Communication: Covalent nature of X⋯H2O (X = F, Cl, and Br) interactions

Infrared Spectroscopy of [H2O-N2O]+-(H2O)n (n = 1 and 2): Microhydration Effects on the Hemibond

The hemibond, a nonclassical covalent bond involving three electrons shared between two centers, has attracted considerable attention due to its significance in radiation chemistry. Water radical cation clusters, [H2O-X]+, exhibit two primary bonding motifs: the hemibond and the hydrogen bond. Although hydrogen bond formation typically dominates, recent studies have identified instances of hemibond formation in some systems involving water molecules. This study focuses on the [H2O-N2O]+ radical cation cluster, a rare system exhibiting hemibond formation. We investigate the stability of this hemibond in [H2O-N2O]+ against microhydration by employing infrared photodissociation spectroscopy and conducting theoretical calculations on [H2O-N2O]+-(H2O)n (n = 1 and 2). By comparing experimental and simulated spectra, we determined the predominant intermolecular bonding motifs in [H2O-N2O]+-(H2O)n (n = 1 and 2). Our analysis revealed that proton-transferred-type isomers are almost exclusively populated for n = 1 and 2, whereas hemibonded-type isomers are energetically unfavorable. These findings indicate that microhydration disrupts the hemibond and shifts the stable structural motifs.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers

Much attention has been paid to the dynamics of both activated gas-phase bimolecular reactions, which feature monotonically increasing integral cross sections and Arrhenius kinetics, and their barrierless capture counterparts, which manifest monotonically decreasing integral cross sections and negative temperature dependence of the rate coefficients. In this Perspective, we focus on the dynamics of gas-phase bimolecular reactions with submerged barriers, which often involve radicals or ions and are prevalent in combustion, atmospheric chemistry, astrochemistry, and plasma chemistry. The temperature dependence of the rate coefficients for such reactions is often non-Arrhenius and complex, and the corresponding dynamics may also be quite different from those with significant barriers or those completely dominated by capture. Recent experimental and theoretical studies of such reactions, particularly at relatively low temperatures or collision energies, have revealed interesting dynamical behaviors, which are discussed here. The new knowledge enriches our understanding of the dynamics of these unusual reactions.

Potential energy profile for the Cl + (H2O)3 → HCl + (H2O)2OH reaction. A CCSD(T) study

Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H₂O)₃ → HCl + (H₂O)₂OH using a standard DFT method. As found for the analogous fluorine reaction, the geometrical and energetic results for the four chlorine pathways are closely related. However, the energetics for the Cl reaction are very different from those for fluorine. In the present paper, we investigate the lowest-energy chlorine pathway using the "gold standard" CCSD(T) method in conjunction with correlation-consistent basis sets up to cc-pVQZ. Structurally, the stationary points for the water trimer reaction Cl + (H₂O)₃ may be compared to those for the water monomer reaction Cl + H₂O and water dimer reaction Cl + (H₂O)₂. Based on the CCSD(T) energies, the title reaction is endothermic by 19.3 kcal mol^-1, with a classical barrier height of 16.7 kcal mol^-1 between the reactants and the exit complex. There is no barrier for the reverse reaction. The Cl⋯(H₂O)₃ entrance complex lies 5.3 kcal mol^-1 below the separated reactants. The HCl⋯(H₂O)₂OH exit complex is bound by 8.6 kcal mol^-1 relative to the separated products. The Cl + (H₂O)₃ reaction is somewhat similar to the analogous Cl + (H₂O)₂ reaction, but qualitatively different from the Cl + H₂O reaction. It is reasonable to expect that the reactions between the chlorine atom and larger water clusters may be similar to the Cl + (H₂O)₃ reaction. The potential energy profile for the Cl + (H₂O)₃ reaction is radically different from that for the valence isoelectronic F + (H₂O)₃ system, which may be related to the different bond energies between HCl and HF.

