Infrared Spectrum and Stability of the H2O−HO Complex: Experiment and Theory

Experimentally Delineating the Catalytic Effect of a Single Water Molecule in the Photochemical Rearrangement of the Phenylperoxy Radical to the Oxepin-2(5H)-one-5-yl Radical

Catalysis plays a pivotal role in both chemistry and biology, primarily attributed to its ability to stabilize transition states and lower activation free energies, thereby accelerating reaction rates. While computational studies have contributed valuable mechanistic insights, there remains a scarcity of experimental investigations into transition states. In this work, we embark on an experimental exploration of the catalytic energy lowering associated with transition states in the photorearrangement of the phenylperoxy radical-water complex to the oxepin-2(5H)-one-5-yl radical. Employing matrix isolation spectroscopy, density functional theory, and post-HF computations, we scrutinize the (photo)catalytic impact of a single water molecule on the rearrangement. Our computations indicate that the barrier heights for the water-assisted unimolecular isomerization steps are approximately 2-3 kcal mol-1 lower compared to the uncatalyzed steps. This decrease directly coincides with the energy difference in the required wavelength during the transformation (Δλ = λ546 nm - λ579 nm ≡ 52.4-49.4 = 3.0 kcal mol-1), allowing us to elucidate the differential transition state energy in the photochemical rearrangement of the phenylperoxy radical catalyzed by a single water molecule. Our work highlights the important role of water catalysis and has, among others, implications for understanding the mechanism of organic reactions under atmospheric conditions.

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation

An organic radical monohydrate complex is detected in vacuum isolation at low temperature by FTIR supersonic jet spectroscopy for the first time. It is shown to exhibit a rich conformational and vibrational coupling dynamics, which can be drastically reduced by appropriate isotope substitution. Its detection with a new gas recycling infrared spectrometer demonstrates the thermal metastability of the gaseous TEMPO radical even under humid gas conditions. Compared to its almost isoelectronic and isostructural, closed shell ketone analogue, the hydrogen bond of the solvating water is found to be less directional, but stronger and more strongly downshifting the bonded water OH stretch vibration. A second solvent water directs the first one into a metastable hydrogen bond position to solvate the nitrogen center and the first water at the same time.

The mobility and solvation structure of a hydroxyl radical in a water nanodroplet: a Born-Oppenheimer molecular dynamics study

Hydroxyl radicals (OH*) play a crucial role in atmospheric chemistry and biological processes. In this study, Born-Oppenheimer molecular dynamics simulations are performed under ambient conditions for a hydroxyl radical in a water nanodroplet containing 191 water molecules. Density functional theory calculations are performed at the BLYP-D3 level with some test calculations at the B3LYP-D3 level. In two 150 ps trajectories, either with OH* initially located in the interior region or at the surface of the water nanodroplet, the OH* radical ends up in the subsurface layer of the nanodroplet, which is different from the "surface preference" predicted from previous empirical force field simulations. The solvation structure of OH* contains fluctuating hydrogen bonds, as well as a two-center three-electron hemibond in some cases. The mobility of OH* is enhanced by hydrogen transfer, which has a free energy barrier of ∼4.6 kcal mol-1. The results presented in this study deepen our understanding of the structure and dynamics of OH* in aqueous solutions, especially around the air-water interface.

G, M.H.A. S. Aqueous Micro-hydration of Na+(H2O)n=1-7 Clusters: DFT Study

Sodium ion micro-solvated clusters, [Na(H2O) n]+, n = 1–7, were completed by (DFT) density functional theory at B3LYP/6-311+G(d,p) level in the gaseous phase. At the ambient situation, the four, five and six micro-solvated configurations can convert from each other. The investigation of the sequential water binding energy on Na+ obviously indicates that the influence of Na+ on the neighboring water molecules goes beyond the first solvation layer with the hydration number of 5. The hydration number of Na+ is 5 and the hydration space (rNa-O) is 2.43 Å. The current study displays that all our simulations have an brilliant harmony with the diffraction result from X-ray scattering study. The vibration frequency of H2O solvent was also determined. This work is important for additional identification of the Na+(H2O)n clusters in aqueous medium.

Role of (H2O) n (n = 1-2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH₃CH₂OH + ^•OH reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH₃CH₂OH, the bared reaction proceeds through three elementary paths, producing CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH₃CHOH is favored over the temperature range of 216.7-425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH₃CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH are computed to be 5.2 × 10^-13, 8.6 × 10^-14, and 9.0 × 10^-11 cm³ molecule^-1 s^-1, respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3-5 and 6-8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH₃CH₂OH + ^•OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it.

Mode-specific quantum dynamics and kinetics of the hydrogen abstraction reaction OH + H2O → H2O + OH

The synergistic effect between the reactant stretching and bending modes on promoting the reaction.

Elucidating the mechanism and kinetics of the water-assisted reaction of nitrous acid with hydroxyl radical

The rate constant of the HONO + OH reaction is slightly increased by hydration.

