The reaction between HO and (H2O) n (n = 1, 3) clusters: reaction mechanisms and tunneling effects

Resolving the amine-promoted hydrolysis mechanism of N2O5 under tropospheric conditions

Hydrolysis of N₂O₅ under tropospheric conditions plays a critical role in assessing the fate of O₃, OH, and NO_x in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N₂O₅ on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N₂O₅ at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH₃-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N₂O₅ on water clusters and droplet. Our results demonstrate that methylation of NH₃ stabilizes the product state and promotes hydrolysis of N₂O₅ by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.

The favorable routes for the hydrolysis of CH2OO with (H2O)n (n = 1-4) investigated by global minimum searching combined with quantum chemical methods

The hydrolysis reaction of CH₂OO with water and water clusters is believed to be a dominant sink for the CH₂OO intermediate in the atmosphere. However, the favorable route for the hydrolysis of CH₂OO with water clusters is still unclear. Here global minimum searching using the Tsinghua Global Minimum program has been introduced to find the most stable geometry of the CH₂OO(H₂O)_n (n = 1-4) complex firstly. Then, based on these stable complexes, favorable hydrolysis of CH₂OO with (H₂O)_n (n = 1-4) has been investigated using the quantum chemical method of CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2d,2p) and canonical variational transition state theory with small curvature tunneling. The calculated results have revealed that, although the contribution of CH₂OO + (H₂O)₂ is the most obvious in the hydrolysis of CH₂OO with (H₂O)_n (n = 1-4), the hydrolysis of CH₂OO with (H₂O)₃ is not negligible in atmospheric gas-phase chemistry as its rate is close to the rate of the CH₂OO + H₂O reaction. The calculated results also show that, in a clean atmosphere, the CH₂OO + (H₂O)_n (n = 1-2) reaction competes well with the CH₂OO + SO₂ reaction at 298 K when the concentrations of (H₂O)_n (n = 1-2) range from 20% relative humidity (RH) to 100% RH, and SO₂ is 2.46 × 10¹¹ molecules per cm³. Meanwhile, when the RH is higher than 40%, it is a new prediction that the CH₂OO + (H₂O)₃ reaction can also compete well with the CH₂OO + SO₂ reaction at 298 K. Besides, Born-Oppenheimer molecular dynamics simulation results show that all the favorable channels of the CH₂OO + (H₂O)_n (n = 1-3) reaction cannot react on a time scale of 100 ps in the NVT simulation. However, the NVE simulation results show that the CH₂OO + (H₂O)₃ reaction can be finished well at 8.5 ps, indicating that the gas phase reaction of CH₂OO + (H₂O)₃ is not negligible in the atmosphere. Overall, the present results have provided a definitive example of how the favorable hydrolysis of important atmospheric species with (H₂O)_n (n = 1-4) takes place, which will stimulate one to consider the favorable hydrolysis of water and water clusters with other Criegee intermediates and other important atmospheric species.

Cryogenic Vibrationally Resolved Photoelectron Spectroscopy of OH-(H2O): Confirmation of Multidimensional Franck-Condon Simulation Results for the Transition State of the OH + H2O Reaction

We present a transition state spectroscopic study of the OH + H₂O reaction using the experimental technique of cryogenic negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum at 193 nm exhibits multiple vibrational progressions that include excitations to the shared H atom antisymmetric stretching mode with an interval of 0.32 eV as well as other progressions, mainly involving the H bending and O···O symmetric stretching modes. The vertical detachment energy (VDE) was measured at 3.53 eV, whereas an upper limit for the adiabatic detachment energy (ADE) was estimated at 2.90 eV. These values are in excellent agreement with the theoretically computed values of 3.51 and 2.87 eV, respectively, obtained at the CCSD(T)/aug-cc-pV5Z level of theory. The recorded NIPE spectrum is in very good agreement when compared to the one recently reported from four-dimensional Franck-Condon simulations, in which a similar spectral profile was predicted. Besides observing the ground state, we identified a charge-transfer excited state in the form of [OH^-(H₂O)⁺] with a relative energy of 1.39 eV, well matching the previous prediction of 1.36 eV.

