OH Dissociation Research Articles

High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis

Water adsorption and dissociation on the Fe(100) surface at different coverages have been calculated using density functional theory methods and ab initio thermodynamics. For the adsorption of (H2O)n clusters on the (3 × 4) Fe(100) surface, the adsorption energy is contributed by direct H2O–Fe interaction and hydrogen bonding. For n = 1–3, direct H2O–Fe interaction is dominant, and hydrogen bonding becomes more important for n = 4–5. For n = 6–8 and 12, structurally different adsorption configurations have very close energies. Monomeric H2O dissociation is more favored on the clean Fe(100) surface than that on H2O or OH precovered surfaces. O-assisted H2O dissociation is favorable kinetically (O + H2O = 2OH), and further OH dissociation is roughly thermo-neutral. With the increase of surface O coverage (nO, n = 2–7), further H2O dissociation has similar potential energy surfaces, and H2 formation from surface adsorbed H atoms becomes easy, while the desorption energy is close to zero for n = 7. The calcul...

Switchable bifunctional stimuli-triggered poly-N-isopropylacrylamide/DNA hydrogels.

DNA-tethered poly-N-isopropylacrylamide copolymer chains, pNIPAM, that include nucleic acid tethers have been synthesized. They are capable of inducing pH-stimulated crosslinking of the chains by i-motif structures or to be bridged by Ag(+) ions to form duplexes. The solutions of pNIPAM chains undergo crosslinking at pH 5.2 or in the presence of Ag(+) ions to form hydrogels. The hydrogels reveal switchable hydrogel-to-solution transitions by the reversible crosslinking of the chains at pH 5.2 and the separation of the crosslinking units at pH 7.5, or by the Ag(+) ion-stimulated crosslinking of the chains and the reverse dissolution of the hydrogel by the cysteamine-induced elimination of the Ag(+) ions. The DNA-crosslinked hydrogels are thermosensitive and undergo reversible temperature-controlled hydrogel-to-solid transitions. The solid pNIPAM matrices are protected against the OH(-) or cysteamine-stimulated dissociation to the respective polymer solutions.

Ab initio reaction pathways for photodissociation and isomerization of nitromethane on four singlet potential energy surfaces with three roaming paths.

Photodissociation pathways of nitromethane following π → π(*) electronic excitation are reported. The potential energy surfaces for four lowest singlet states are explored, and structures of many intermediates, dissociation limits, transition states, and minimum energy conical intersections were determined using the automated searching algorism called the global reaction route mapping strategy. Geometries are finally optimized at CASSCF(14e,11o) level and energies are computed at CAS(14o,11e)PT2 level. The calculated preferable pathways and important products qualitatively explain experimental observations. The major photodissociation product CH3 and NO2 ((2)B2) is formed by direct dissociation from the S1 state. Important pathways involving S1 and S0 states for production of various dissociation products CH3NO + O ((1)D), CH3O(X(2)E) + NO (X(2)Π), CH2NO + OH, and CH2O + HNO, as well as various isomerization pathways have been identified. Three roaming processes also have been identified: the O atom roaming in O dissociation from CH3NO2, the OH radical roaming in OH dissociation from CH2N(O)(OH), and the NO roaming in NO dissociation from CH3ONO.

Investigation of oxidation attack sites in sterols: Thermodynamics of hydrogen atom transfer

B3LYP study of 7 naturally occurring sterols was carried out in terms of OH and CH bond dissociation enthalpies (BDE) to identify thermodynamically favored oxidation attack site. In all compounds, OH BDEs are significantly higher than CH BDEs. Found results showed that sterol nuclei are more susceptible to the oxidation than side chains. Lowest BDE values were found for the C14H bond in the majority of molecules with C14 atom in the α-position to a CC double bond. C5C6C7C8 alternating double bonds cause further drop in C14H BDE. Because the nuclei are responsible for the reported oxidation instability of sterols, 15 various steroid nuclei were also investigated.

