Formation Of OH Bonds Research Articles

Effect of protonation and deprotonation on oxygen-containing groups functionalized graphene for boron adsorption removal

This study investigated the adsorption mechanisms of boron on protonated and deprotonated graphene with oxygen-containing functional groups. Six representative adsorption conformations of boron predominant species (B(OH)3 (B3) and B(OH)4− (B4)) on graphene were proposed. Protonation and deprotonation of graphene had significant effects on boron adsorption. Deprotonation of hydroxyl-modified (G20-OH) and carboxyl-modified graphene (G20-COOH) with 20 carbon rings decreased B3 and B4 adsorption. Protonation of G20-OH enhanced B3 adsorption. The adsorption interface, mainly around hydrogen atoms associated with protonated groups, exhibited a sharp gradient, due to the increased electron density from OH bond formation. B3 showed stronger electron sharing, emphasizing robust interactions. Hydrogen bonding dominated G20-OH2-B3, while G20-O-B4 exhibited more substantial hydrogen bond interactions. In summary, hydrogen bond strength was crucial in B3 adsorption, while electrostatic repulsion hindered B4 adsorption on deprotonated graphene. These findings highlight the importance of graphene protonated and deprotonated states in effectively removing various pKa contaminants.

Computational insights into the reaction mechanism of the synthesis of quinazoline derivatives via the cyclocondensation reaction between methyl 2-amino-4-(2-diethylaminoethoxy)-5-methoxybenzoate and formamide

In this work, the cyclocondensation reaction mechanism between methyl 2-amino-4-(2-diethylaminoethoxy)-5-methoxybenzoate (MAD, 1) and formamide (FME, 2) was theoretically explored at the DFT level using the B3LYP/6-311++G(d,p) computational method. The two paths (a and b) involved in this reaction take place through four steps. CDFT results show the most favourable electrophilic/nucleophilic interaction between the C3 electrophilic center of MAD (1) and the N4 nucleophilic center of FME (2). Activation energy analysis predicts the formation of quinazoline derivative as the main product, in reasonable agreement with experimental results. BET allowed a deeper understanding of the reaction mechanism by revealing the existence of four structural stability domains (SSD) during the processes of CN and OH bond formation. The bond process is as follows: depopulation of V(N) basin with the formation of first CN bond (appearance of V(C,N) basin), cleavage of NH bond with the creation of V(N) and V(H) monosynaptic basins, and finally the appearance of V(H,O) disynaptic basin related to the OH bond. In the case of dehydration, the transition from SSD-I to SSD-II corresponds to the cleavage of the CO bond, whose electron population is transferred to the V(O3) basin. The cleavage of N7-H9 bond with the formation of V(N) and V(H) basins is the first stage, followed by the formation of the OH bond as the second stage.

Mechanistic and kinetics insights into Cu size effects on catalytic hydrogen combustion

The removal of excess hydrogen in a catalytic process is a safety measure, which should be incorporated into fuel cell technologies. In this study, the size effects of Cu catalysts on the catalytic hydrogen combustion are examined by investigating the impact of Cu–Cu and Cu–O coordination numbers. A distinct volcano-shaped trend in catalytic performance is observed, reaching its peak when the Cu loading is at 5 wt% with a reaction rate of 0.55 molH2·molCu−1·min−1. It is found that the lower Cu loading leads to strong metal-support interactions and a high Cu–O coordination, whereas the higher Cu loading hinders reduction and increases the Cu–Cu coordination. DFT calculations emphasize the significance of Cu(111) facets in facilitating HH bond dissociation and OH bond formation. However, excessively large Cu nanoparticles limit catalytic activity due to the reduced active sites. As a result, the 5 wt% Cu/γ-Al2O3 catalyst, which strikes a balance between Cu–Cu and Cu–O coordination numbers, exhibits the highest catalytic performance for this reaction. This study not only provides valuable insights into the size effects of Cu–based catalysts on hydrogen combustion, but also offers guidance for designing highly active Cu–based catalysts.

Binary alloys for electrocatalytic CO2 conversion to hydrocarbons and alcohols

Bimetallic alloys are expected to create superior catalytic performance that cannot be achieved on monometallic catalysts by regulations of both electronic and surface structures. In this work, the reaction mechanism and catalytic kinetics of electrochemical CO2 conversion on nine ⅠB group metal binary alloys are studied by using DFT computations. Various reduction products, from CO to CO-beyond products and from C1 to C2 hydrocarbons and alcohols, can be produced on different bimetallic catalysts by controlling their reaction kinetics of competing OH bond formation, CH bond formation, and CC coupling. Among all the studied binary alloys, Au-Ag alloys show enhanced CO production ability with respect to elemental Au and Ag. Cu1/4Au3/4 gives the best performance to catalyze CO2 conversion to methane and methanol as a result of simultaneously promoting CORR activity and suppressing HER side reaction. By adjusting the CORR pathway through COH* intermediate, the CC coupling barriers are lowered on Cu3/4Ag1/4 and Cu3/4Au1/4 catalysts, which deliver potential capacity to catalyze CO2 transformation to C2 products.

