Hydroxyl Migration Research Articles

Raman and X-ray photoelectron spectroscopic characterizations of thermal stability of 3 mol% yttria stabilized zirconia ceramics

Structural variations and phase stability in 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) subjected to long-term aging at room temperature and thermal treatments in air were investigated by Raman and X-ray photoelectron spectroscopies, and compared with previous cases obtained by hydrothermal treatments. The results revealed a significant difference in the kinetics of zirconia polymorphic transformation in different environments due to the critical roles played by surface and lattice hydroxyl defects. Thermal treatments in air of long-term stored 3Y-TZP revealed a gradual recovery to tetragonal phase due to the ocurrence of dehydroxylation, while thermal treatments of pristine 3Y-TZP caused tetragonal-to-monoclinic polymorphic transformation at low temperature, but then an inverse transformation at higher tempertures due to a competition of hydroxylation and dehydroxylation. Treatments of 3Y-TZP in different environments were found to lead to distinct paths of off-stoichiometric chemistry, including hydroxylation, hydroxyl migration, surface reconstruction, and dehydroxylation during the treatments.

Insights into Hydrogen Transport Behavior on Perovskite Surfaces: Transition from the Grotthuss Mechanism to the Vehicle Mechanism.

Hydrogen transport on transition-metal oxides is a shared process in many important physical and chemical changes of interest. In this work, DFT + U calculations have been carried out to explore the mechanism for hydrogen migration on the defect-free and oxygen-deficient LaMO3(001) (M = Cr, Mn, and Fe) surfaces. The calculated results indicate that hydrogen is preferentially adsorbed at the oxygen sites on all surfaces other than the defective LaCrO3(001), where the occupation of vacancies is energetically most favorable. The resultant O-H bonds would be weakened when oxygen vacancies are formed in their immediate vicinity because the increased electron density on the remaining ions would limit the ability of O to withdraw electrons from H. On defect-free LaMO3(001), hydrogen prefers to migrate along the [010] axis, during which the O-H bond is reoriented at the oxygen site for the hopping to proceed by the Grotthuss mechanism. In the presence of oxygen vacancies, the vehicle mechanism in which hydrogen hops together with the underlying oxygen would dominate on LaMnO3 and LaFeO3, whereas on the defective LaCrO3(001) the Grotthuss mechanism prevails. The linear scaling relations established show that the hydrogen and hydroxyl migration barriers decrease and increase, respectively, with increasing the strength of ionic bonding in perovskites, which provides a rational interpretation of the change in the preferred hydrogen migration mechanism.

First-principles study on the hydroxyl migration from inner to surface in hydroxyapatite

Understanding to the surface structure of hydroxyapatite (HAp) is crucial for the design of HAp-based biomaterials. While most bioactive HAp materials have disordered structures, the ordered surface structures relaxed from HAp crystal are commonly used for studying their interaction with biomolecules. Using a combined approach of molecular dynamics simulated annealing simulations and density functional theory calculations, we proposed a disordered surface structure model for HAp. In this model hydroxyls beneath the surface move outward and cover the exposed calcium ions. The destabilization caused by hydroxyl vacancy is compensated by the stabilization of hydroxyl coverage. The Ca-exposed surface with strong electrophilicity changes into the hydroxyl- and phosphate-covered surface with moderate activity. The new surface model would be useful for regulating the surface activity of HAp-based biomaterials.

Hydroxyl migration disorders the surface structure of hydroxyapatite nanoparticles

The surface structure of nano-hydroxyapatite (HAP) was investigated using a combined simulated annealing and molecular dynamics method. The stationary structures of nano-HAP with 4–7nm in diameter and annealed under different temperatures were analyzed in terms of pair distribution function, structural factor, mean square displacement and atomic coordination number. The particles possess different structures from bulk crystal. A clear radial change in their atomic arrangements was noted. From core to surface the structures change from ordered to disordered. A three-shell model was proposed to describe the structure evolution of nano-HAP. Atoms in the core zone keep their arrangements as in crystal, while atoms in the surface shell are in short-range order and long-range disorder, adopting a typically amorphous structure. Atoms in the middle shell have small displacements and/or deflections but basically retain their original locations as in crystal. The disordered shell is about 1nm in thickness, in agreement with experimental observations. The disordering mainly stems from hydroxyl migration during which hydroxyls move to the surface and bond with the exposed Ca ions, and their left vacancies bring about a rearrangement of nearby atoms. The disordering is to some extent different for particles unannealed under different temperatures, resulting from fewer number of migrated hydroxyls at lower temperatures. Particles with different sizes have similar surface structures, and their surface energy decreases with increasing size. Moreover, the surface energy is reduced by hydroxyl migration because the exposed Ca ions on the surface are ionically bonded with the migrated hydroxyls. Our calculations proposed a new structure model for nano-HAP, which indicates a surface structure with activities different from those without surface reorganization. This is particularly interesting because most bioactivities of biomaterials are dominated by their surface activity.