The Role of Zero-Point Vibration and Reactant Attraction in Exothermic Bimolecular Reactions with Submerged Potential Barriers: Theoretical Studies of the R + HBr → RH + Br (R = CH3, HO) Systems

The dynamics of the reactions CH₃ + HBr → CH₄ + Br and HO + HBr → H₂O + Br have been studied using the quasiclassical trajectory method to explore the interplay of the vibrational excitation of the breaking bond and the potential energy surface characterized by a prereaction van der Waals well and a submerged barrier to reaction. The attraction between the reactants is favorable for the reaction, because it brings together the reactants without any energy investment. The reaction can be thought to be controlled by capture. The trajectory calculations indeed provide excitation functions typical to capture: the reaction cross sections diverge when the collision energy is reduced toward zero. Excitation of reactant vibration accelerates both reactions. The barrier on the potential surface is so early that the coupling between the degrees of freedom at the saddle point geometry is negligible. However, the trajectory calculations show that when the breaking bond is stretched at the time of the encounter, an attractive force arises, as if the radical approached a HBr molecule whose bond is partially broken. As a result, the dynamics of the reaction are controlled more by the temporary "dynamical", vibrationally induced than by the "static" van der Waals attraction even when the reactants are in vibrational ground state. The cross sections are shown to drop to very small values when the amplitude of the breaking bond's vibration is artificially reduced, which provides an estimate of the reactivity due to the "static" attraction. Without zero-point vibration these reactions would be very slow, which is a manifestation of a unique quantum effect. Reactions where the reactivity is determined by dynamical factors such as the vibrationally enhanced attraction are found to be beyond the range of applicability of Polanyi's rules.

Infrared Spectroscopy and Anharmonic Vibrational Analysis of (H2O-Krn)+ (n = 1-3): Hemibond Formation of the Water Radical Cation

The hemibond is a nonclassical covalent bond formed between a radical (cation) and a closed shell molecule. The hemibond formation ability of water has attracted great interest, concerning its role in ionization of water. While many computational studies on the water hemibond have been performed, clear experimental evidence has been hardly reported because the hydrogen bond formation overwhelms the hemibond formation. In the present study, infrared photodissociation spectroscopy is applied to (H₂O-Kr_n)⁺ (n = 1-3) radical cation clusters. The observed spectra of (H₂O-Kr_n)⁺ are well reproduced by the anharmonic vibrational simulations based on the hemibonded isomer structures. The firm evidence of the hemibond formation ability of water is revealed.

A Time-Independent Quantum Approach to Ro-vibrationally Inelastic Scattering between Atoms and Triatomic Molecules

A full-dimensional time-independent quantum mechanical theory for ro-vibrationally inelastic scattering of triatomic molecules with atoms is formulated. The Jacobi-Radau coordinate system used in the calculation allows not only a near perfect description of the vibrational problem but also the adaptation of the exchange symmetry for A₂B type triatoms. The S-matrix elements are obtained by solving the close-coupling equations with contracted basis using the log-derivative method. This method is applied to the inelastic scattering of the water molecule by a chlorine atom, which sheds light on the energy gap law in energy transfer in atom-triatom collisions.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl^•(H₂O)_n=1-4. Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H₂O)_n-1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical-water complexes.

Influence of the microsolvation on hemibonded and protonated hydrogen sulfide: infrared spectroscopy of [(H2S)n(X)1]+ and H+(H2S)n(X)1 (n = 1 and 2, X = water, methanol, and ethanol)

Changes of the excess charge accommodation motif in hemibonded and protonated hydrogen sulfide by microsolvation are studied by infrared spectroscopy of [(H2S)n(X)1]+ and H+(H2S)n(X)1 (n = 1 and 2, X = water, methanol, and ethanol) clusters. While the hemibond in the (H2S)2+ ion core is stable to the microhydration by a single water molecule, the hemibond is broken by the proton transfer with the microsolvation by a single methanol or ethanol molecule. Hetero hemibond formation between hydrogen sulfide and these solvent molecules is not observed. On the other hand, the excess proton in H+(H2S)n can be easily transferred to the solvent molecule, even though the proton affinity of the solvent molecule is lower than that of hydrogen sulfide. Implications of these results to the charge accommodation by sulfur under the biological conditions are discussed.

