Cavity Ringdown Spectroscopy of cis-cis HOONO and the HOONO/HONO2 Branching Ratio in the Reaction OH + NO2 + M

UV-spectrum and photodecomposition of peroxynitrous acid in the troposphere

The UV spectrum of peroxynitrous acid, HOONO, was computed at the B3LYP/AVTZ and MCSCF/AVTZ levels using the fewest switches surface hopping algorithm. Due to large-amplitude vibrational motions of this molecule, the maxima in the simulated spectra are displaced from the positions of vertical excitations. The three lowest excited electronic singlet states, which are all repulsive, can be reached by UV absorption. The photolysis products are determined, and the photolysis rate constant is provided for the first time. We found that near the tropopause the photolysis rate constant J ≈ 6 × 10-4 s-1, exceeds that for thermal decomposition by two orders of magnitude. The photolysis lifetime is about 30 minutes. Thus, photolysis is an important process and should be included in atmospheric models.

Yields of HONO2 and HOONO Products from the Reaction of HO2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy

The reaction of HO₂ with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO₂) directly from HO₂ + NO. Atmospheric models previously treated HO₂ + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO₂ and ozone (O₃), among others. Here, the yield of HONO₂, φ_HONO2, from the reaction of HO₂ + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO₂, produced by pulsed laser photolysis of Cl₂ in the presence of methanol, reacted with NO in a buffer gas mixture of N₂ and CO between 300 and 700 Torr at 278 and 300 K. HONO₂ and its weakly bound isomer HOONO were directly detected by their v₁ absorption bands in the mid-IR region. CO was used to suppress HONO₂ produced from OH + NO₂ and exploit a chemical amplification scheme, converting OH back to HO₂. Under the experimental conditions described here, no evidence for the formation of either HONO₂ or HOONO was observed from HO₂ + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO₂ detected in the system could be accounted for by OH + NO₂. At 700 ± 14 Torr and 300 ± 3 K, φ_HONO2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO₂ was attributed to the HO₂ + NO reaction, φ_HONO2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ_HONO2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.

An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO2: The Limiting High-Pressure Rate Coefficients as a Function of Temperature

The kinetics of the reaction OH/OD(v = 1,2,3) + SO₂ were studied using a photolysis/laser-induced fluorescence technique. The rate coefficients OH/OD(v = 1,2,3) + SO₂, k₁, over the temperature range of 295-810 K were used to determine the limiting high-pressure limit k₁^∞. This method is usually applicable if the reaction samples the potential well of the adduct HOSO₂ and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients showed an additional fast removal contribution as evidenced by the increase in k₁ with vibrational level; this behavior together with its temperature dependence is consistent with the existence of a weakly bound complex on the potential energy surface prior to adduct formation. The data were analyzed using a composite mechanism that incoporates energy-transfer mechanisms via both the adduct and the complex, and yielded a value of k₁^∞(295 K) equal to (7.2 ± 3.3) × 10^-13 cm³ molecule^-1 s^-1 (errors at 1σ), a factor of between 2 and 3 smaller than the current recommended IUPAC and JPL values of (2.0_-1.0^+2.0) and (1.6 ± 0.4) × 10^-12 cm³ molecule^-1 s^-1 at 298 K, respectively, although the error bars do overlap. k₁^∞ was observed to only depend weakly on temperature. Further evidence for a smaller k₁^∞ is presented in the companion paper.

Detection and structure of HOON: microwave spectroscopy reveals an O-O bond exceeding 1.9 Å

Nitric oxide (NO) reacts with hydroxyl radicals (OH) in the gas phase to produce nitrous acid, HONO, but essentially nothing is known about the isomeric nitrosyl-O-hydroxide (HOON), owing to its perceived instability. We report the detection of gas-phase HOON in a supersonic molecular beam by Fourier transform microwave spectroscopy and a precise determination of its molecular structure by further spectroscopic analysis of its (2)H, (15)N, and (18)O isotopologs. HOON contains the longest O-O bond in any known molecule (1.9149 ± 0.0005 Å) and appears surprisingly stable, with an abundance roughly 3% that of HONO in our experiments.

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.

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.