Density functional theory study into H2O dissociative adsorption on the Fe5C2(0 1 0) surface

Spin-polarized density functional theory calculations (GGA–PBE) have been carried out to study H2O adsorption and dissociation on the Fe5C2(010) surface. It is found that the iron region on the Fe5C2(010) surface is active for H2O adsorption and dissociation, while the carbon region is inactive. For H2O adsorption in the iron region, H2O prefers the top site of the surface iron atoms, and significant hydrogen bonding interaction has been found at high H2O coverage on the basis of the computed adsorption energies and the intermolecular O–H distances. In the iron region H2O dissociation (H2O→H+OH; OH→H+O) is favored both kinetically and thermodynamically. On one O pre-covered surface, O-assisted H2O dissociation becomes favorable kinetically (O+H2O→OH+OH) and further OH dissociation (OH→H+O) becomes difficult thermodynamically. Upon the increase of surface O coverage, H2O dissociation becomes difficult, while H2 formation from the surface adsorbed H atoms becomes easy. On the potential energy surface, the dissociation of four H2O molecules into four surface O and four H2 molecules (4H2O(g)→4O(s)+4H2(g)) is still thermodynamically favorable by 0.63eV, and the iron region is fully covered by surface oxygen atoms. Thermodynamic analysis reveals clearly that the catalyst surface has always adsorbed oxygen atoms under water environment and their number in the iron region depends on temperature and water content; and high temperature and low H2O partial pressure can maintain the catalyst stability and excess H2O partial pressure will result in full oxidation. For the oxidation of one surface carbon atom, it is necessary to migrate one of the four adsorbed oxygen atoms from the iron region to the carbon region, and the H2O assisted CO2 formation is more favorable than the direct CO2 formation. The overall surface carbon oxidation, FexCy+4H2O(g)→O2FexCy–1+CO2(g)+4H2(g), is thermodynamically accessible. Detailed comparisons show that the Fe(100) and Fe5C2(010) surfaces are very similar in H2O adsorption and dissociation at low coverage.

Photodissociation Dynamics of Enolic 1,2-Cyclohexanedione at 266, 248, and 193 nm: Mechanism and Nascent State Product Distribution of OH

The photodissociation dynamics of 1,2-cyclohexanedione (CHD), which exists in enolic form in gas phase, is studied using pulsed laser photolysis (LP)-laser induced fluorescence (LIF) "pump-and-probe" technique at room temperature. The nascent state distribution of the OH radical, formed after initial photoexcitation of the molecule to it is (π, π*) and Rydberg states, is determined. The initial (π, π*) and Rydberg states are prepared by excitation with the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in the vibrationally cold state at all the above wavelengths of excitation but differ in rotational state distributions. At 266 nm photolysis, the rotational population of OH shows a curvature in Boltzmann plot, which is fairly described by two types of Boltzmann-like distributions characterized by rotational temperatures of 3100 ± 100 and 900 ± 80 K. However, at 248 nm photolysis, the rotational distribution is described by a single rotational temperature of 950 ± 80 K. The spin-orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy in the center-of-mass coordinate, partitioned into the photofragment pairs of the OH formation channels, is determined to be 12.5 ± 3.0, 12.7 ± 3.0, and 12.0 ± 3.0 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH formation channel, detailed ab initio calculations are performed using configuration interaction-singles (CIS) method. It is proposed that at 266 nm photolysis, the OH fragment is formed from two different excited state structures, one with a strong H bonding, similar to that in the ground state, and another without effective H bonding, whereas, at 248 nm photodissociation, it seems that the OH formation occurs mainly from the excited state, which lacks effective H-bonding. At 193 nm excitation, the initially prepared population in the Rydberg state crosses over to a nearby σ* repulsive state along the C-O bond, from where the dissociation takes place. The exit barrier for the OH dissociation channel is estimated to be 14 kcal/mol. The existence of dynamical constraint due to strong hydrogen bond in the ground state is effectively present in the dissociation process at 266 and somewhat deficient at 248 nm photolysis.