Quantum dynamics of ClH2O- photodetachment: Isotope effect and impact of anion vibrational excitation

Photodetachment of the ClH₂O^- anion is investigated using full-dimensional quantum mechanics on accurate potential energy surfaces of both the anion and neutral species. Detailed analysis of the photoelectron spectrum and the corresponding wavefunctions reveals that the photodetachment leads to, in the product channel of the exothermic HCl + OH → Cl + H₂O reaction, the formation of numerous Feshbach resonances due apparently to slow energy transfer from H₂O vibrational modes to the dissociation coordinate. These long-lived resonances can be grouped into two broad peaks in the low-resolution photoelectron spectrum, which is in good agreement with available experiments, and they are assigned to the ground and first excited OH stretching vibrational manifolds of H₂O complexed with Cl. In addition, effects of isotope substitution on the photoelectron spectrum were small. Finally, photodetachment of the vibrationally excited ClH₂O^- in the ionic hydrogen bond mode is found to lead to Feshbach resonances with higher stretching vibrational excitations in H₂O.

Correction: Spectroscopic observation of two-center three-electron bonded (hemi-bonded) structures of (H2S)n+ clusters in the gas phase

Correction for ‘Spectroscopic observation of two-center three-electron bonded (hemi-bonded) structures of (H₂S)_n⁺ clusters in the gas phase’ by Dandan Wang et al., Chem. Sci., 2017, 8, 2667–2670.

Accurate Determination of Tunneling-Affected Rate Coefficients: Theory Assessing Experiment

The thermal rate coefficients of a prototypical bimolecular reaction are determined on an accurate ab initio potential energy surface (PES) using ring polymer molecular dynamics (RPMD). It is shown that quantum effects such as tunneling and zero-point energy (ZPE) are of critical importance for the HCl + OH reaction at low temperatures, while the heavier deuterium substitution renders tunneling less facile in the DCl + OH reaction. The calculated RPMD rate coefficients are in excellent agreement with experimental data for the HCl + OH reaction in the entire temperature range of 200-1000 K, confirming the accuracy of the PES. On the other hand, the RPMD rate coefficients for the DCl + OH reaction agree with some, but not all, experimental values. The self-consistency of the theoretical results thus allows a quality assessment of the experimental data.

Kinetics and dynamics of the C(3P) + H2O reaction on a full-dimensional accurate triplet state potential energy surface

A full-dimensional accurate PES for the C(³P) + H₂O reaction is developed using the PIP-NN method.

Dynamics of transient speciesviaanion photodetachment

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

A global coupled cluster potential energy surface for HCl + OH ↔ Cl + H2O

A new and more accurate full-dimensional global potential energy surface (PES) for the ground electronic state of the ClH₂O system is developed by using the permutation invariant polynomial-neural network (PIP-NN) method to fit 15 777 points obtained using an explicitly correlated unrestricted coupled-cluster method with single, double, and perturbative triple excitations (UCCSD(T)-F12b).

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.

Theoretical calculation of the vibrational state dependent photodetachment spectra of X-H2O, X = F, Cl, Br

Vibrational spectra of X^-H₂O (X = F, Cl, Br) were simulated using full dimensional vibrational calculations using quantum chemistry potential energy surfaces. Furthermore, utilizing the reflection approximation, we simulated the photodetachment spectra obtained from different vibrational excited states. From these spectra, we can observe changes in the hydrogen bond interaction between the anion and the neutral XH₂O system. Notably, for F^-H₂O, the excitation of the ionic hydrogen bonded (IHB) OH stretching vibration generates a large tail on the low energy side of the photodetachment spectra compared to the detachment from the zero-point vibration state. This shows that the IHB OH stretching vibration of F^-H₂O causes charge delocalization from F^- to the oxygen atom in H₂O, and that the photodetachment from FHOH^- occurs at lower energies.