Photochemical processes induced by vibrational overtone excitations: dynamics simulations for cis-HONO, trans-HONO, HNO3, and HNO3-H2O

Photochemical processes in HNO3, HNO3-H2O, and cis- and trans-HONO following overtone excitation of the OH stretching mode are studied by classical trajectory simulations. Initial conditions for the trajectories are sampled according to the initially prepared vibrational wave function. Semiempirical potential energy surfaces are used in "on-the-fly" simulations. Several tests indicate at least semiquantitative validity of the potential surfaces employed. A number of interesting new processes and intermediate species are found. The main results include the following: (1) In excitation of HNO3 to the fifth and sixth OH-stretch overtone, hopping of the H atom between the oxygen atoms is found to take place in nearly all trajectories, and can persist for many picoseconds. H-atom hopping events have a higher yield and a faster time scale than the photodissociation of HNO3 into OH and NO2. (2) A fraction of the trajectories for HNO3 show isomerization into HOONO, which in a few cases dissociates into HOO and NO. (3) For high overtone excitation of HONO, isomerization into the weakly bound species HOON is seen in all trajectories, in part of the events as an intermediate step on the way to dissociation into OH + NO. This process has not been reported previously. Well-established processes for HONO, including cis-trans isomerization and H hopping are also observed. (4) Only low overtone levels of HNO3-H2O have sufficiently long liftimes to be spectrocopically relevant. Excitation of these OH stretching overtones is found to result in the dissociation of the cluster H hopping, or dissociation of HNO3 does not take place. The results demonstrate the richness of processes induced by overtone excitation of HNO(x) species, with evidence for new phenomena. Possible relevance of the results to atmospheric processes is discussed.

Vibrational overtone spectrum of matrix isolated cis, cis-HOONO

Cis, cis-peroxynitrous acid is known to be an intermediate in atmospheric reactions between OH and NO2 as well as HOO and NO. The infrared absorption spectra of matrix-isolated cc-HOONO and cc-DOONO in argon have been observed in the range of 500-8000 cm-1. Besides the seven fundamental vibrational modes that have been assigned earlier for this molecule [Zhang et al., J. Chem. Phys. 124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for cc-HOONO and cc-DOONO. Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cis-HOONO that go as high as 8000 cm-1 and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to +/-35 cm-1. The success of the theoretically predicted anharmonic frequencies {upsilon} in assigning overtone spectra of HOONO up to 8000 cm-1 suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.

Quantum study on the branching ratio of the reaction NO2+OH

A reduced dimensionality (RD) approximation is developed for the title reaction which treats the angle of approach of the hydroxyl radical to the nitrogen dioxide molecule and the radial distance between the two species explicitly. All other degrees of freedom are treated adiabatically. Electronic structure calculations at the complete active space self-consistent field level are used to fit a potential energy surface (PES) in these two coordinates. Within this RD model the adiabatic capture centrifugal sudden approximation is used to calculate the high pressure limit rate constant. A correction for reflection from the PES due to rotationally nonadiabatic transitions is applied using the wave packet capture approximation. The branching ratio for the title reaction is calculated for the atmospherically significant temperature range of 200-400 K at 20 Torr without distinguishing between the conformers of HOONO. The result is k(HOONO)k(HNO(3) )=0.051 at 20 Torr and 300 K, which is in good agreement with the measured branching ratio between cis-cis-HOONO and nitric acid. This suggests that most of the different conformers of HOONO were converted to the most stable cis-cis conformer on the time scale of the measurements made.

Constraining the Mechanism and Kinetics of OH + NO2 and HO2 + NO Using the Multiple-Well Master Equation

Several recent experimental studies have provided substantial new constraints for the mechanisms on the HNO3 potential energy surface. These include observations of biexponential OH decay over short time scales from OH + NO2, which constrain key properties of the short-lived HOONO intermediate, observations of both conformers of the HOONO intermediate itself, isotopic scrambling data for 18OH + NO2, and observations of HONO2 production from the HO2 + NO reaction. We combine all of these recent data in a master-equation simulation of the system. This simulation is initialized with computational values for both stable species (wells) and transition states, but parameters are then adjusted to fit the observations. All parameters are kept within limits defined by experimental and theoretical uncertainty, and all converge away from their bounds. The primary fitting is carried out on the OH kinetic data-we first fit the biexponential kinetics, then address the isotopic scrambling. Isotopic scrambling is shown to be rapid but not complete at low pressure, while at least two parameter sets are shown to be consistent with the biexponential data. Of these two parameter sets, one is far more consistent with recent observations of trans-HOONO decay, isotopic scrambling, and HONO2 production from HO2 + NO. This we regard as the most probable potential energy surface for the reaction. On this PES, cis-trans isomerization for HOONO is slow but isomerization of trans-HOONO to HONO2 is rapid. This has significant implications for observed HOONO behavior and also HONO2 formation in the atmosphere from both HO2 + NO and OH + NO2.

