanisms are needed to assess their potential environmental and human impacts. In this study, the gas-phase reaction of hydrazine (N2H4), monomethyldrazine (MMH) and unsymmetrical dimethyhydrazine (UDMH) with O3 have been studied at varying reactant ratios, both in the presence and absence of a radical trap. Gas chromatography-mass spectroscopy (GC-MS) has been implied to follow reactant consumption and product formation. Apart from the reported products detected by Fourier transform infrared spectroscopy (FT-IR), the newly found compounds (hydrazones, formamides, dimethylamine, 1,1,4,4-tetramethyl-1,2-tetrazene,dimethylamino-acetonitrile, N2, H2O, et al.) are identified by GC-MS. The relative yields of the organic products vary considerably at different O3/MMH or UDMH ratios. UDMH and MMH are confirmed as high potential precursors of N-nitrosodimethylamine (NDMA). The presence of hydroxyl radicals (HO·) hinders NDMA formation in MMH-O3 system. Meanwhile, it increases NDMA formation in UDMH-O3 system. The suggested reaction mechanisms which account for the observed products are discussed.

Weakly Bound Clusters in Astrochemistry? Millimeter and Submillimeter Spectroscopy of trans-HO3 and Comparison to Astronomical Observations

The emergence of chemical complexity during star and planet formation is largely guided by the chemistry of unstable molecules that are reaction intermediates in terrestrial chemistry. Our knowledge of these intermediates is limited by both the lack of laboratory studies and the difficulty in their astronomical detection. In this work, we focus on the weakly bound cluster HO3 as an example of the connection between laboratory spectroscopic study and astronomical observations. Here, we present a fast-sweep spectroscopic technique in the millimeter and submillimeter range to facilitate the laboratory search for trans-HO3 and DO3 transitions in a discharge supersonic jet and report their rotational spectra from 70 to 450 GHz. These new measurements enable full determination of the molecular constants of HO3 and DO3. We also present a preliminary search for trans-HO3 in 32 star-forming regions using this new spectroscopic information. HO3 is not detected, and column density upper limits are reported. This work provides additional benchmark information for computational studies of this intriguing radical, as well as a reliable set of molecular constants for extrapolation of the transition frequencies of HO3 for future astronomical observations.

Weakly bound molecules in the atmosphere: a case study of HOOO

Weakly bound molecules--particularly hydrated complexes of abundant atmospheric species--have long been postulated to play an important role in atmospherically relevant reactions. For example, such complexes could seed cloud formation and alter the global radiation budget. In this Account, we initially describe the current data on weakly bound species produced in association reactions of the hydroxyl radical (OH) with molecular partners, particularly oxygen (O(2)), nitric acid (HONO(2)), and nitrogen dioxide (NO(2)). Researchers have identified weakly bound association products of these reactions as the hydrogen trioxy (HOOO) radical, the doubly hydrogen-bonded OH-HONO(2) complex, and peroxynitrous acid (HOONO), respectively. In each case, previous kinetic studies of the reaction or OH vibrational relaxation processes have indicated unusual, non-Arrhenius behavior. Under the temperature-pressure conditions of the Earth's lower atmosphere, these processes exhibit a negative temperature dependence, indicative of an attractive interaction, or a pressure dependence. Researchers have subsequently carried out extensive theoretical studies of the properties of these weakly bound molecules, but the theoretical studies have lacked experimental validation. Next, we describe experimental studies to determine the vibrational frequencies and stability of HOOO as a prototypical example of these weakly bound molecules. We then use these data to assess its importance in the atmosphere. We discuss the efficient production of the HOOO radical from OH and O(2) under laboratory conditions and its subsequent detection using infrared action spectroscopy, a highly sensitive and selective double resonance technique. Using excitation of OH stretch and combination bands comprising OH stretch with lower frequency modes, we obtain detailed spectroscopic information on the vibrational modes of the two conformers of HOOO. In addition, we infer fundamental information about the dissociation dynamics from the OH product state distribution, which provides insight into the chemical bonding in HOOO. Perhaps most importantly, we utilize a simple conservation of energy relationship based on the highest energetically open OH product state to derive a rigorous upper limit for the stability of HOOO relative to the OH + O(2) asymptote of 5.3 kcal mol(-1). When combined with previous experimental rotational constants that reflect the structure of the HOOO radical, our laboratory characterization of its stability and vibrational frequencies provides critical information to assess its thermochemical properties. Using standard statistical mechanics approaches, we can calculate the likely atmospheric abundance of HOOO. We estimate that up to 25% of the OH radicals in the vicinity of the tropopause may be associated with O(2) as a weakly bound molecule.