The water trimer reaction OH + (H2O)3→ (H2O)2OH + H2O

All important stationary points on the potential energy surface (PES) for the reaction OH + (H₂O)₃→ (H₂O)₂OH + H₂O have been fully optimized using the "gold standard" CCSD(T) method with the large Dunning correlation-consistent cc-pVQZ basis sets. Three types of pathways were found. For the pathway without hydrogen abstraction, the barrier height of the transition state (TS1) is predicted to lie 5.9 kcal mol^-1 below the reactants. The two major complexes (H₂O)₃OH (CP1 and CP2a) are found to lie 6.3 and 11.0 kcal mol^-1, respectively, below the reactants [OH + (H₂O)₃]. For one of the H-abstraction pathways the lowest classical barrier height is predicted to be much higher, 6.1 kcal mol^-1 (TS2a) above the reactants. For the other H-abstraction pathway the barrier height is even higher, 15.0 (TS3) kcal mol^-1. Vibrational frequencies and the zero-point vibrational energies connected to the PES are also reported. The energy barriers for the H-abstraction pathways are compared with those for the OH + (H₂O)₂ and OH + H₂O reactions, and the effects of the third water on the energetics are usually minor (0.2 kcal mol^-1).

Tracking the energy flow in the hydrogen exchange reaction OH + H2O → H2O + OH

The prototypical hydrogen exchange reaction OH + H2O → H2O + OH has attracted considerable interest due to its importance in a wide range of chemically active environments. In this work, an accurate global potential energy surface (PES) for the ground electronic state was developed based on ∼44 000 ab initio points at the level of UCCSD(T)-F12a/aug-cc-pVTZ. The PES was fitted using the fundamental invariant-neural network method with a root mean squared error of 4.37 meV. The mode specific dynamics was then studied by the quasi-classical trajectory method on the PES. Furthermore, the normal mode analysis approach was employed to calculate the final vibrational state distribution of the product H2O, in which a new scheme to acquire the Cartesian coordinates and momenta of each atom in the product molecule from the trajectories was proposed. It was found that, on one hand, excitation of either the symmetric stretching mode or the asymmetric stretching mode of the reactant H2O promotes the reaction more than the translational energy, which can be rationalized by the sudden vector projection model. On the other hand, the relatively higher efficacy of exciting the symmetric stretching mode than that of the asymmetric stretching mode is caused by the prevalence of the indirect mechanism at low collision energies and the stripping mechanism at high collision energies. In addition, the initial collision energy turns ineffectively into the vibrational energy of the products H2O and OH while a fraction of the energy transforms into the rotational energy of the product H2O. Fundamental excitation of the stretching modes of H2O results in the product H2O having the highest population in the fundamental state of the asymmetric stretching mode, followed by the ground state and the fundamental state of the symmetric stretching mode.

Impact of the water dimer on the atmospheric reactivity of carbonyl oxides

The reactions of twelve carbonyl oxides or Criegee intermediates with the water monomer and with the water dimer have been investigated employing high level theoretical methods. The study includes all possible carbonyl oxides arising from the isoprene ozonolysis and the methyl and dimethyl carbonyl oxides that originated from the reaction of ozone with several hydrocarbons. These reactions have great significance in the chemistry of the atmosphere because Criegee intermediates have recently been identified as important oxidants in the troposphere and as precursors of secondary organic aerosols. Moreover, water vapor is one of the most abundant trace gases in the atmosphere and the water dimer can trigger the atmospheric decomposition of Criegee intermediates. Our calculations show that the nature and position of the substituents in carbonyl oxides play a very important role in the reactivity of these species with both the water monomer and the water dimer. This fact results in differences in rate constants of up to six orders of magnitude depending on the carbonyl oxide. In this work we have defined an effective rate constant (keff) for the atmospheric reaction of carbonyl oxides with water vapor, which depends on the temperature and on the relative humidity as well. With this keff we show that the water dimer, despite its low tropospheric concentration, enhances the atmospheric reactivity of Criegee intermediates, but its effect changes with the nature of carbonyl oxide, ranging between 59 and 295 times in the most favorable case (syn-methyl carbonyl oxide), and between 1.4 and 3 times only in the most unfavorable case.

