Transition state dynamics of the OH+OH→O+H2O reaction studied by dissociative photodetachment of H2O2−

A Full-Dimensional Potential Energy Surface and Dynamics of the Multichannel Reaction between H and HO2

In addition to its vital significance in combustion and atmospheric chemistry, the reaction between H' and HO₂ on the ground triplet state represents a prototype with multiple product channels, including H₂ + O₂, OH + OH, O + H₂O, and H + H'O₂. In this work, a full-dimensional accurate potential energy surface (PES) for the title reaction was developed to provide reliable descriptions for all dynamically relevant regions. Using this PES, we adopted the quasi-classical trajectory approach to study the corresponding reaction dynamics, including the reactivity of each product channel and the associated product branching ratio, the product energy distributions, product angular distributions, and associated microscopic mechanisms. For representing distributions of the product energies, such as product translational energy as well as product rotational and vibrational energies, both the traditional histogram and the kernel density estimation (KDE) methods were used and compared. It seems that the features of the resulting distributions in this work are very similar to each other among different methods. The KDE method is suggested for statistics, particularly for those populations with small oscillations in the histogram plot.

Disproportionation Channel of the Self-reaction of Hydroxyl Radical, OH + OH → H2O + O, Revisited

The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → H₂O + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value (k_1a = 1.48 × 10^-12 cm³ molecule^-1 s^-1, discharge flow system, Bedjanian et al. J. Phys. Chem. A 1999, 103, 7017) and a factor of ca. 1.8 higher value by pulsed laser photolysis (2.7 × 10^-12 cm³ molecule^-1 s^-1, Bahng et al. J. Phys. Chem. A 2007, 111, 3850, and 2.52 × 10^-12 cm³ molecule^-1 s^-1, Altinay et al. J. Phys. Chem. A 2014, 118, 38). To resolve this discrepancy, the rate constant of the title reaction was remeasured in three laboratories using two different experimental techniques, namely, laser-pulsed photolysis-transient UV absorption and fast discharge flow system coupled with mass spectrometry. Two different precursors were used to generate OH radicals in the laser-pulsed photolysis experiments. The experiments confirmed the low value of the rate constant at ambient temperature (k_1a = (1.4 ± 0.2) × 10^-12 cm³ molecule^-1 s^-1 at 295 K) as well as the V-shaped temperature dependence, negative at low temperatures and positive at high temperatures, with a turning point at 427 K: k_1a = 8.38 × 10^-14 × (T/300)^1.99 × exp(855/T) cm³ molecule^-1 s^-1 (220-950 K). Recommended expression over the 220-2384 K temperature range: k_1a = 2.68 × 10^-14 × (T/300)^2.75 × exp(1165/T) cm³ molecule^-1 s^-1 (220-2384 K).

Reaction Profiles and Kinetics for Radical-Radical Hydrogen Abstraction via Multireference Coupled Cluster Theory

Radical-radical abstractions in hydrocarbon oxidation chemistry are disproportionation reactions that are generally exothermic with little or no barrier yet are underappreciated and poorly studied. Such challenging multireference electronic structure problems are tackled here using the recently developed state-specific multireference coupled cluster methods Mk-MRCCSD and Mk-MRCCSD(T), as well as the companion perturbation theory Mk-MRPT2 and the established MRCISD, MRCISD+Q, and CASPT2 approaches. Reaction paths are investigated for five prototypes involving radical-radical hydrogen abstraction: H + BeH → H₂+ Be, H + NH₂ → H₂ + NH, CH₃ + C₂H₅ → CH₄ + C₂H₄, H + C₂H₅ → H₂ + C₂H₄, and H + HCO → H₂ + CO. Full configuration interaction (FCI) benchmark computations for the H + BeH, H + NH₂, and H + HCO reactions prove that Mk-MRCCSD(T) provides superior accuracy for the interaction energies in the entrance channel, with mean absolute errors less than 0.3 kcal mol^-1 and percentage deviations less than 10% over the fragment separations of relevance to kinetics. To facilitate combustion studies, energetics for the CH₃ + C₂H₅, H + C₂H₅, and H + HCO reactions were computed at each level of theory with correlation-consistent basis sets (cc-pVXZ, X = T, Q, 5) and extrapolated to the complete basis set (CBS) limit. These CBS energies were coupled with CASPT2 projected vibrational frequencies along a minimum energy path to obtain rate constants for these three reactions. The rigorous Mk-MRCCSD(T)/CBS results demonstrate unequivocally that these three reactions proceed with no barrier in the entrance channel, contrary to some earlier predictions. Mk-MRCCSD(T) also reveals that the economical CASPT2 method performs well for large interfragment separations but may deteriorate substantially at shorter distances.

Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction

The mechanism of the O₂^⋅− and H₂O₂ reaction (Haber–Weiss) under solvent‐free conditions has been characterized at the DFT and CCSD(T) level of theory to account for the ease of this reaction in the gas phase and the formation of two different set of products (Blanksby et al., Angew. Chem. Int. Ed. 2007, 46, 4948). The reaction is shown to proceed through an electron‐transfer process from the superoxide anion to hydrogen peroxide, along two pathways. While the O₃^⋅− + H₂O products are formed from a spin‐allowed reaction (on the doublet surface), the preferred products, O^⋅−(H₂O)+³O₂, are formed through a spin‐forbidden reaction as a result of a favorable crossing point between the doublet and quartet surface. Plausible reasons for the preference toward the latter set are given in terms of the characteristics of the minimum energy crossing point (MECP) and the stability of an intermediate formed (after the MECP) in the quartet surface. These unique results show that these two pathways are associated with a bifurcation, yielding spin‐dependent products.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

An accurate multi-channel multi-reference full-dimensional global potential energy surface for the lowest triplet state of H2O2

A full dimensional potential energy surface for the lowest triplet state of H₂O₂ was developed at the MRCI-F12 level.