Atmospheric Chemistry of Perfluoro-3-methyl-2-butanone [CF3C(O)CF(CF3)2]: Photodissociation and Reaction with OH Radicals

Perfluoro-3-methyl-2-butanone (CF₃C(O)CF(CF₃)₂, abbreviated as C5) is a potentially excellent fire suppression alternative to halons and a promising dielectric gas for SF₆ replacement. As a prototypical perfluorinated asymmetrical ketone, photodissociation and reaction with hydroxyl radicals of C5 have been investigated theoretically to gain insights into its atmospheric chemistry and environmental impact. C5 has a broad UV absorption band in the range 260-360 nm with a maximum at 302 nm and the maximal photolysis rate coefficient is 8.3 × 10^-5 s^-1. Photoexcitation from S₀ through the perpendicular n → π* transition produces the excited S₁ species, which can either dissociate straightforwardly via the bifurcated α-CC bond cleavage or be trickled down to T₁ via the S₁/T₁ intersystem crossing (ISC) pathway. In the Franck-Condon region of the S₁ surface, the long-lived S₁ species exists and the slow ISC pathway is dominant, followed by the α-cleavage through T₁ barriers to form perfluoroalkyl and perfluoroacetyl radicals. While the excitation energy exceeds 286 nm, the direct dissociation of C5 though the S₁ barriers takes over before the ISC occurs. Several pathways for regeneration of the ground-state S₀ from S₁ and T₁ via seams of crossing or internal conversion were revealed. The C5 + OH reaction occurs via direct carbonyl addition mechanism followed by the rapid displacement of one of the alkyl groups. Although it can be accelerated considerably by the H₂O-mediated catalysis or the intercepted vibrationally excited quantum states in the hot S₀*, the degradation of C5 by OH radicals is too slow to compete with the photolysis pathways.

Role of water on the H-abstraction from methanol by ClO

The influence of a single water molecule on the reaction mechanism and kinetics of hydrogen abstraction from methanol (CH₃OH) by the ClO radical has been investigated using ab initio calculations. The reaction proceeds through two channels: abstraction of the hydroxyl H-atom and methyl H-atom of CH₃OH by ClO, leading to the formation of CH₃O+HOCl (+H₂O) and CH₂OH+HOCl (+H₂O), respectively. In both cases, pre- and post-reactive complexes were located at the entrance and exit channel on the potential energy surfaces. Results indicate that the formation of CH₂OH+HOCl (+H₂O) is predominant over the formation of CH₃O+HOCl (+H₂O), with ambient rate constants of 3.07×10^-19 and 3.01×10^-23cm³/(molecule·sec), respectively, for the reaction without water. Over the temperature range 216.7-298.2K, the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases. It is therefore concluded that water plays an inhibitive role on the CH₃OH+ClO reaction under tropospheric conditions.

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.

Electronic Structures and OH-Induced Atmospheric Degradation of CF3NSF2: A Potential Green Dielectric Replacement for SF6

Electronic structures of [(trifluoromethyl)imino]sulfur difluoride (CF₃NSF₂) and degradation mechanisms by hydroxyl radical have been investigated using density functional theory (M06-2X), the complete basis set quadratic CBS-Q, and the explicitly correlated coupled-cluster methods [CCSD(T)-F12]. The d-function augmented correlation-consistent basis sets including triple- and quadruple-ξ were employed for the sulfur-containing species. It was found that CF₃NSF₂ exists as two conformations connected by the internal rotation of CF₃ around the central NS bond. The distorted syn conformer is more stable than the symmetrical anti conformer. The nitrogen-sulfur link in CF₃NSF₂ was revealed to be predominantly ionic CF₃N^--⁺SF₂ in structure rather than the conventional N═S double bond on the basis of natural bond orbital analysis. OH radical prefers to attach on the S atom of CF₃NSF₂ along the opposite direction of the S-F bond via a nucleophilic addition mechanism with a barrier of 2.9 kcal/mol whereas the ON association pathway is negligible. Although many product channels are thermodynamically favorable, none of them is kinetically accessible because of the significant barriers along the reaction routes. However, the degradation of CF₃NSF₂ by OH can be accelerated considerably in the presence of a single water molecule, which acts as a bridge for the consecutive proton migration within the floppy cyclic geometries. The half-life of CF₃NSF₂ was estimated to be 2.5 year, and the final products are exclusively CF₃NH and SF₂O. Theoretical calculation supports that CF₃NSF₂ is an environment-friendly green gas. It is worthy of testing its dielectric properties to replace SF₆ for practical use.

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.