Quantum Chemical Molecular Dynamics Study of the Water–Gas Shift Reaction on a Pd/MgO(100) Catalyst Surface

The water–gas shift (WGS) reaction on a Pd/MgO(100) catalyst surface was studied using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. Molecular adsorption of CO was observed. In contrast, we observed that H2O adsorption occurs first molecularly but the molecule then dissociates on the surface. The resultant hydroxyl group reacts with preadsorbed CO to form an OCOH intermediate and a single H atom. This process is relevant as the initial hydroxylation step, and it is part of the catalyzed hydrolysis mechanism. During the molecular dynamics simulation the OCOH intermediate inverted into an H–CO2 like molecule and finally HCO2 decomposed to CO2 and H. Later on, the resultant H interacts with the previously dissociated single H atom (H released from the H–OH dissociation) and forms the WGS product H–H molecule. It was observed that the CO2 desorbed from the supported Pd cluster while the H2 molecule remains attached to the Pd cluster during the simulation. The geometries and dissociation energies of water molecules were obtained and the type of adsorption assessed. Chemical changes, changes in electronic and adsorption states, and structural changes were also investigated through TB-QCMD calculations, which indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of the OCOH intermediate. The present study indicates that the MgO(100) support has a strong interaction with the Pd catalyst, which may cause an increase in Pd activity as well as enhancement of the metal catalyst dispersion, hence, increasing the rate of the WGS reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of the WGS reaction.

Ionization dynamics of the water trimer: A direct ab initio MD study

Ionization dynamics of the cyclic water trimer (H2O)3 have been investigated by means of direct ab initio molecular dynamics (AIMD) method. Two reaction channels, complex formation and OH dissociation, were found following the ionization of (H2O)3. In both channels, first, a proton was rapidly transferred from H2O+ to H2O (time scale is ∼15fs after the ionization). In complex channel, an ion–radical contact pair (H3O+–OH) solvated by the third water molecule was formed as a long-lived H3O+(OH)H2O complex. In OH dissociation channel, the second proton transfer further takes place from H3O+(OH) to H2O (time scale is 50–100fs) and the OH radical is separated from the H3O+. At the same time, the OH dissociation takes place when the excess energy is efficiently transferred into the kinetic energy of OH radical. The OH dissociation channel is significantly minor, and almost all product channels were the complex formation. The reaction mechanism was discussed on the basis of theoretical results.

The role of an oxometallic complex in OH dissociation during water oxidation: a microscopic insight from DFT study

The uncatalyzed atomic dissociation of water requires breaking of a strong O–H bond with an enthalpy of 494 kJ mol−1, which necessitates the understanding and designing of appropriate catalysts. Here we employ transition state theory within quantum chemical density functional theory to understand the role of metal-oxide inorganic complexes in the OH → O + H process, the most important reaction in water oxidation. We study the effect of (a) chemical bonding in different M4O4 (M = Mn, Co) cubane complexes, (b) heterocubane geometry containing Ca, in addition to a transition metal ion, (c) dimensionality by considering both three-dimensional and two-dimensional geometry of the oxometallic unit, and (d) connectivity between two oxometallic cubane units, corner shared versus edge shared geometry. Analysis of our density functional theory based calculations singles out a robust microscopic quantity among various plausible and competing factors, which elucidates the important role of metal–oxygen covalency at the oxidized site. The M–O bonding strength inversely determines the strength of the O–H bond, and thus the energy required for OH dissociation. This provides one with an important microscopic design principle for a metal-oxide complex catalyst responsible for water oxidation.

Photodissocaition Dynamics of Propiolic Acid at 212 nm: The OH Production Channel

transition by time-dependent density functional theory calculations. Potential energy surfacesof both the ground and electronically excited states were obtained employing quantum chemical calculations.It was suggested that the dissociation of OH from propiolic acid excited at 212 nm should take place along theS

Dissociative Adsorption and Aggregation of Water on the Fe(100) Surface: A DFT Study