Insights on the dissolution of water in an albite melt at high pressures and temperatures from a direct structural analysis

The water dissolution mechanism in silicate melts under high pressures is not well understood. Here we present the first direct structure investigation of a water-saturated albite melt to monitor the interactions between water and the network structure of silicate melt at the molecular level. In situ high-energy X-ray diffraction was carried out on the NaAlSi3O8-H2O system at 800 °C and 300 MPa, at the Advanced Photon Source synchrotron facility. The analysis of the X-ray diffraction data was augmented with classical Molecular Dynamics simulations of a hydrous albite melt, incorporating accurate water-based interactions. The results show that metal–oxygen bond breaking at the bridging sites occurs overwhelmingly at the Si site upon reaction with H2O, with subsequent Si–OH bond formation and negligible Al–OH formation. Furthermore, we see no evidence for the dissociation of the Al3+ ion from the network structure upon breaking of the Si–O bond in the hydrous albite melt. The results also indicate that the Na+ ion is an active participant in the modifications of the silicate network structure of the albite melt upon water dissolution at high P–T conditions. We do not find evidence for the Na+ ion dissociating from the network structure upon depolymerization and subsequent formation of NaOH complexes. Instead, our results show that the Na+ ion persists as a structure modifier with a shift away from Na–BO bonding to an increase in the extent of Na-NBO bonding, in parallel with pronounced depolymerization of the network. Our MD simulations show that the Si–O and Al–O bond lengths are expanded by about 6% in the hydrous albite melt compared to those of the dry melt at high P–T conditions. The changes in the network silicate structure of a hydrous albite melt at high pressure and temperature, as revealed in this study, must be considered in the advancement of water dissolution models of hydrous granitic (or alkali aluminosilicate) melts.

Hydrophilic-hydrophilic mixed micellar system: effect on solubilization of drug

Mixed micellar systems have been tried with the aim of achieving higher solubility of drugs compared to single micellar systems. Hydrophobic-hydrophilic mixed micellar systems have been used for the above purpose for the drug ciprofloxacin in the past. In the present study, a hydrophilic-hydrophilic binary micellar system comprising of pluronic copolymers pluronic F127 and pluronic L64 has been studied for its effect on solubilization of the drug Ciprofloxacin. The solutions of the two individual pluronic and their mixed micellar system with drugs were subjected to characterizations viz. UV-spectrophotometry, fluorimetry, FT-IR, dynamic light scattering (DLS), rheology, and partition coefficient. The mixed pluronic–drug system displayed greater solubility of the drug compared with the neat pluronic-drug systems in most of the characterizations. New C–OH bond formation was evidenced by FT-IR spectra due to drug micelle interaction. The values of free energy changes of micellization were found to be −25 kJ mol−1 for pluronic F127, −74.5kJmol−1 for L-64, and −170.4 kJ mol−1 for the mixed pluronic. This is suggestive of spontaneous and stronger binding of drug ciprofloxacin with mixed pluronic in comparison with that in single micellar systems.Graphic abstract

First-principle investigation on catalytic hydrogenation of benzaldehyde over Pt-group metals

Understanding the hydrogenation of organic compounds in the aqueous phase has always been fundamentally important for improving carbon neutral pathways to fuels and value-added chemicals. In this study, we investigated both thermodynamic and kinetic profiles of benzaldehyde hydrogenation over the Pd(111) and Pt(111) metal surfaces using density functional theory (DFT) and ab initio molecular dynamic (AIMD) simulations. The adsorption of H2 shows the mixed preference of H adsorption sites on the Pt(111), while the fcc adsorption site is dominant for H on the Pd(111). When benzaldehyde is added to the systems, we observe a strong reduction of benzaldehyde on charged Pd (111) surface compared with that on neutral surface. In contrast, charged state of the Pt(111) surface does not change their interaction. Subsequent hydrogenation reaction of benzaldehyde over Pd(111), proceeding via Langmuir-Hinshelwood mechanism, is affected by two major factors: the presence of H2O solvent and surface charge. The presence of H2O solvent greatly reduces the activation energy of CH and OH bond formation during the hydrogenation process. Furthermore, the hydrogenation step via CH bond formation is preferred thermodynamically and kinetically over OH bond formation during thermocatalytic hydrogenation, while the opposite trend holds true during electrocatalytic hydrogenation.

