Measurements of the Rate Constant of HO2 + NO2 + N2 → HO2NO2 + N2 Using Near-Infrared Wavelength-Modulation Spectroscopy and UV−Visible Absorption Spectroscopy

HO2 + NO2: Kinetics, Thermochemistry, and Evidence for a Bimolecular Product Channel

A master equation (ME) analysis of available experimental data has been carried out on the reaction HO₂ + NO₂ + M ⇋ HO₂NO₂ + M (1a)/(-1a). The analysis, based on the ME code MESMER, uses both the association and dissociation kinetic data from the literature, and provides improved thermochemistry on reaction 1a. Our preferred model assigns two low-frequency vibrations of HO₂NO₂ as hindered rotors and couples these to the external rotations. This model gives Δ_rH°₀(1a) = -93.9 ± 1.0 kJ mol^-1, which implies that Δ_fH°₀ HO₂NO₂ = -42.0 ± 1.0 kJ mol^-1 (uncertainties are 2σ). A significant contributor to the uncertainty derives from modeling the interaction between the internal and external rotors. Using this improved kinetics for reaction 1a/-1a, data at elevated temperatures, 353-423 K, which show no evidence of the expected equilibration, have been reanalyzed, indicating that an additional reaction is occurring that masks the equilibration. Based on a published ab initio study, this additional channel is assigned to the bimolecular reaction HO₂ + NO₂ → H-NO₂ + O₂ (1b); H-NO₂ is nitryl hydride and has not previously been directly observed in experiments. The output of the master equation analysis has been parametrized and Troe expressions are provided for an improved description of k_1a(p,T) and k_-1a(p,T).

Sensitive detection of HO radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a well-established, highly sensitive absorption technique whose sensitivity and selectivity for trace radical sensing can be further enhanced by measuring the polarization rotation of the intracavity light by the paramagnetic samples in the presence of a magnetic field. In this paper, we highlight the use of this Faraday rotation cavity ring-down spectroscopy (FR-CRDS) for the detection of HO₂ radicals. In particular, we use a cold atmospheric pressure plasma jet as a highly efficient source of HO₂ radicals and show that FR-CRDS in the near-infrared spectral region (1506 nm) has the potential to be a useful tool for studying radical chemistry. By simultaneously measuring ring-down times of orthogonal linearly polarized light, measurements of Faraday effect-induced rotation angles (θ) and absorption coefficients (α) are retrieved from the same data set. The Faraday rotation measurement exhibits better long-term stability and enhanced sensitivity due to its differential nature, whereby highly correlated noise between the two channels and slow drifts cancel out. The bandwidth-normalized sensitivities are α_min=2.2×10^-11 cm^-1 Hz^-1/2 and θ_min=0.62 nrad Hz^-1/2. The latter corresponds to a minimum detectable (circular) birefringence of Δn_min=5×10^-16 Hz^-1/2. Using the overlapping ^qQ₃(N = 4-9) transitions of HO₂, we estimate limits of detection of 3.1 × 10⁸ cm^-3 based on traditional (absorption) CRDS methods and 6.7 × 10⁷ cm^-3 using FR-CRDS detection, where each point of the spectrum was acquired during 2 s. In addition, Verdet constants for pertinent carrier (He, Ar) and bulk (N₂, O₂) gases were recorded in this spectral region for the first time. These show good agreement with recent measurements of air and values extrapolated from reported Verdet constants at shorter wavelengths, demonstrating the potential of FR-CRDS for measurements of very weak Faraday effects and providing a quantitative validation to the computed rotation angles.

Temperature Dependence of the Reaction of Chlorine Atoms with CH3OH and CH3CHO

Rate constants of the reactions Cl + CH₃OH → CH₂OH + HCl ( k₁) and Cl + CH₃CHO → CH₃C(O) + HCl ( k₃) were measured at 100 Torr over the temperature range 230.3-297.1 K. Radical chemistry was initiated by pulsed laser photolysis of Cl₂ in mixtures of CH₃OH and CH₃CHO in a flow reactor. Heterodyne near-IR wavelength modulation spectroscopy was used to directly detect HO₂ produced from the subsequent reaction of CH₂OH with O₂ in real time to determine the rate of reaction of Cl with CH₃OH. The rate of Cl + CH₃CHO was measured relative to that of the Cl + CH₃OH reaction. Secondary chemistry, including that of the adducts HO₂·CH₃OH and HO₂·CH₃CHO, was taken into account. The Arrhenius expressions were found to be k₁( T) = 5.02_-1.5^+1.8 × 10^-11 exp[(20 ± 88)/ T] cm³ molecule^-1 s^-1 and k₃( T) = 6.38_-2.0^+2.4 × 10^-11 exp[(56 ± 90)/ T] cm³ molecule^-1 s^-1 (2σ uncertainties). The average values of the rate constants over this temperature range were k₁ = (5.45 ± 0.37) × 10^-11 cm³ molecule^-1 s^-1 and k₃ = (8.00 ± 1.27) × 10^-11 cm³ molecule^-1 s^-1 (2σ uncertainties), consistent with current literature values.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy

The HO₂ + CH₃C(O)O₂ reaction consists of three product channels: CH₃C(O)OOH + O₂ (R1a), CH₃C(O)OH + O₃ (R1b), and OH + CH₃C(O)O + O₂ (R1c). The overall rate constant ( k₁) and product yields (α_1a, α_1b, and α_1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N₂. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO₂ and CH₃C(O)O₂ were formed by Cl-atom reactions with CH₃OH and CH₃CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO₂ and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O₃). These experimental techniques enabled direct measurements of α_1c and α_1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO₂ and CH₃C(O)O₂ concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k₁( T) = 1.38_-0.63^+1.17 × 10^-12 exp[(730 ± 170)/ T] cm³ molecule^-1 s^-1. α_1a was temperature-independent while α_1b and α_1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO₂ with the precursors, HO₂·CH₃OH and HO₂·CH₃CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO₂ self-reaction due to reactions of the adducts with HO₂ were also measured.

Derivation of Hydroperoxyl Radical Levels at an Urban Site via Measurement of Pernitric Acid by Iodide Chemical Ionization Mass Spectrometry

Hydroperoxyl radical (HO₂) is a key species to atmospheric chemistry. At warm temperatures, the HO₂ and NO₂ come to a rapid steady state with pernitric acid (HO₂NO₂). This paper presents the derivation of HO₂ from observations of HO₂NO₂ and NO₂ in metropolitan Atlanta, US, in winter 2014 and summer 2015. HO₂ was observed to have a diurnal cycle with morning concentrations suppressed by high NO from the traffic. At night, derived HO₂ levels were nonzero and exhibited correlations with O₃ and NO₃, consistent with previous studies that ozonolysis and oxidation by NO₃ are sources of nighttime HO₂. Measured and model calculated HO₂ were in reasonable agreement: Without the constraint of measured HO₂NO₂, the model reproduced HO₂ with a model-to-observed ratio (M/O) of 1.27 (r = 0.54) for winter, 2014, and 0.70 (r = 0.80) for summer, 2015. Adding measured HO₂NO₂ as a constraint, the model predicted HO₂ with M/O = 1.13 (r = 0.77) for winter 2014 and 0.90 (r = 0.97) for summer 2015. These results demonstrate the feasibility of deriving HO₂ from HO₂NO₂ measurements in warm regions where HO₂NO₂ has a short lifetime.

Effects of a single water molecule on the OH + H2O2 reaction

The effect of a single water molecule on the reaction between H(2)O(2) and HO has been investigated by employing MP2 and CCSD(T) theoretical approaches in connection with the aug-cc-PVDZ, aug-cc-PVTZ, and aug-cc-PVQZ basis sets and extrapolation to an ∞ basis set. The reaction without water has two elementary reaction paths that differ from each other in the orientation of the hydrogen atom of the hydroxyl radical moiety. Our computed rate constant, at 298 K, is 1.56 × 10(-12) cm(3) molecule(-1) s(-1), in excellent agreement with the suggested value by the NASA/JPL evaluation. The influence of water vapor has been investigated by considering either that H(2)O(2) first forms a complex with water that reacts with hydroxyl radical or that H(2)O(2) reacts with a previously formed H(2)O·OH complex. With the addition of water, the reaction mechanism becomes much more complex, yielding four different reaction paths. Two pathways do not undergo the oxidation reaction but an exchange reaction where there is an interchange between H(2)O(2)·H(2)O and H(2)O·OH complexes. The other two pathways oxidize H(2)O(2), with a computed total rate constant of 4.09 × 10(-12) cm(3) molecule(-1) s(-1) at 298 K, 2.6 times the value of the rate constant of the unassisted reaction. However, the true effect of water vapor requires taking into account the concentration of the prereactive bimolecular complex, namely, H(2)O(2)·H(2)O. With this consideration, water can actually slow down the oxidation of H(2)O(2) by OH between 1840 and 20.5 times in the 240-425 K temperature range. This is an example that demonstrates how water could be a catalyst in an atmospheric reaction in the laboratory but is slow under atmospheric conditions.

Near-infrared kinetic spectroscopy of the HO2 and C2H5O2 self-reactions and cross reactions

The self-reactions and cross reactions of the peroxy radicals C2H5O2 and HO2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO2, and UV absorption monitored C2H5O2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221-296 K. The Arrhenius expression determined for the cross reaction, k2(T) = (6.01(-1.47)(+1.95)) x 10(-13) exp((638 +/- 73)/T) cm3 molecules(-1) s(-1) is in agreement with other work from the literature. The measurements of the HO2 self-reaction agreed with previous work from this lab and were not further refined. The C2H5O2 self-reaction is complicated by secondary production of HO2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO2. The Arrhenius expression for the self-reaction rate constant is k3(T) = (1.29(-0.27)(+0.34)) x 10(-13)exp((-23 +/- 61)/T) cm3 molecules(-1) s(-1), and the branching fraction value is alpha = 0.28 +/- 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO2 self-reactions are required.