Unusual hydrogen and hydroxyl migration in the fragmentation of excited doubly-positively-charged amino acids in the gas phase

SynopsisWe present a combined experimental and theoretical study of the fragmentation of doubly-positively- charged amino acids in the gas phase. The combination of ab initio molecular dynamics simulations with ion- molecule collisions followed by multiple-coincidence mass spectrometric techniques, allows us to obtain a complete picture of the fragmentation dynamics. In addition to the expected Coulomb explosion, we have found evidence of hydrogen and hydroxyl-group migration processes, which leads to unusual fragmentation products.

Unusual hydroxyl migration in the fragmentation of β-alanine dication in the gas phase

We present a combined experimental and theoretical study of the fragmentation of doubly positively charged β-alanine molecules in the gas phase. The dissociation of the produced dicationic molecules, induced by low-energy ion collisions, is analysed by coincidence mass spectrometric techniques; the coupling with ab initio molecular dynamics simulations allows rationalisation of the experimental observations. The present strategy gives deeper insights into the chemical mechanisms of multiply charged amino acids in the gas phase. In the case of the β-alanine dication, in addition to the expected Coulomb explosion and hydrogen migration processes, we have found evidence of hydroxyl-group migration, which leads to unusual fragmentation products, such as hydroxymethyl cation, and is necessary to explain some of the observed dominant channels.

Hydroxyl migration in heterotrimetallic clusters: an assessment of fluxionality pathways.

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H2 liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W3O6(-) core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W2TcO6(-) (2) and W2OsO6(-) (3) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H2 liberation from H2O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium.

Proton Hop Paving the Way for Hydroxyl Migration: Theoretical Elucidation of Fluxionality in Transition-Metal Oxide Clusters

The reactions of chemisorbed water on W3O6 - and Mo3O6 - clusters have been investigated to explore the phenomenon of fluxionality in transition- metal oxide clusters. The net observed phenomenon here is a hydroxyl migration. However, mechanistic studies using electronic structure theory reveal that the hydroxyl migration occurs by a synergistic pathway led by a proton hop and assisted by an interconversion between a bridging oxygen and a terminal oxygen. Theprotonhopprovidesaccesstotwoisomers,whichdifferintherelativeposition and orientation of the hydroxyl groups, thereby generating the scope for an enhanced catalytic activity in important processes such as hydrogen evolution from water. SECTION Dynamics, Clusters, Excited States

Hydrogenolytic opening of 4,4-dimethyl-4a,5-epoxy-a-homocholestane derivatives and mass spectra of some reaction products

The hydrogenolytic opening of the epoxide ring of all four stereoisomeric 3-acetoxy-4,4-dimethyl-4a,5-epoxy-A-homocholestanes was investigated. The reaction proceeds at the more substituted carbon atom C(5), giving rise to 4a-hydroxy-5,6-unsaturated derivatives. The hydrogenation of the 5,6-double bond of these allylic alcohols affords 4,4-dimethyl-A-homo-5β-cholestane derivatives. The mass spectra of 3,4a-diketones XI and XVII and 4a-ketones XVI and XXVIII to XXX were also studied. In the case of the saturated diketone XVII it was observed that the most abundant metastable peak corresponds to a two-step fragmentation process. The decomposition of the molecular ion of the unsaturated diketone XI is associated with ring A expansion. The mass spectra of 3-acetoxy ketones XVI and XXVIII are characterized by an elimination of a C4H8 molecule from the contracted ring A of the ion [M-CH3COOH]+. The unexpected elimination of the species C6H11, C6H12, and C6H13 from the ring A of the molecular ions of 3-hydroxy ketones XXIX and XXX is shown to be preceded by hydroxyl migration and ring A rearrangement.