Spectroscopic Characterization of Peroxynitrous Acid in cis-perp Configurations

This paper presents experimental evidence, supported by two-dimensional theoretical calculations, that HOONO can be observed in cis-perp (cp) configurations in a pulsed supersonic expansion. The spectral properties (transition frequency, rotational constants, and transition type) of OH overtone transitions originating from a state with predominately cp character are predicted theoretically and compared with those associated with a weak feature at 6996.2 cm(-1) observed experimentally using infrared action spectroscopy. This spectral feature is attributed to HOONO in cp configurations based on its vibrational frequency, rotational band contour, and resultant OH product state distribution.

Reaction of OH + NO2: High Pressure Experiments and Falloff Analysis

High pressure experiments on the OH + NO2 reaction are presented for 3 different temperatures. At 300 K, experiments in He (p = 2-500 bar) as well as in Ar (p = 2-4 bar) were performed. The rate constants obtained in Ar agree well with values which have been reported earlier by our group (Forster, R.; Frost, M.; Fulle, D.; Hamann, H. F.; Hippler, H.; Schlepegrell, A.; Troe, J. J. Chem. Phys. 1995, 103, 2949. Fulle, D.; Hamann, H. F.; Hippler, H.; Troe, J. J. Chem. Phys. 1998, 108, 5391). In contrast, the rate coefficients determined in He were found to be 15-25% lower than the values given in our earlier publications. Additionally, results for He as bath gas at elevated temperatures (T = 400 K, p = 3-150 bar; T = 600 K, p = 3-150 bar) are reported. The results obtained at elevated pressures are found to be in good agreement with existing literature data. The observed falloff behavior is analyzed in terms of the Troe formalism taking into account two reaction channels: one yielding HNO3 and one yielding HOONO. It is found that the extracted parameters are in agreement with rate constants for vibrational relaxation and isotopic scrambling as well as with experimentally determined branching ratios. Based on our analysis we determine falloff parameters to calculate the rate constant for atmospheric conditions.

Rotational spectroscopy and dipole moment of cis-cis HOONO and DOONO

The rotational spectrum of cis-cis HOONO has been studied over a broad range of frequencies, 13-840 GHz, using pulsed beam Fourier-transform microwave spectroscopy and room-temperature flow cell submillimeter spectroscopy. The rotational spectrum of the deuterated isotopomer, cis-cis DOONO, has been studied over a subset of this range, 84-640 GHz. Improved spectroscopic constants have been determined for HOONO, and the DOONO spectrum is analyzed for the first time. Weak-field Stark effect measurements in the region of 84-110 GHz have been employed to determine the molecular dipole moments of cis-cis HOONO [mu(a) = 0.542(8) D, mu(b) = 0.918(15) D, mu = 1.07(2) D] and DOONO [mu(a) = 0.517(9) D, mu(b) = 0.930(15) D, mu = 1.06(2) D]. The quadrupole coupling tensor in the principal inertial axis system for the 14N nucleus has been determined to be chi(aa) = 1.4907(25) MHz, chi(bb) = -4.5990(59) MHz, chi(ab) = 3.17(147) MHz, and chi(cc) = 3.1082(59) MHz. Coordinates of the H atom in the center-of-mass frame have been determined with use of the Kraitchman equations, /aH/ = 0.516 A and /bH/ = 1.171 A. The inertial defects of HOONO and DOONO are consistent with a planar equilibrium structure with significant out-of-plane H atom torsional motion. Comparisons of the present results are made to ab initio calculations.

Infrared absorption spectra of matrix-isolated cis, cis-HOONO and its ab initio CCSD(T) anharmonic vibrational bands

The infrared absorption spectra of matrix-isolated cis, cis-peroxynitrous acid (HOONO and DOONO) in argon have been observed. Six of the nine fundamental vibrational modes for cis, cis-HOONO have been assigned definitively, and one tentatively. Coupled-cluster, ab initio anharmonic force field calculations were used to help guide some of the assignments. The experimental matrix frequencies (cm(-1)) for cis, cis-HOONO are (a' modes) nu1 = 3303+/-1, nu2 = 1600.6+/-0.6, nu3 = 1392+/-1, nu4 = 922.8+/-0.5, nu5 = 789.7+/-0.4, nu6 = 617+/-1; and (a" mode) nu8 = 462+/-1. The fundamentals for the deuterated isotopomer, cis, cis-DOONO, are (a' modes) nu1 = 2447.2+/-0.6, nu2 = 1595.7+/-0.7, nu3 = 1089.1+/-0.4, nu4 = 888.1+/-0.4, nu5 = 786.6+/-0.5, nu6 = 613.9+/-0.9; and (a" mode) nu8 = 456.5+/-0.5.

