Hemibonded Structure Research Articles

Isolation and Infrared Spectroscopic Characterization of Hemibonded Water Dimer Cation in Superfluid Helium Nanodroplets.

The structure of the minimum unit of the radical cationic water clusters, the (H2O)2+ dimer, has attracted much attention because of its importance for the radiation chemistry of water. Previous spectroscopic studies indicated that the dimers have a proton-transferred structure (H3O+·OH), though the alternate metastable hemibonded structure (H2O·OH2)+ was also predicted based on theoretical calculations. Here, we produce (H2O)2+ dimers in superfluid helium nanodroplets and study their infrared spectra in the range of OH stretching vibrations. The observed spectra indicate the coexistence of the two structures in the droplets, supported by density functional theory calculations. This is the first spectroscopic identification of the hemibonded isomer of water radical cation dimers. The observation of the higher-energy isomer reveals efficient kinetic trapping for metastable ionic clusters due to the rapid cooling in helium droplets.

The role of exact exchange on the structure of water dimer radical cation: Hydrogen bond vs hemibond.

Understanding the structure and chemical bonding in water dimers is central to the study of many (photo-)electrochemical oxidation reactions. Two structures of the water dimer radical cation, namely, proton-transfer and hemi-bonded structures, have been suggested using density functional theory (DFT) and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]. Both structures are identified by us as local minima, and their relative stability strongly depends on the level of theory. The exact exchange correlates linearly to the energy difference between both local minima. DFT functionals with less than 20 percent exact exchange predict the hemi-bonded structure to be more stable, while more than 20 percent of the exact exchange stabilizes the proton-transfer structure. The latter structure is also confirmed by CCSD(T) benchmark computations. These computations, furthermore, indicate that the oxidized water dimer consists of a hydronium cation (H3O+) and an HO· radical. These results are reproduced by DFT functionals with more than 50% of exact exchange (BHandH, M06-2X, and M06-HF). The transition barrier for the interconversion from the proton-transfer to the hemi-bonded structure is 0.6eV, while the reverse reaction has a barrier of 0.1eV.

Investigation of OH radical in the water nanodroplet during vapor freezing process: An ab initio molecular dynamics study

The hydroxyl radical (OH*) plays a crucial role in chemical processes of earth’s atmosphere. Previous studies have focused on gas-phase OH* or OH* in aqueous solution. However, the vapor freezing process of water nanodroplet as a crucial part of cloud formation in the atmosphere is the subject of lively debate. The vapor freezing process was applied in this work in order to gain insight into the sustainability of the first solvation shell of OH* in water nanodroplet. In this study, we constructed three spherical systems consist of 191 H2O molecules and one OH radical initially placed in different positions of the water nanodroplet. The vapor freezing of nanodroplet revealed the existence of hemibonded structure at low temperatures. Several hydrogen transfers occurred during increasing temperature. Moreover, the structural analysis indicated that in addition to hemibonded water molecule, the other two hydrogen-bonded water molecules located at the opposite sides of the hydroxyl oxygen have key roles in the stability of OH* during the freezing process.

OH radical in water from ab initio molecular dynamics simulation employing hybrid functionals

This work presents for the first time ab initio molecular dynamics simulations for the OH⋆-(H2O)n cluster with n = 0–5 and the OH radical in the bulk phase, using B3LYP as a functional. Furthermore, for OH⋆-w31, simulations with PBE0 and HSE03 are also investigated. In all systems, the OH radical is a stronger hydrogen bond donor than acceptor; a stronger hydrogen bond donor than water and a weaker hydrogen bond acceptor than water. Radial distribution functions (RDFs) reveal that for all systems, neither a hemibond between radical and water nor hydrogen abstraction is present. Comparisons with past simulations indicate that BLYP leads to artifacts, such as overstructuring of water in OH⋆-w31 and the hemibonded structure. In order to have strong hydrogen bonds with an extensive hydrogen bond network, at least four water molecules are necessary. RDFs as well as continuous dimer existence autocorrelation functions show that the OH radical is not disrupting the hydrogen bond network of water. In the bulk phase, the acceptor interaction is a very low probable interaction, whereas in the gas phase, it has a higher probability. The orientation of the water molecule around the OH radical is in the bulk liquid phase much less and in the gas phase enhanced, especially for the OH radical acting as a hydrogen bond donor toward one water molecule. PBE0 results for OH⋆-w31 in a strong hydrogen bond donor interaction compared to HSE03 and B3LYP, which has the weakest interaction. HSE03 leads to strong O–H interactions in OH⋆-w31, followed by PBE0 and B3LYP.

The Hydrogen Abstraction Reaction H2S + OH → H2O + SH: Convergent Quantum Mechanical Predictions.

