High levelab initiomolecular orbital theory study of the structure, vibrational spectrum, stability, and low-lying excited states of HOONO

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.

Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface

We report a full-dimensional potential energy surface (PES) for the OH+NO(2) reaction based on fitting more than 55,000 energies obtained with density functional theory-B3LYP6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), and the fragments OH+NO(2) and HO(2)+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO(2) association reactions to form HOONO and HONO(2) are done using this PES. The cross section to form HOONO is larger than the one to form HONO(2) at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) for the ground rovibrational states in the high-pressure limit.

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.

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.

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.

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.

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.

High level ab initio study of the structure and vibrational spectra of HO2NO2

A high-level ab initio study has been performed on the conformational structure and vibrational spectra of HO(2)NO(2). Calculations carried out with coupled-cluster methods using a series of Pople and Dunning basis sets reveal that there is a significant basis set dependence on the predicted ab initio structure. Higher angular momentum basis sets are shown to be necessary in order to bring the calculated structure into agreement with experimental rotational constants. Harmonic vibrational frequencies of HO(2)NO(2) are computed at the CCSD(T)/aug-cc-pVTZ level of theory while the corresponding vibrational anharmonicities are calculated at the MP2/cc-pVTZ level. In addition, the absorption cross sections of OH stretching overtones in HO(2)NO(2) are calculated using a dipole function computed at the QCISD level of theory and found to be in good agreement with the available experimental data.

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.