Matrix-isolation studies of noncovalent interactions: more sophisticated approaches

Noncovalent interactions are crucial for many physical, chemical, and biological phenomena. Matrix isolation is a powerful method to study noncovalent interactions, including hydrogen-bonded species, and it has been extensively used in this field. However, there are difficult situations, such as in the case of species that are impossible to prepare in the gas phase. In this article, we describe some advanced approaches allowing studies of complexes that are problematic for the traditional methods. Photolysis of a suitable precursor in a matrix can lead to a large concentration of 1:1 complexes, which are otherwise very difficult to prepare (e.g., the H2O···O complex). Photolysis of species combined with annealing can lead to complexes of molecules with mobile atoms (e.g., the same H2O···O complex). Simultaneous photolysis of two species combined with annealing can produce complexes of radicals via reactions of the photogenerated complexes with mobile atoms (e.g., the H2O···HCO complex). Interaction of noble-gas (Ng) hydrides with other species is another topic (e.g., the N2···HArF complex) and very large blue shifts of the H-Ng stretching modes are normally observed for these systems. Complexes and dimers of the higher-energy conformer of formic acid have been prepared by using selective vibrational excitation of the ground-state conformer. The higher-energy conformer of formic acid can be efficiently stabilized in the complexes with strong hydrogen bonding. We also consider some problematic cases when the changes in the vibrational frequencies of the 1:1 complexes are very small (e.g., the phenol···Xe complex) and when the complex formation is prevented by strong solvation in the matrix (e.g., species in solid xenon).

Proton transfer rates in ionized water clusters (H2O)n (n = 2–4)

A proton transfer process is usually dominant in several biological phenomena such as the energy relaxation of photo-excited DNA base pairs and a charge relay process in Ser-His-Glu.

The exothermic HCl + OH·(H2O) reaction: removal of the HCl + OH barrier by a single water molecule

The entrance complex, transition state, and exit complex for the title reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ. The stationary point geometries for the reaction are related to but different from those for the water monomer reaction HCl + OH → Cl + H2O. Our most important conclusion is that the hydrogen-bonded water molecule removes the classical barrier entirely. For the endothermic reverse reaction Cl + (H2O)2, the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by about 4 kcal/mol. The title reaction is exothermic by 17.7 kcal/mol. The entrance complex HCl⋯OH·(H2O) is bound by 6.9 kcal/mol relative to the separated reactants. The classical barrier height for the reverse reaction is predicted to be 16.5 kcal/mol. The exit complex Cl⋯(H2O)2 is found to lie 6.8 kcal/mol below the separated products. The potential energy surface for the Cl + (H2O)2 reaction is radically different from that for the valence isoelectronic F + (H2O)2 system.

Full-dimensional MCTDH/MGPF study of the ground and lowest lying vibrational states of the bihydroxide H3O2(-) complex

In this study, we present a full-dimensional (9D) quantum dynamical analysis of the lowest vibrational eigenstates of H3O2(-). We have made use of the Multiconfiguration Time-Dependent Hartree method in conjunction with both an analytical and a numerical representation of the Kinetic Energy Operator and the newly developed Multigrid POTFIT [D. Peláez, H.-D. Meyer, J. Chem. Phys. 138 (2013) 014108], an algorithm which performs the transformation of a high-dimensional (up to ~12D) Potential Energy tensor into product form. Many sets of top-down Multigrid POTFIT expansions, differing in the system coordinate definition (valence and Jacobi), as well as in the number of terms in the expansion, have been analyzed. Relaxations for the computation of the ground states energies have been carried out on these potentials, obtaining an excellent overall agreement with accurate previous Diffusion Monte Carlo (DMC) calculations, irrespective of the coordinate choice. The 24 lowest excited vibrational states of H3O2(-) have been computed by Block Relaxation and assigned for the first time. This has been carried out in two different pictures, namely: a 7D reduced dimensional one, in which the OH distances have been frozen at the Potential Energy Surface minimum, and a 9D full-dimensional one. The agreement between both descriptions is remarkable. The following fundamental modes have been characterized: OH torsion, OO stretching, OH wagging, OH rocking, and the elusive bridging H stretching. In particular, we provide a very accurate description of the latter in perfect agreement with experiment.

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.

Mobility mechanism of hydroxyl radicals in aqueous solution via hydrogen transfer

The hydroxyl radical (OH*) is a highly reactive oxygen species that plays a salient role in aqueous solution. The influence of water molecules upon the mobility and reactivity of the OH* constitutes a crucial knowledge gap in our current understanding of many critical reactions that impact a broad range of scientific fields. Specifically, the relevant molecular mechanisms associated with OH* mobility and the possibility of diffusion in water via a H-transfer reaction remain open questions. Here we report insights into the local hydration and electronic structure of the OH* in aqueous solution from Car-Parrinello molecular dynamics and explore the mechanism of H-transfer between OH* and a water molecule. The relatively small free energy barrier observed (~4 kcal/mol) supports a conjecture that the H-transfer can be a very rapid process in water, in accord with very recent experimental results, and that this reaction can contribute significantly to OH* mobility in aqueous solution. Our findings reveal a novel H-transfer mechanism of hydrated OH*, resembling that of hydrated OH(-) and presenting hybrid characteristics of hydrogen-atom and electron-proton transfer processes, where local structural fluctuations play a pivotal role.