Aggregation, molecular adsorption, and dissociation of water on the Fe(100) surface were investigated using spin-polarized density functional calculations. The preferential sites for H2O, HO, O, and H were carefully investigated on this surface. Also, the dissociation of H2O into H + OH species, and further OH into O + H species, was examined. The charge transfer mechanism during these dissociation processes, as well as of small water aggregates at different orientations on the Fe(100) surface, was studied within the Bader charge analysis. The coverage dependence on the adsorption properties was examined by comparing the results of a (2 × 2) with a (3 × 3) supercell. These calculations predicted that H2O is weakly adsorbed (physisorption) on hollow, bridge, and on-top sites, with the on-top site being slightly preferred for both coverages of 0.11 and 0.25 monolayer. As expected, OH was predicted to be strongly adsorbed (chemisorption) on the Fe(100) sites, producing a large charge transfer from the surface to p-orbitals of the O atom. A dissociation barrier of about 1.0 eV, for the dissociation H2O → OH + H, was calculated from the on-top site to the next most stable bridge and hollow sites, respectively. In contrast, a smaller barrier of ca. 0.8 eV was calculated for the dissociation of OH. Regarding the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule.

Electron-impact dissociation and ionization of OH+ and OD+ ions

Absolute cross sections for electron impact ionization and dissociation of OH+ and OD+ leading to the formation of the OH2+, O+, O2+, O3+ and D+ ions have been measured by applying the animated electron-ion beam method in the energy range from the respective reaction thresholds up to 2.5 keV. The maximum of the single ionization cross section is found to be (0.95 ± 0.02) × 10-19 cm2 at 155 eV. The maximum total cross sections for O+ and D+ fragments production are observed to be (15.7 ± 0.2) × 10-17 cm2 at 95 eV and (10.8 ± 0.5) × 10-17 cm2 at 95 eV, respectively. The cross sections for O2+ and O3+ are much smaller, (5.37 ± 0.04) × 10-18 cm2 at 135 eV and (7.95 ± 0.23) × 10-20 cm2 at 315 eV, respectively. The collected data are analyzed in details in order to determine separately the contributions of dissociative excitation and of dissociative ionization to the O+ and D+ fragments production.

Theoretical study of the excited-state reaction paths of the OH and NH dissociation processes in barbituric acid

We studied the OH and NH dissociation mechanisms of eight tautomers of barbituric acid via excited states. The theoretical research was performed at the TD DFT level of theory (B3LYP). It was found that all mechanisms pass through the repulsive 1πσ* excited state whose excited-state reaction paths showed conical intersections S0–S1. Some OH dissociation mechanisms showed barrierless 1πσ* reaction paths which indicate an ultrafast relaxation of the 1πσ* excited states to the ground state. The study of the NH dissociation mechanisms revealed that almost all 1πσ* reaction paths show low energy barriers and minima before the conical intersections S0–S1.

Kinetics of Catalytic OH Dissociation on Metal Surfaces

A tunnel mechanism of OH dissociation reaction on metal surfaces is suggested and analyzed. Expressions for the rate constant and the activation energy are obtained by calculating the transition probability on semiempirical potential energy surfaces in the framework of the nonadiabatic perturbation theory. The analysis of the transition probability leads to a linear Brønsted–Evans–Polanyi (BEP) relationship between the activation energies and the surface reaction energies (ΔI), correlated by Ea = 0.73ΔI + 18 (kcal/mol). This relationship is compared with published literature data. The effect of the electron coupling on the pre-exponential factor is discussed, and the kinetic isotope effect of hydrogen for the reaction is predicted.

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).

Reaction of Hydroquinone and p-Benzoquinone with the Ge(100)-2 × 1 Surface

The adsorption behavior of two bifunctional molecules, hydroquinone (HO–C6H4–OH) and p-benzoquinone (O═C6H4═O), on the germanium (100) surface is studied. A combination of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy experiments together with density functional theory calculations were used to identify the products. The distinct functionalities of the reactants result in different reaction pathways, namely, OH dissociation for hydroquinone and cycloaddition for p-benzoquinone. Yet, the major product formed, a dually tethered surface hydroquinoxy group with an aromatic ring (−OC6H4O–), is the same for both molecules. Dual dissociation of hydroquinone was highly selective in stereochemical configuration. Minor singly tethered products are also found for both precursors. The results suggest that selective attachment of aromatic groups on semiconductor surfaces can be achieved by choosing proper functionalities of the reactants.