Eliminierung von Aceton und Bildung von protoniertem Aceton bei der chemischen Ionisation von [2.2.6.6]-Tetramethylcyclohexanon/ Elimination of Acetone and Formation of Protonated Acetone from [2,2,6,6]-Tetramethylcyclohexanone under the Condition of Chemical Ionisation

Abstract By means of appropriately D and 13C labelled precursors it is shown that the proton catalysed degradation of [2,2,6,6]-tetramethyl cyclohexanone {e.g. elimination of acetone and formation of protonatecl acetone from [MH]+) proceeds via two distinct pathways. The energetically favoured one (pathway A, Scheme 1) involves a sequence of [1,2]-methyl migration, ring contraction and methyl migration, whereas the energetically less attractive path B commences with a Wagner-Meerwein ring contraction, followed by hydroxyl and methyl migration. Semi-empirical quantum mechanical calculations (MNDO) are em-ployed to explore computationally relevant parts of the potential energy surface. The syntheses of the specifically 13C labelled title compounds (1 a, b) are described in detail.

Ion migration energies and color center absorption in hydroxyapatite.

By modelling a hydroxyapatite crystal lattice as a finite cluster of atoms surrounded by an array of point ions, molecular orbital MS-Xalpha calculations point to differences in fluoride and hydroxyl migration energies. Despite the fluoride ion being more tightly bound to the lattice, it is in fact more mobile. Color center optical absorption is predicted to be in the infrared region of the spectrum for hydroxyapatite, whereas for fluorapatite the absorption peak of a vacancy is predicted to within 2% of the observed peak in the optical region. This result is discussed in terms of experimental studies of bonding between the calcified and organic phases of bone.

ChemInform Abstract: ELECTRON IMPACT INDUCED FRAGMENTATION OF ACETYLENE COMPOUNDS. XXIII. (1,3)‐HYDROXYL MIGRATION IN CATION RADICALS OF ALLENE AND ACETYLENE ALCOHOLS

Chemischer InformationsdienstVolume 11, Issue 2 Physical Organic Chemistry ChemInform Abstract: ELECTRON IMPACT INDUCED FRAGMENTATION OF ACETYLENE COMPOUNDS. XXIII. (1,3)-HYDROXYL MIGRATION IN CATION RADICALS OF ALLENE AND ACETYLENE ALCOHOLS B. CIOMMER, B. CIOMMERSearch for more papers by this authorH. SCHWARZ, H. SCHWARZSearch for more papers by this author B. CIOMMER, B. CIOMMERSearch for more papers by this authorH. SCHWARZ, H. SCHWARZSearch for more papers by this author First published: January 15, 1980 https://doi.org/10.1002/chin.198002060Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume11, Issue2January 15, 1980 RelatedInformation

1.3] -Hydroxylwanderung bei Radikalkationen von Allen-und Acetylenalkoholen/[1,3]-Hydroxyl Migration in Cation Radicals of Allene and Acetylene Alcohols

Abstract Contrary to previous conclusions it is shown that the molecular ions of 1, 2, and 3 do not interconvert prior to dissociation. Investigation of labelled compounds establishes a new typ of intramolecular [1,3]-hydroxyl migration to allenic and acetylenic carbons. It is shown that both 1 and 2 undergo irreversible rearrangement to an a,β-unsaturated ketone 4. Methyl loss from the intermediate 4, which accounts for 90% of the total methyl elimination, is preceded by partial hydrogen exchange processes between the three methyl groups of 4. The actual dissociation can be described as an a-cleavage to the carbonyl function. The unusual formation of CH3CO+ " is shown to occur exclusively from 1. However, it is demonstrated that prior to CH3CO+ formation 1 rearranges to m. The mechanistic details of this as well as alternative processes are discussed.

Skeletal-rearrangement fragments in the mass spectra of β-thiopropionic acid derivatives

The mass spectra of S-alkyl-β-thiopropionic acids contain skeletal- rearrangement ions which arise by hydroxyl migration to a carbonium-ion centre followed by elimination of a neutral molecule (e.g. ketene or formaldehyde). In contrast, those of methyl S-alkyl-β-thiopropionates contain peaks which are formed by methoxyl migration to sulphur, followed by loss of ketene.