The structures of ozone and HOx radicals in aqueous solution from combined quantum/classical molecular dynamics simulations

Ozone in aqueous solution decomposes through a complex mechanism that involves initial reaction with a hydroxide ion followed by formation of a variety of oxidizing species such as HO, HO(2), and HO(3) radicals. Though a number of hydrogen-bonded complexes have been described in the gas phase, both theoretically and experimentally, the structures of ozone and HO(x) in liquid water remain uncertain. In this work, combined quantum/classical computer simulations of aqueous solutions of these species have been reported. The results show that ozone undergoes noticeable electron polarization but it does not participate in hydrogen bonds with liquid water. The main contribution of the solvation energy comes from dispersion forces. In contrast, HO(x) radicals form strong hydrogen bonds. They are better proton donors but weaker proton acceptors than water. Their electronic and geometrical structures are significantly modified by the solvent, especially in the case of HO(3). In all cases, fluctuations in amplitudes of electronic properties are considerable, suggesting that solvent effects might play a crucial role on oxidation mechanisms initiated by ozone in liquid water. These mechanisms are important in a broad range of domains, such as atmospheric processes, plant response to ambient ozone, and medical and industrial applications.

Stability of the Hydrogen Trioxy Radical via Infrared Action Spectroscopy

The hydrogen trioxy radical (HO3) has been proposed as an intermediate in several important chemical reactions and relaxation processes involving OH in the atmosphere. In this work, the gas-phase infrared action spectrum of HO3 is obtained in the OH overtone region, along with the product state distribution of the OH fragment following dissociation. The highest observed OH product channel sets an upper limit for the HO-O2 binding energy of 6.12 kcal mol(-1). The experimental stability of HO3 and derived equilibrium constant imply that up to 66% of atmospheric OH may be converted into HO3 in the tropopause region.

Mechanistical studies on the formation of isotopomers of hydrogen peroxide (HOOH), hydrotrioxy (HOOO), and dihydrogentrioxide (HOOOH) in electron-irradiated H218O/O2 ice mixtures

In order to investigate the chemical reactions inside water-oxygen ice mixtures in extreme environments, and to confirm the proposed reaction mechanisms in pure water ice, we conducted a detailed infrared spectroscopy and mass spectrometry study on the electron irradiation of H(2)(18)O/O(2) ice mixtures. The formation of molecular hydrogen, isotopically substituted oxygen molecules (18)O(18)O and (16)O(18)O, ozone ((16)O(16)O(16)O, (16)O(16)O(18)O, and (16)O(18)O(16)O), hydrogen peroxide (H(18)O(18)OH, H(16)O(16)OH and H(16)O(18)OH), hydrotrioxy (HOOO), and dihydrogentrioxide (HOOOH) were detected. Kinetic models and reaction mechanisms are proposed to form these molecules in water and oxygen-rich solar system ices.

Ab initio study on the decomposition of first excited state HOOO radicals

The HOOO radical plays a crucial role in atmospheric processes involving the OH radical and O(2) molecule. We present an ab initio molecular orbital theory study on the decomposition reaction of the first excited state HOOO((2)A') with respect to OH and O(2). The geometries and harmonic vibrational frequencies of all stationary points are calculated at the CASSCF and MRCI levels of theory in conjunction with the 6-31+G(d,p) basis set. The potential energy profile of the decomposition reaction is studied at the CASSCF/6-31+G(d,p) level of theory, in which the complete valence orbitals and electrons are included in the active space. The energies of the potential energy profile are further refined at the CASPT2 and MRCI levels of the theory. Additionally, we have determined the interesting reaction process: the HOOO((2)A') radical with C(s) symmetry does not dissociate to OH((2)Pi) and O(2)((3)Sigma(-)(g)) directly as this is forbidden by orbital symmetry, but dissociates to OH((2)Pi) and O(2)((3)Sigma(-)(g)) via the change in symmetry from C(s) to C(infinity v) symmetry with a low barrier.

Infrared Detection of HO2 and HO3 Radicals in Water Ice

Infrared spectroscopy has been used to detect HO(2) and HO(3) radicals in H(2)O + O(2) ice mixtures irradiated with 0.8 MeV protons. In these experiments, HO(2) was formed by the addition of an H atom to O(2) and HO(3) was formed by a similar addition of H to O(3). The band positions observed for HO(2) and HO(3) in H(2)O-ice are 1142 and 1259 cm(-1), respectively, and these assignments were confirmed with (18)O(2). HO(2) and HO(3) were also observed in irradiated H(2)O + O(3) ice mixtures, as well as in irradiated H(2)O(2) ice. The astronomical relevance of these laboratory measurements is discussed.

Molecular complexes in close and far away

In this review, gas-phase chemistry of interstellar media and some planetary atmospheres is extended to include molecular complexes. Although the composition, density, and temperature of the environments discussed are very different, molecular complexes have recently been considered as potential contributors to chemistry. The complexes reviewed include strongly bound aggregates of molecules with ions, intermediate-strength hydrogen bonded complexes (primarily hydrates), and weakly bonded van der Waals molecules. In low-density, low-temperature environments characteristic of giant molecular clouds, molecular synthesis, known to involve gas-phase ion-molecule reactions and chemistry at the surface of dust and ice grains is extended here to involve molecular ionic clusters. At the high density and high temperatures found on planetary atmospheres, molecular complexes contribute to both atmospheric chemistry and climate. Using the observational, laboratory, and theoretical database, the role of molecular complexes in close and far away is discussed.