The OH-stretching and OOH-bending overtone spectrum of HOONO

We have simulated the HOONO vibrational overtone spectrum with use of a local mode Hamiltonian that includes the OH-stretching, OOH-bending, and NOOH-torsional modes and coupling between all three modes. The local mode parameters and the dipole moment function are calculated with coupled-cluster ab initio theory and an augmented Dunning-type triple-zeta basis set. We investigate the accuracy of the local mode parameters obtained from two different potential-energy fitting routines, as well as the sensitivity of these parameters to the basis set employed. We compare our simulated spectra to previously published action spectra in the first and second OH-stretching overtone regions. In addition we have recorded the spectrum in the OH-stretch and OOH-bend combination region around 7700 cm-1 and we also compare to this. Our simulated spectrum is in qualitative agreement with experiment in the first and second OH-stretching overtone and in the stretch-bend regions.

Formation of Nitric Acid in the Gas-Phase HO2 + NO Reaction: Effects of Temperature and Water Vapor

A high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer was used to investigate the minor channel (1b) producing nitric acid, HNO3, in the HO2 + NO reaction for which only one channel (1a) is known so far: HO2 + NO --> OH + NO2 (1a), HO2 + NO --> HNO3 (1b). The reaction has been investigated in the temperature range 223-298 K at a pressure of 200 Torr of N2 carrier gas. The influence of water vapor has been studied at 298 K. The branching ratio, k1b/k1a, was found to increase from (0.18(+0.04/-0.06))% at 298 K to (0.87(+0.05/-0.08))% at 223 K, corresponding to k1b = (1.6 +/- 0.5) x 10(-14) and (10.4 +/- 1.7) x 10(-14) cm3 molecule(-1) s(-1), respectively at 298 and 223 K. The data could be fitted by the Arrhenius expression k1b = 6.4 x 10(-17) exp((1644 +/- 76)/T) cm3 molecule(-1) s(-1) at T = 223-298 K. The yield of HNO3 was found to increase in the presence of water vapor (by 90% at about 3 Torr of H2O). Implications of the obtained results for atmospheric radicals chemistry and chemical amplifiers used to measure peroxy radicals are discussed. The results show in particular that reaction 1b can be a significant loss process for the HO(x) (OH, HO2) radicals in the upper troposphere.

Internal rotation in peroxynitrous acid (ONOOH)

Using higher levels of wave-function-based electronic structure theory than previously applied, as well as density functional theory (B-LYP and B3-LYP functionals), all theoretical models conclude that three ONOOH conformers are stationary point minima, in disagreement with some of the previous studies that we survey. In order of increasing energy, these are the cis-cis, cis-perp, and trans-perp conformers. Basis sets including diffuse functions seem to be needed to obtain a qualitatively correct representation of the internal rotation potential energy surface at higher levels of theory. Internal rotation about the peroxide bond involving the cis-cis, cis-gauche transition structure (TS), cis-perp, and cis-trans TS conformers is studied in detail. To help ascertain the relative stability of the cis-perp conformer, multireference configuration interaction energy calculations are carried out, and rule of thumb estimates of multireference character in the ground-state wave functions of the ONOOH conformers are considered. CCSD(T)/aug-cc-pVTZ physical properties (geometries, rotational constants, electric dipole moments, harmonic vibrational frequencies, and infrared intensities) are compared with the analogous experimental data wherever possible, and also with density functional theory. Where such experimental data are nonexistent, the CCSD(T) and B3-LYP results are useful representations. For example, the electric dipole moment |mu(e)| of the cis-cis conformer is predicted to be 0.97+/-0.03 D. CCSD(T) energies, extrapolated to the aug-cc-pVNZ limit, are employed in isodesmic reaction schemes to derive zero Kelvin heats of formation and bond dissociation energies of the ONOOH stationary point minima. In agreement with recent gas-phase experiments, the peroxide bond dissociation energies of the cis-cis and trans-perp conformers are calculated as 19.3+/-0.4 and 16.0+/-0.4 kcalmol, respectively. The lowest energy cis-cis conformer is less stable than nitric acid by 28.1+/-0.4 kcalmol at 0 K.

Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

A joint theoretical and experimental investigation is undertaken to study the effects of OH-stretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of cis-cis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ). The two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/cc-pVTZ and QCISD/AUG-cc-pVTZ (QCISD-quadratic configuration interaction with single and double excitation and AUG-augmented with diffuse functions) level of theory/basis, respectively. The OH-stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at approximately 90 degrees , the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nu(OH) region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO(2) and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D(0)=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Deltan=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm(-1) is assigned to a series of Deltan=+1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300-6850 cm(-1)) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm(-1) and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.

State-resolved unimolecular dissociation ofcis-cisHOONO: Product state distributions and action spectrum in the 2νOH band region

Nascent OH fragment product state distributions arising from unimolecular dissociation of room temperature HOONO, initiated by excitation in the region of the 2nu(OH) band, are probed using laser-induced fluorescence at sub-Doppler resolution. Phase-space simulations of the measured OH rotational distributions are consistent with the dissociation dynamics being statistical and confirm that all major features in the room temperature action spectrum belong to the cis-cis conformer. The phase-space simulations also allow us to estimate the HO-ONO bond dissociation energy of cis-cis HOONO to be D(0)=19.9+/-0.5 kcal/mol, which when combined with the known heat-of-formation data for the OH and NO(2) fragments gives DeltaH(f) (0)(cis-cis HOONO)=-2.5 kcal/mol. In addition to fragment energy release, spectral features in the cis-cis HOONO action spectrum are examined with respect to their shifts upon (15)N isotope substitution and through ab initio spectral simulation using a two-dimensional dipole surface that takes into account the influence of HOON torsional motion on the OH stretching overtone. The two-dimensional spectral simulations, using CCSD(T)/cc-pVTZ dipole surface, qualitatively reproduces features appearing in the action spectrum and suggest that the strong broad feature occurring approximately 570 cm(-1) to the blue of the cis-cis HOONO 2nu(OH) peak, likely involve excitation of HOON-torsion/OH-stretch combination bands originating from thermally populated excited torsional states. A closer examination of the predictions of the two-dimensional model with experiments also reveals its limitations and suggests that a more elaborate treatment, one which includes several additional modes, will likely be required in order to fully explain the room temperature action spectrum. Ab initio calculations of the HOON torsional potential at the CCSD(T)/cc-pVTZ level of theory are also presented and confirm that cis-perp configuration does not correspond to a bound localized minimum on the HOONO potential energy surface.

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.

Kinetic and mechanistic studies of the recombination of OH with NO2: Vibrational deactivation, isotopic scrambling and product isomer branching ratios

The kinetics and mechanism of the three-body recombination of OH with NO2 were studied using a pulsed laser photolysis pulsed laser induced fluorescence technique. The rate coefficients for deactivation of vibrationally excited OH (v = 1-5) by NO2 were found to be independent of vibrational level with a value of (6.4 +/- 0.3) x 10(-11) cm3 molecule s (-1) at 298 K. The rate coefficient for reaction of 18OH with NO2 was measured and found to be much faster than for unlabeled OH with a "zero pressure" rate of 1 x 10(-11) cm3 molecule(-1) s(-1) at 298 K and 273 K. Observation of temporal profiles of 16OH and 18OH suggest that isotopic scrambling in the initially formed [H18ON16O2] complex is complete on the microsecond time scale of our experiments. The rate coefficient for reaction of unlabeled OH with NO2 was measured at 413 K in 400 Torr of He. Biexponential temporal profiles were obtained and are consistent with a 10 +/- 3% yield of the weakly bound HOONO isomer.