Role of the (H2O)n (n = 1–3) cluster in the HO2 + HO → 3O2 + H2O reaction: mechanistic and kinetic studies

Upon incorporation of the catalyst (H₂O)_n (n = 1–3) into the reaction HO₂ + HO → H₂O + ³O₂, the catalytic effects of water, water dimer, and water trimer mainly arise from the contribution of a single molecule of water vapor.

The water dimer reaction OH + (H2O)2 → (H2O)-OH + H2O

The stationary points, including the entrance complex, transition states, and the exit complex, for the reaction OH + (H₂O)₂ → (H₂O)OH + H₂O have been carefully examined using the "gold standard" CCSD(T) method with the correlation-consistent basis sets up to cc-pVQZ. The complex (H₂O)₂OH is found to lie 10.8 kcal mol^-1 below the separated reactants. This complex should be observable in the gas phase via vibrational or microwave spectroscopy. Seven unique transition states were found. One pathway for the title reaction has no barrier, in which the OH radical captures a whole water molecule from the water dimer. For the hydrogen abstraction pathways the lowest classical barrier height is predicted to be 5.9 kcal mol^-1 (TS1) relative to separated reactants, and the other pathways are of higher barriers, i.e., 17.8 (TS2) and 18.4 (TS3) kcal mol^-1. The harmonic vibrational frequencies and the zero-point vibrational energies of the stationary points for the reaction are also reported.

The atmospheric oxidation of CH3OOH by the OH radical: the effect of water vapor

The relative humidity can enhance the atmospheric oxidation of CH₃OOH by OH into CH₃O₂ + H₂O up to 19% whereas the formation of H₂CO + OH + H₂O is enhanced up to 5% only under the same conditions.

Quasi-classical trajectory studies on the full-dimensional accurate potential energy surface for the OH + H2O = H2O + OH reaction

The first accurate PES for the OH + H₂O reaction is developed by using the permutation invariant polynomial-neural network method to fit ∼48 000 CCSD(T)-F12a/AVTZ calculated points.

The catalytic effect of water, water dimers and water trimers on H2S +3O2formation by the HO2+ HS reaction under tropospheric conditions

Catalyst X (X = H₂O, (H₂O)₂and (H₂O)₃) is incorporated into the channel of H₂S +³O₂formation and the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule.

The hemibond as an alternative condensed phase structure for the hydroxyl radical

Despite the critical importance of the hydroxyl radical in major scientific fields, there are still open questions on the behavior of this species in the aqueous phase. In particular, there has been much debate on the existence of a hemibonded interaction between the hydroxyl radical and water molecules. While some reports indicate that the hemibonded radical might explain some experimental data, others have claimed that this interaction is simply a density functional theory (DFT) artifact. Here, we provide results from high level (basis set limit of coupled-cluster levels up to single, double, triple excitations (CCSD(T)) and beyond) ab initio calculations of different OH•(H2O)n clusters in the gas phase to accurately explore the existence of the hemibonded interaction and its energy difference with respect to other well-defined hydrogen bond interactions. Additional comparisons with second order perturbation theory (MP2) and DFT are also presented. Constrained molecular dynamics was applied to determine the free energy for the formation/disruption and ice systems. Overall, our findings confirm that the hemibond can be an alternative structure for the hydroxyl radical in the condensed phase when the formation of hydrogen bonds is impeded. These results will aid the understanding of theoretical and experimental data and help future experimental designs for the detection of this important species.

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.