Unexpected effect of potassium ions on the copigmentation in red wines

The effect of potassium ions on the Glories color parameters of red wine has been investigated. The results show that temperature dependence of color intensity is high, while no considerable effect of temperature on color hue was obtained. Enhanced color intensity was observed in the presence of potassium ions. Color intensity reaches a maximum, when potassium is added in concentrations of about 0.5g/dm3. Furthermore, the temperature-dependence of the color intensity was decreased at higher potassium ion concentrations. According to the possible dissociation of phenolic OH, independent measurements validate that changes in pH itself could not be responsible for this unexpected phenomenon.

Dynamics of the CH3 + OH reaction

We study dynamics of the CH3 + OH reaction over the temperature range of 300–2500 K using a quasiclassical method for the potential energy composed of explicit forms of short-range and long-range interactions. The explicit potential energy used in the study gives minimum energy paths on potential energy surfaces showing barrier heights, channel energies, and van der Waals well, which are consistent with ab initio calculations. Approximately, 20% of CH3 + OH collisions undergo OH dissociation in a direct-mode mechanism on a subpicosecond scale (<50 fs) with the rate coefficient as high as ∼10−10 cm3 molecule−1 s−1. Less than 10% leads to the formation of excited intermediates CH3OH† with excess vibrational energies in CO and OH bonds. CH3OH† stabilizes to CH3OH, redissociates back to reactants, or forms one of various products after intramolecular energy redistribution via bond dissociation and formation on the time scale of 50–200 fs. The principal product is 1CH2 (k being ∼10−11), whereas ks for CH2OH, CH2O, and CH3O are ∼10−12. The minor products are HCOH and CH4 (k∼10−13). The total rate coefficient for CH3 + OH CH3OH† products is ∼10−11 and is weakly dependent on temperature. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 455–466, 2011

Properties and applications of calcium hydroxide in endodontics and dental traumatology

Calcium hydroxide has been included within several materials and antimicrobial formulations that are used in a number of treatment modalities in endodontics. These include, inter-appointment intracanal medicaments, pulp-capping agents and root canal sealers. Calcium hydroxide formulations are also used during treatment of root perforations, root fractures and root resorption and have a role in dental traumatology, for example, following tooth avulsion and luxation injuries. The purpose of this paper is to review the properties and clinical applications of calcium hydroxide in endodontics and dental traumatology including its antibacterial activity, antifungal activity, effect on bacterial biofilms, the synergism between calcium hydroxide and other agents, its effects on the properties of dentine, the diffusion of hydroxyl ions through dentine and its toxicity. Pure calcium hydroxide paste has a high pH (approximately 12.5-12.8) and is classified chemically as a strong base. Its main actions are achieved through the ionic dissociation of Ca(2+) and OH(-) ions and their effect on vital tissues, the induction of hard-tissue deposition and the antibacterial properties. The lethal effects of calcium hydroxide on bacterial cells are probably due to protein denaturation and damage to DNA and cytoplasmic membranes. It has a wide range of antimicrobial activity against common endodontic pathogens but is less effective against Enterococcus faecalis and Candida albicans. Calcium hydroxide is also an effective anti-endotoxin agent. However, its effect on microbial biofilms is controversial.

Ultrafast photo-excitation dynamics in isolated, neutral water clusters

Using the efficient nonlinear conversion scheme which was recently developed in our group [M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 134, 1491 (2010); M. Ghotbi, M. Beutler, and F. Noack, ibid 35, 3492 (2010)] to provide intense sub-50 fs vacuum ultraviolet laser pulses we have performed the first real time study of ultrafast, photo-induced dynamics in the electronically excited Ã-state of water clusters (H(2)O)(n) and (D(2)O)(n) , n=2-10. Three relevant time scales, 1.8-2.5, 10-30, and 50-150 fs, can be distinguished which-guided by the available theoretical results-are attributed to H (D)-ejection, OH (OD) dissociation, and a nonadiabatic transition through a conical intersection, respectively. While a direct quantitative comparison is only very preliminary, the present results provide a crucial test for future modeling of excited state dynamics in water clusters, and should help to unravel some of the many still unresolved puzzles about water.