Cis-cis and trans-perp HOONO: Action spectroscopy and isomerization kinetics

The weakly bound HOONO product of the OH+NO2+M reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2nu1). We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO 2nu1 band is observed under thermal (223-238 K) conditions at 6971 cm(-1). We assign the previously published (warmer temperature) HOONO spectrum to the 2nu1 band at 6365 cm(-1) and 2nu1-containing combination bands of the cis-cis conformer of HOONO. The band shape of the trans-perp HOONO spectrum is in excellent agreement with the predicted rotational contour based on previous experimental and theoretical results, but the apparent origin of the cis-cis HOONO spectrum at 6365 cm(-1) is featureless and significantly broader, suggesting more rapid intramolecular vibrational redistribution or predissociation in the latter isomer. The thermally less stable trans-perp HOONO isomerizes rapidly to cis-cis HOONO with an experimentally determined lifetime of 39 ms at 233 K at 13 hPa (in a buffer gas of predominantly Ar). The temperature dependence of the trans-perp HOONO lifetime in the range 223-238 K yields an isomerization barrier of 33+/-12 kJ/mol. New ab initio calculations of the structure and vibrational mode frequencies of the transition state perp-perp HOONO are performed using the coupled cluster singles and doubles with perturbative triples [CCSD(T)] model, using a correlation consistent polarized triple zeta basis set (cc-pVTZ). The energetics of cis-cis, trans-perp, and perp-perp HOONO are also calculated at this level [CCSD(T)/cc-pVTZ] and with a quadruple zeta basis set using the structure determined at the triple zeta basis set [CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ]. These calculations predict that the anti form of perp-perp HOONO has an energy of DeltaE0=42.4 kJ/mol above trans-perp HOONO, corresponding to an activation enthalpy of DeltaH298 (double dagger 0)=41.1 kJ/mol. These results are in good agreement with statistical simulations based on a model developed by Golden, Barker, and Lohr. The simulated isomerization rates match the observed decay rates when modeled with a trans-perp to cis-cis HOONO isomerization barrier of 40.8 kJ/mol and a strong collision model. The quantum yield of cis-cis HOONO dissociation to OH and NO2 is also calculated as a function of photon excitation energy in the range 3500-7500 cm(-1), assuming D0=83 kJ/mol. The quantum yield is predicted to vary from 0.15 to 1 over the observed spectrum at 298 K, leading to band intensities in the action spectrum that are highly temperature dependent; however, the observed relative band strengths in the cis-cis HOONO spectrum do not change substantially with temperature over the range 193-273 K. Semiempirical calculations of the oscillator strengths for 2nu1(cis-cis HOONO) and 2nu1(trans-perp HOONO) are performed using (1) a one-dimensional anharmonic model and (2) a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory (B3LYP), Møller-Plesset pertubation theory truncated at the second and third order (MP2 and MP3), and quadratic configuration interaction theory using single and double excitations (QCISD). The QCISD level calculated ratio of 2nu1 oscillator strengths of trans-perp to cis-cis HOONO is 3.7:1. The observed intensities indicate that the concentration of trans-perp HOONO early in the OH+NO2 reaction is significantly greater than predicted by a Boltzmann distribution, consistent with statistical predictions of high initial yields of trans-perp HOONO from the OH+NO2+M reaction. In the atmosphere, trans-perp HOONO will isomerize nearly instantaneously to cis-cis HOONO. Loss of HOONO via photodissociation in the near-IR limits the lifetime of cis-cis HOONO during daylight to less than 45 h, other loss mechanisms will reduce the lifetime further.

Relative vibrational overtone intensity of cis–cis and trans–perp peroxynitrous acid

The vibrational overtone spectrum of HOONO is examined in the region of the 2 nu(OH) and 3 nu(OH) bands using action spectroscopy in conjunction with ab initio intensity calculations. The present measurements indicate that the oscillator strength associated with the higher energy trans-perp conformer of HOONO is stronger relative to the lower energy cis-cis conformer for both these vibrational overtone levels. Ab initio intensity calculations carried out at the QCISD level of theory suggest that this disparity in oscillator strength apparently arises from differences in the second derivative of the transition dipole moment function of the two isomers. The calculations indicate that the oscillator strength for the trans-perp isomer is approximately 5.4 times larger than that of the cis-cis isomer for the 2 nu(OH) band and approximately 2 times larger for 3 nu(OH) band. The band positions and intensities predicted by the calculations are used to aid in the assignment of features in the experimental action spectra associated with the OH stretching overtones of HOONO. The observed relative intensities in the experimental action spectra when normalized to the calculated oscillator strengths appears to suggest that the concentration of the higher energy trans-perp isomer is comparable to the concentration of the cis-cis isomer in these room temperature experiments.

Rotational spectrum of cis–cis HOONO

The pure rotational spectrum of cis-cis peroxynitrous acid, HOONO, has been observed. Over 220 transitions, sampling states up to J'=67 and Ka'=31, have been fitted with an rms uncertainty of 48.4 kHz. The experimentally determined rotational constants agree well with ab initio values for the cis-cis conformer, a five-membered ring formed by intramolecular hydrogen bonding. The small, positive inertial defect Delta=0.075667(60) amu A2 and lack of any observable torsional splittings in the spectrum indicate that cis-cis HOONO exists in a well-defined planar structure at room temperature.