Structure of the HOOO Radical in Liquid Water: A Theoretical Study

The HOOO radical is supposed to play a role in ozone chemistry, both in the gas phase and aqueous media. We discuss the influence of the solvent on the electronic and geometrical structure of this radical using density functional and high-level ab initio calculations together with continuum, discrete, and discrete-continuum solvent models. Solute-solvent electrostatic interactions are shown to be fundamental, and lead to a noticeable stabilization of the radical, which should adopt a trans conformation in aqueous media. In fact, no energy minimum for the cis conformation is predicted in these conditions.

The Rotational Spectrum and Structure of the HOOO Radical

The adduct of the hydroxyl radical with oxygen has been studied theoretically, in connection with atmospheric reactions, but its stability and structure remained an open question. Pure rotational spectra of the HOOO and DOOO radicals have now been observed in a supersonic jet by using a Fourier-transform microwave spectrometer with a pulsed discharge nozzle. The molecular constants extracted from 12 rotational transitions with fine and hyperfine splittings support a trans planar molecular structure, in contrast to the cis planar structure predicted by most ab initio calculations. The bond linking the HO and O2 moieties is fairly long (1.688 angstroms) and comparable to the F-O bond in the isoelectronic FOO radical.

Peroxone chemistry: formation of H2O3 and ring-(HO2)(HO3) from O3/H2O2

The recent observation [Wentworth, P., Jones, L. H., Wentworth, A. D., Zhu, X. Y., Larsen, N. A., Wilson, I. A., Xu, X., Goddard, W. A., Janda, K. D., Eschenmoser, A. & Lerner, R. A. (2001) Science 293, 1806-1811] that antibodies form H(2)O(2) from (1)O(2) plus H(2)O was explained in terms of the formation of the H(2)O(3) species that in the antibody reacts with a second H(2)O(3) to form H(2)O(2). There have been few reports of the chemistry for forming H(2)O(3), but recently Engdahl and Nelander [Engdahl, A. & Nelander, B. (2002) Science 295, 482-483] reported that photolysis of the ozone-hydrogen peroxide complex in argon matrices leads to significant concentrations of H(2)O(3). We report here the chemical mechanism for this process, determined by using first-principles quantum mechanics. We show that in an argon matrix it is favorable (3.5 kcal/mol barrier) for H(2)O(2) and O(3) to form a [(HO(2))(HO(3))] hydrogen-bonded complex [head-to-tail seven-membered ring (7r)]. In this complex, the barrier for forming H(2)O(3) plus (3)O(2) is only 4.8 kcal/mol, which should be observable by means of thermal processes (not yet reported). Irradiation of the [(HO(2))(HO(3))-7r] complex should break the HO-OO bond of the HO(3) moiety, eliminating (3)O(2) and leading to [(HO(2))(HO)]. This [(HO(2))(HO)] confined in the matrix cage is expected to rearrange to also form H(2)O(3) (observed experimentally). We show that these two processes can be distinguished isotopically. These results (including the predicted vibrational frequencies) suggest strategies for synthesizing H(2)O(3) and characterizing its chemistry. We suggest that the [(HO(2))(HO(3))-7r] complex and H(2)O(3) are involved in biological, atmospheric, and environmental oxidative processes.

Peculiar structure of the HOOO(-) anion

The HOOO(-) anion (1) can adopt a triplet state (T-1) or a singlet state (S-1), where the former is 9.8 kcal/mol (DeltaH(298) = 10.3 kcal/mol) more stable than the latter. S-1 possesses a strong O-OOH bond with some double bond character and a weakly covalent OO-OH bond (1.80 A) according to CCSD(T)/6-311++G(3df,3pd) calculations (the longest O-O bond ever found for a peroxide). In aqueous solution, S-1 adopts a geometry closely related to that of HOOOH (OO(O), 1.388 A; (O)OO(H), 1.509 A; tau(OOOH), 78.3 degrees ), justifying that S-1 is considered the anion of HOOOH. Dissociation into HO anion and O(2)((1)Delta(g)) requires 15.4 (DeltaH(298) = 14.3; DeltaG(298) = 8.9) kcal/mol. Structure T-1 corresponds to a van der Waals complex between HO anion and O(2)((3)Sigma(g)(-)) having a binding energy of 2.7 (DeltaH(298) = 2.1) kcal/mol. Modes of generating S-1 in aqueous solution are discussed, and it is shown that S-1 represents an important intermediate in ozonation reactions.

The vibrational spectrum of H2O3

This report presents positive infrared spectroscopic identification of H2O3, a higher oxide of hydrogen of importance for the understanding of the chain formation ability of atomic oxygen and a possible intermediate in hydrogen oxygen radical chemistry. All fundamental vibrations of H2O3, isolated in an argon matrix, have been observed. In addition, several bands of HDO3, D2O3, and H2(16)O2(18)O have been measured. One particular mode, the antisymetric O-O stretch at 776 cm-1, should be observable even in the presence of high water concentrations.