Visible spectrum photofragmentation of O₃⁻(H₂O)n, n ≤ 16

Photofragmentation of ozonide solvated in water clusters, O3(-)(H2O)n, n ≤ 16, has been studied as a function of photon energy as well as the degree of solvation. Using mass selection, the effect of the presence of the solvent molecule on the O3(-) photodissociation process is assessed one solvent molecule at a time. The O3(-) acts as a visible light chromophore within the water cluster, namely the O3(-)(H2O) total photodissociation cross-section exhibits generally the same photon energy dependence as isolated O3(-) throughout the visible wavelength range studied (430-620 nm). With the addition of a single solvent molecule, new photodissociation pathways are opened, including the production of recombined O3(-). As the degree of solvation of the parent anion increases, recombination to O3(-)-based products accounts for close to 40% of photoproducts by n = 16. The remainder of the photoproducts exist as O(-)-based; no O2(-)-based products are observed. Upper bounds on the O3(-) solvation energy (530 meV) and the O(-)-OO bond dissociation energy in the cluster (1.06 eV) are derived.

Accretion disc origin of the Earth's water

Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻⁸ bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.

Determination of the Rate Constant for the OH(X2Π) + OH(X2Π) → O(3P) + H2O Reaction over the Temperature Range 293−373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.

Structures and Energetics of Hydrated Oxygen Anion Clusters

Hydration of the atomic oxygen radical anion is studied with computational electronic structure methods, considering (O(-))(H(2)O)(n) clusters and related proton-transferred (OH(-))(OH)(H(2)O)(n)(-)(1) clusters having n = 1-5. A total of 67 distinct local-minimum structures having various interesting hydrogen bonding motifs are obtained and analyzed. On the basis of the most stable form of each type, (O(-))(H(2)O)(n)) clusters are energetically favored, although for n > or = 3, there is considerable overlap in energy between other members of the (O(-))(H(2)O)(n) family and various members of the (OH(-))(OH)(H(2)O)(n)(-)(1) family. In the lower-energy (O(-))(H(2)O)(n) clusters, the hydrogen bonding arrangement about the oxygen anion center tends to be planar, leaving the oxygen anion p-like orbital containing the unpaired electron uninvolved in hydrogen bonding with any water molecule. In (OH(-))(OH)(H(2)O)(n)(-)(1) clusters, on the other hand, nonplanar arrangements are the rule about the anionic oxygen center that accepts hydrogen bonds. No instances are found of OH(-) acting as a hydrogen bond donor. Those OH bonds that form hydrogen bonds to an anionic O(-) or OH(-) center are significantly stretched from their equilibrium value in isolated water or hydroxyl. A quantitative inverse correlation is established for all hydrogen bonds between the amount of the OH bond stretch and the distance to the other oxygen involved in the hydrogen bond.

Potential surfaces and dynamics of the O(P3)+H2O(XA111)→OH(XΠ2)+OH(XΠ2) reaction

We present global potential energy surfaces for the three lowest triplet states in O(3P)+H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction using these surfaces. The surfaces are spline-based fits of approximately 20,000 fixed geometry ab initio calculations at the complete-active-space self-consistent field+second-order perturbation theory (CASSCF+MP2) level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1000-2500 K range using these surfaces. We also compute the total, rovibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approximately 4 km s(-1) (16.9 kcal mol(-1) collision energy) to 11 km s(-1) (122.5 kcal mol(-1) collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approximately 40 kcal mol(-1) collision energy rovibrationally excited OH(X2pi) products are a significant and perhaps dominant contributor to the observed 1-5 micro spectral emission from O(3P)+H2O(X1A1) collisions. Another important result is that OH(X2pi) products are formed in two distinct rovibrational distributions. The "active" OH products are formed with the reagent O atom, and their rovibrational distributions are extremely hot. The remaining "spectator" OH is relatively rovibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approximately 50 kcal mol(-1) collision energy, vibration dominates the OH internal energy, and above approximately 50 kcal mol(-1) rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2pi)+OH(X2pi) collisions.

Degradation of N-Acetyl Tryptophan by Low-Energy (<12 eV) Electrons

Secondary low-energy electrons are abundantly created during the early moments following the deposition of energy by radiation into cells. Here we show the ability of slow (<12 eV) electrons to effectively decompose gas-phase N-acetyl tryptophan (NAT) which can model a simple protein. The fragmentation of NAT, initiated via a resonant electron-molecule interaction exclusively at the peptide bridge, produces a large variety of negative species. The present findings contribute to the molecular description of the initial step in the radiation-induced damage.

Probing the transition state via photoelectron and photodetachment spectroscopy of H(3)O(-)

The H(3)O(-) anion has stable and metastable structures that resemble configurations in the vicinity of the transition state for the neutral reactions OH + H(2) <--> H(2)O + H. Photoelectron spectroscopy of this anion probes the neutral reaction dynamics in the critical transition-state region. Accurate quantum dynamics calculations of the photoelectron intensity and photodissociation product energies are shown to provide a quantitatively reliable means of interpreting such experimental observations and reveal a detailed picture of the reaction dynamics.