Sensitive Absorption Methods for Quantitative Gas Phase Kinetic Measurements. Part 1: Frequency Modulation Spectroscopy

Frequency modulation spectroscopy (FMS) and cavity ringdown spectroscopy (CRDS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 1 of this review, the theoretical foundations of FMS are addressed with a special emphasis on time-resolved quantitative measurements of concentration profiles. A complementary review of CRDS can be found in part 2 (Z. Phys. Chem. 222 (2008) 31–61). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Kinetic studies based on FMS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with more than a hundred entries is included to facilitate the access to the large body of original literature.

Enthalpies of hydration of N-methylacetamide by one, two, and three waters and the effect upon the C=O stretching frequency. An Ab initio DFT study

DFT calculations at the B3LYP/D95++(d,p) level of clusters of N-methylacetamide (NMA) with one, two, and three waters that were geometrically optimized on the counterpoise-corrected potential energy surfaces show that the gas-phase enthalpy of the interactions of NMA with three waters is -14.11 kcal/mol. Since the interactions between the three waters is 0.99 kcal/mol, the interaction enthalpy would become -15.10 if these interactions were subtracted. The internal geometry of the NMA molecule is sensitive to the degree of hydration, as are the H-bond lengths. Changes in the internal bond lengths with degree of hydration are approximately additive. The calculated C=O stretching frequencies correlate extremely well with the calculated C=O bond lengths, which suggests that the solvent effect upon this stretch could not be a purely electrostatic interaction. The calculated C=O stretch for NMA solvated by three waters in the gas phase agrees very well with that experimentally observed in aqueous solution.

Water dependence of the HO2 self reaction: kinetics of the HO2-H2O complex

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <--> HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.

Experimental and ab Initio Study of the HO2·CH3OH Complex: Thermodynamics and Kinetics of Formation

Near-infrared spectroscopy was used to monitor HO2 formed by pulsed laser photolysis of Cl2-O2-CH3OH-N2 mixtures. On the microsecond time scale, [HO2] exhibited a time dependence consistent with a mechanism in which [HO2] approached equilibrium via HO2 + HO2.CH3OH (3, -3). The equilibrium constant for reaction 3, K(p), was measured between 231 and 261 K at 50 and 100 Torr, leading to standard reaction enthalpy and entropy values (1 sigma) of delta(r) = -37.4 +/- 4.8 kJ mol(-1) and delta(r) = -100 +/- 19 J mol(-1) K(-1). The effective bimolecular rate constant, k3, for formation of the HO2.CH3OH complex is .10(-15).exp[(1800 +/- 500)/T] cm3 molecule(-1) s(-1) at 100 Torr (1 sigma). Ab initio calculations of the optimized structure and energetics of the HO2.CH3OH complex were performed at the CCSD(T)/6-311++G(3df,3pd)//MP2(full)/6-311++G(2df,2pd) level. The complex was found to have a strong hydrogen bond (D(e) = 43.9 kJ mol(-1)) with the hydrogen in HO2 binding to the oxygen in CH3OH. The calculated enthalpy for association is delta(r) = -36.8 kJ mol(-1). The potentials for the torsion about the O2-H bond and for the hydrogen-bond stretch were computed and 1D vibrational levels determined. After explicitly accounting for these degrees of freedom, the calculated Third Law entropy of association is delta(r) = -106 J mol(-1) K(-1). Both the calculated enthalpy and entropy of association are in reasonably good agreement with experiment. When combined with results from our previous study (Christensen et al. Geophys. Res. Lett. 2002, 29; doi:10.1029/2001GL014525), the rate coefficient for the reaction of HO2 with the complex, HO2 + HO2.CH3OH, is determined to be (2.1 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1). The results of the present work argue for a reinterpretation of the recent measurement of the HO2 self-reaction rate constant by Stone and Rowley (Phys. Chem. Chem. Phys. 2005, 7, 2156). Significant complex concentrations are present at the high methanol concentrations used in that work and lead to a nonlinear methanol dependence of the apparent rate constant. This nonlinearity introduces substantial uncertainty in the extrapolation to zero methanol.

Thermal Decomposition of HO2NO2 (Peroxynitric Acid, PNA): Rate Coefficient and Determination of the Enthalpy of Formation

Rate coefficients for the gas-phase thermal decomposition of HO(2)NO(2) (peroxynitric acid, PNA) are reported at temperatures between 331 and 350 K at total pressures of 25 and 50 Torr of N(2). Rate coefficients were determined by measuring the steady-state OH concentration in a mixture of known concentrations of HO(2)NO(2) and NO. The measured thermal decomposition rate coefficients k(-)(1)(T,P) are used in combination with previously published rate coefficient data for the HO(2)NO(2) formation reaction to yield a standard enthalpy for reaction 1 of Delta(r)H degrees (298K) = -24.0 +/- 0.5 kcal mol(-1) (uncertainties are 2sigma values and include estimated systematic errors). A HO(2)NO(2) standard heat of formation, Delta(f)H degrees (298K)(HO(2)NO(2)), of -12.6 +/- 1.0 kcal mol(-1) was calculated from this value. Some of the previously reported data on the thermal decomposition of HO(2)NO(2) have been reanalyzed and shown to be in good agreement with our reported value.