Control of chemical reactivity by transition-state and beyond

It has been long established that the transition state for an activated reaction controls the overall reactivity, serving as the bottleneck for reaction flux. However, the role of the transition state in regulating quantum state resolved reactivity has only been addressed more recently, thanks to advances in both experimental and theoretical techniques. In this perspective, we discuss some recent advances in understanding mode-specific reaction dynamics in bimolecular reactions, mainly focusing on the X + H2O/CH4 (X = H, F, Cl, and O(3P)) systems, extensively studied in our groups. These advances shed valuable light on the importance of the transition state in mode-specific and steric dynamics of these prototypical reactions. It is shown that many mode-specific phenomena can be understood in terms of a transition-state based model, which assumes in the sudden limit that the ability of a reactant mode for promoting the reaction stems from its coupling with the reaction coordinate at the transition state. Yet, in some cases the long-range anisotropic interactions in the entrance (or exit) valley, which govern how the trajectories reach (or leave) the transition state, also come into play, thus modifying the reactive outcomes.

Reactive and Nonreactive Feshbach Resonances Accessed by Photodetachment of FH2O(-)

The photodetachment of the FH2O(-) anion is investigated quantum mechanically on accurate full-dimensional potential energy surfaces of the two lowest-lying electronic states of FH2O. The calculated photoelectron spectrum possesses both broad and sharp features, corresponding to reactive and nonreactive Feshbach resonances. The former extend to both reactant and product channels over the transition state, while the latter are supported by a hydrogen bonded HO-HF well in the product channel. Many of the resonances are assignable with quantum numbers for the stretching and bending modes of the HO-HF complex as well as the H-F vibration. The implications of these resonances in the F + H2O ↔ HF + HO reaction are discussed.

Modulations of Transition-State Control of State-to-State Dynamics in the F + H2O → HF + OH Reaction

The full-dimensional quantum dynamics of the F + H2O → HF + OH reaction is investigated at the state-to-state level for the first time using a transition-state wave packet method on an accurate global potential energy surface. It is found that the H2O rotation enhances the reactivity and the product-state distribution is dominated by HF vibrational excitation while the OH moiety serves effectively as a spectator. These observations underscore the transition-state control of the reaction dynamics, as both the H2O rotational and HF vibrational modes are strongly coupled to the reaction coordinate at the transition state. It is also shown that the transition-state dominance of the reaction dynamics is modulated by other features on the potential energy surface, such as the prereaction well.

Insights into the bond-selective reaction of Cl + HOD(nOH) → HCl + OD

The theoretical study of the title reaction reveals strong bond selectivity, vibrational enhancement, and unique product distributions, in agreement with the experiment.

Energy disposal and thermal rate constants for the OH + HBr and OH + DBr reactions: quasiclassical trajectory calculations on an accurate potential energy surface

We report reaction cross sections, energy disposal, and rate constants for the OH + HBr → Br + H2O and OH + DBr → Br + HDO reactions from quasiclassical trajectory calculations using an ab initio potential energy surface [ de Oliveira-Filho , A. G. S. ; Ornellas , F. R. ; Bowman , J. M. J. Phys. Chem. Lett. 2014 , 5 , 706 - 712 ]. Comparison with available experiments are made and generally show good agreement.

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.

Quasiclassical Trajectory Calculations of the Rate Constant of the OH + HBr → Br + H2O Reaction Using a Full-Dimensional Ab Initio Potential Energy Surface Over the Temperature Range 5 to 500 K

We report a permutationally invariant, ab initio potential energy surface (PES) for the OH + HBr → Br + H2O reaction. The PES is a fit to roughly 26 000 spin-free UCCSD(T)/cc-pVDZ-F12a energies and has no classical barrier to reaction. It is used in quasiclassical trajectory calculations with a focus on the thermal rate constant, k(T), over the temperature range 5 to 500 K. Comparisons with available experimental data over the temperature range 23 to 416 K are made using three approaches to treat the OH rotational and associated electronic partition function. All display an inverse temperature dependence of k(T) below roughly 160 K and a nearly constant temperature dependence above 160 K, in agreement with experiment. The calculated rate constant with no treatment of spin-orbit coupling is overall in the best agreement with experiment, being (probably fortuitously) within 20% of it.

Aqueous production of oxygen atoms from hydroxyl radicals

Car–Parrinello MD simulations and advanced visualization techniques of OH*-pair encounters in water demonstrate the formation of the triplet oxygen atom.