The hydrogen abstraction reaction H2S + OH → H2O + SH has been studied using the "gold standard" CCSD(T) method along with the Dunning's aug-cc-pVXZ (up to 5Z) basis sets. For the reactant (entrance) complex, the CCSD(T) method predicts a HSH···OH hydrogen-bonded structure to be lowest-lying, and the other lower-lying isomers, including the two-center three-electron hemibonded structure H2S···OH, have energies within 2 kcal/mol. The similar situation is for the product (exit) complex. With the aug-cc-pV5Z single point energies at the aug-cc-pVQZ geometry, the dissociation energy for the reactant complex to the reactants (H2S + OH) is predicted to be 3.37 kcal/mol, and that for the product complex to the products (H2O + SH) is 2.92 kcal/mol. At the same level of theory, the classical barrier height is predicted to be only 0.11 kcal/mol. Thus, the OH radical will react promptly with H2S in the atmosphere. We have also tested the performance of 29 density functional theory (DFT) methods for this reaction. Most of them can reasonably predict the reaction energy, but the different functional give quite different energy barriers, ranged from -10.3 to +2.8 kcal/mol, suggesting some caution in choosing density functionals to explore the PES of chemical reactions.

Hemibonding between Water Cation and Water.

The hemibonding interaction in the water dimer cation is studied using coupled cluster electronic structure methods. The hemibonded dimer cation geometry is a local minimum structure characterized by the two participating monomers having both a very short separation and a near parallel relative orientation. It is shown that the vertically ionized dimer at its optimum neutral geometry can convert to the hemibonded dimer cation structure with essentially no energetic hindrance. Direct conversion to the hemibonded structure is therefore an energetically facile alternative to the minimum energy path that connects the vertically ionized neutral water dimer to the global minimum proton-transferred structure. A substantial barrier must be surmounted to convert the hemibonded dimer cation to the proton-transferred structure. The optical absorption spectrum of the hemibonded dimer cation is characterized by three excited near-UV states, two of which have very large oscillator strengths. Relative resonance Raman intensities are estimated for the hemibonded dimer cation vibrational modes, finding the intermolecular stretching mode to be the most strongly enhanced when in near resonance with each of the near-UV excited states, and the anharmonicity and overtones of this mode are estimated. These results provide guidance for the possible observation of hemibonded cations in irradiated liquid water.

Structures and stabilities of asymmetrical dimer radical cation systems (AH3–H2O)+ (A=N, P, As)

Structures, stabilities, binding energies, and bonding nature of the dimer radical cation systems (AH3–H2O)+ (A=N, P, As) have been studied using B3LYP, MP2, and CCSD methods. The local minima of (NH3–H2O)+ include a hemi-bonded structure (H3N+···OH2), a proton-transferred hydrogen-bonded structure (H3NH+···OH), and a non-proton-transferred hydrogen-bonded structure (H2N–H+···OH2) which is the most stable conformer. Both the P and As complexes have four local minima. In addition to the three structures found for the nitrogen analog, a second proton-transferred hydrogen-bonded structure (H2A···H–OH2 +) was found. For the P complexes, the hemi-bonded structure (H3P+···OH2) is the most stable complex, while for the As compounds the hemi-bonded structure (H3As+···OH2) and hydrogen-bonded structure (H2As···H–OH2 +) are the most stable ones. The atomic spin density and charge population and properties at the bond critical points have been calculated to study the structures and bonding characters of these dimer radical cations.

The structure and bonding of mixed component radical cation clusters

Basin hopping in conjunction with second order Møller–Plesset perturbation theory is used to characterise the lowest energy isomers of mixed component radical cation clusters of the form [H2O-X]+, [(H2O)2-X]+ and [H2O-X2]+, where X=PH3, H2S and HCl, with the relative energies refined using coupled cluster theory calculations. For the dimers where X=H2S or HCl, a proton transfer based structure comprising H3O+ and SH or Cl radicals has the lowest energy structure whereas for X=PH3 a hemibonded structure is most stable. For the trimers, a much wider range of possible isomers based upon both proton transfer and hemibonded structural motifs is observed.

Stability of Hemi-Bonded vs Proton-Transferred Structures of (H2O)2+, (H2S)2+, and (H2Se)2+ Studied with Projected Hartree–Fock Methods

Hartree-Fock (HF) is known to suffer from drawbacks in the description of the relative stabilities between the hemi-bonded (HB) and proton-transferred (PT) isomers of the water dimer cation, (H2O)2(+). The energy difference predicted by HF is too large, approximately 27 kcal/mol, which is lowered to 7 kcal/mol when correlation effects are added. The error in HF has been previously attributed to the large dynamic correlation effects in the HB structure as well to the large symmetry breaking this structure exhibits. In this study we use the recently developed projected Hartree-Fock (PHF) methods to study the relative stability of the two isomers of (H2O)2(+) as well as its second and third row analogs, namely, (H2S)2(+) and (H2Se)2(+). In PHF, symmetries are broken and restored in a variation-after-projection approach and thus can deal easily with systems for which HF itself spontaneously breaks symmetry. We use different flavors of PHF (SUHF, KSUHF, SGHF, and KSGHF) to explore their ability in capturing dynamic correlation effects and to compare their performance to different wave function based methods. We study the role of the symmetry-breaking in the above systems, using wave function based methods with unrestricted and restricted wave functions as well as performing a single-shot symmetry restoration (a projection-after-variation scheme).

Hydrogen Bonding in Neutral and Cation Dimers of H2Se with H2O, H2S, and H2Se

Structures, hydrogen bonding, and binding energies of H(4)SeA (A = O, S, Se) dimers and their cation radicals have been studied using DFT-B3LYP, MP2, and CCSD methods with 6-31++G(d,p), cc-pVTZ, and aug-cc-pVTZ basis sets. The binding energy (BE) order of the most stable neutral and cationic dimers have been found to be (H(2)Se···HOH) > (H(2)Se···HSH) > (H(2)Se···HSeH), and (H(2)Se···SeH(2))(+) > (H(2)Se···SH(2))(+) > (HSe···HOH(2))(+), respectively, by B3LYP/6-31++G(d,p) and MP2/aug-cc-pVTZ methods. Higher electronegativity of the heteroatom has been found to result in more stability of the neutral dimer but less of the cationic dimer. Among neutral dimers, structure with more electronegative heteroatom acting as proton donor has been found to be more stable. However, the hemibonded structure has been found to be more stable for the dimer cation radical unless the ionization potentials of the involved heteroatoms are very different, e.g., H(4)SeO(+). Vibrational frequency calculation suggests that an increase in electronegativity of A-atom results in a decrease in Se-H bond strength in H(4)SeA and H(4)SeA(+) dimers. The calculated values of Mulliken atomic charge/spin and hydrogen bond lengths of the dimers and their radical cations have also been discussed.

Assessment of density functional approximations for the hemibonded structure of the water dimer radical cation

Due to the severe self-interaction errors associated with some density functional approximations, conventional density functionals often fail to dissociate the hemibonded structure of the water dimer radical cation (H(2)O)(2)(+) into the correct fragments: H(2)O and H(2)O(+). Consequently, the binding energy of the hemibonded structure (H(2)O)(2)(+) is not well-defined. For a comprehensive comparison of different functionals for this system, we propose three criteria: (i) the binding energies, (ii) the relative energies between the conformers of the water dimer radical cation, and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional, ωB97X-2(LP) [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2009, 131, 174105], is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.

Hydroxyl Radical and Hydroxide Ion in Liquid Water: A Comparative Electron Density Functional Theory Study

Ab initio density functional theory molecular dynamics simulations of the solvated states of the hydroxyl radical and hydroxide ion are performed using the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional (Becke, A. D. Phys. Rev. A 1988, 38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785). The structures of the solvation shells of the two species are examined. It is found that the OH radical forms a relatively well-defined solvation complex with four neighboring water molecules. Three of these molecules are hydrogen bonded to the OH, while the fourth is hemibonded via a three-electron two-centered bond between the oxygen atoms of the OH and water. The activity and the diffusion mechanism of the OH radical in water is discussed in comparison with the OH- ion. Although the results are partially influenced by the tendency of the BLYP density functional to overestimate hemibonded structure, the present simulations suggest that the widely accepted picture of rapid diffusion of OH radical in water through hydrogen exchange reaction may need to be reconsidered.

Experimental and theoretical characterization of H(2)OOO(+).

This report presents the preparation and characterization of H2OOO+, an important intermediate in water-oxygen chemistry. The H2OOO+ cation was produced by co-deposition of H2O/Ar with radio frequency discharged O2/Ar at 4 K and was identified by four fundamental infrared absorptions. Quantum chemical calculations indicate a doublet ground state with a H2O-O2 hemi-bonded Cs structure.

Ab Initio CASSCF and DFT Investigations of (H2O)2+and (H2S)2+: Hemi-Bonded vs Proton-Transferred Structure

High level ab initio calculations using a complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange−correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H2O)2+ and (H2S)2+ species. For (H2O)2+, DFT calculation using conventional exchange−correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree−Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H2S)2+ system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange−correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorporated through MCQDPT2 is found to be much stronger in (H2O)2+ as compared to (H2S)2+. An analysis of the nature of interactions involved in the (H2O)2+ and (H2S)2+ systems within the framework of Bader's topological theory of atoms in molecules is also presented through the plots of the Laplacian ∇2ρ of the electron density ρ(r) and also other related quantities at the bond critical points with the objective of rationalizing the relative stability of the two isomers in both (H2O)2+ and (H2S)2+.

Ab initio study of the phenol-water cation radical

The phenol-water cation radical has been investigated by ab initio theory using the spin-restricted open-shell Hartree–Fock and spin-restricted open-shell second-order Mo/ller–Plesset theories with 3-21G*(O) and 6-31G* basis sets. The full geometrical optimization was performed for several hydrogen-bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol-localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol-water cation radical are compared with those of the corresponding neutral complex.