Control Valence and Luminescence Properties of Cerium Ions in Al2O3-SiO2 Glasses Prepared by Sol–Gel Method

In this study, the synthesis and luminescence properties (LP) of Al2O3-SiO2 glasses doped with different concentrations of Ce3+, namely, 0.05 mol.%, 0.1 mol.%, and 0.3 mol.% Ce3+, were investigated. The LP of the obtained samples were characterized by ultraviolet–visible absorption (UV–VIS) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and photoluminescence (PL) spectroscopy. The results showed that the valence states of the cerium ions can be controlled by heating the synthesized glasses in hydrogen atmosphere leading to change their LP. The existence of the absorption band at about 295 nm and of a broad band spectrum with a maximum at 250 nm can be attributed to Ce3+ ion and the charge transfer band of Ce4+ ion, respectively. A broad emission at 366 nm observed under excitation of UV light at 310 nm originated from the well-known 4f–5d transition of Ce3+ ion. The role of Al3+ ions is also considered when glass samples were treated under the hydrogen atmosphere. It is revealed that the presence of aluminum ions enabled controlling the valence states of cerium via the formation of OH bonds that can reduce Ce4+ to Ce3+ ions in glass sample treated in reduced atmosphere. The increasing of PL peaks appeared during the treatment under hydrogen atmosphere at high temperature.

Theory of chemical bonds in metalloenzymes XXI. Possible mechanisms of water oxidation in oxygen evolving complex of photosystem II

ABSTRACTPossible mechanisms for water cleavage in oxygen evolving complex (OEC) of photosystem II (PSII) have been investigated based on broken-symmetry (BS) hybrid DFT (HDFT)/def2 TZVP calculations in combination with available XRD, XFEL, EXAFS, XES and EPR results. The BS HDFT and the experimental results have provided basic concepts for understanding of chemical bonds of the CaMn4O5 cluster in the catalytic site of OEC of PSII for elucidation of the mechanism of photosynthetic water cleavage. Scope and applicability of the hybrid DFT (HDFT) methods have been examined in relation to relative stabilities of possible nine intermediates such as Mn-hydroxide, Mn-oxo, Mn-peroxo, Mn-superoxo, etc., in order to understand the O–O (O–OH) bond formation in the S3 and/or S4 states of OEC of PSII. The relative stabilities among these intermediates are variable, depending on the weight of the Hartree–Fock exchange term of HDFT. The Mn-hydroxide, Mn-oxo and Mn-superoxo intermediates are found to be preferable in the weak, intermediate and strong electron correlation regimes, respectively. Recent different serial femtosecond X-ray (SFX) results in the S3 state are investigated based on the proposed basic concepts under the assumption of different water-insertion steps for water cleavage in the Kok cycle. The observation of water insertion in the S3 state is compatible with previous large-scale QM/MM results and previous theoretical proposal for the chemical equilibrium mechanism in the S3 state . On the other hand, the no detection of water insertion in the S3 state based on other SFX results is consistent with previous proposal of the O–OH (or O–O) bond formation in the S4 state . Radical coupling and non-adiabatic one-electron transfer (NA-OET) mechanisms for the OO-bond formation are examined using the energy diagrams by QM calculations and by QM(UB3LYP)/MM calculations . Possible reaction pathways for the O–O and O–OH bond formations are also investigated based on two water-inlet pathways for oxygen evolution in OEC of PSII. Future perspectives are discussed in relation to post HDFT calculations of the energy diagrams for elucidation of the mechanism of water oxidation in OEC of PSII.

Reduction Mechanisms of Cu2+-Doped Na2O-Al2O3-SiO2 Glasses during Heating in H2 Gas.

Controlling valence state of metal ions that are doped in materials has been widely applied for turning optical properties. Even though hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, few comprehensive studies focus on practical applications because of the low diffusion rate of hydrogen in solids and the limited reaction near sample surfaces. Here, we investigated the reactions of hydrogen with Cu2+-doped Na2O-Al2O3-SiO2 glass and found that a completely different reduction from results reported so far occurs, which is dominated by the Al/Na concentration ratio. For Al/Na < 1, Cu2+ ions were reduced via hydrogen to metallic Cu, distributing in glass body. For Al/Na > 1, on the other hand, the reduction of Cu2+ ions occurred simultaneously with the formation of OH bonds, whereas the reduced Cu metal moved outward and formed a metallic film on glass surface. The NMR and Fourier transform infrared results indicated that the Cu2+ ions were surrounded by Al3+ ions that formed AlO4, distorted AlO4, and AlO5 units. The diffused H2 gas reacted with the Al-O-···Cu+ units, forming Al-OH and metallic Cu, the latter of which moved freely toward glass surface and in return enhanced H2 diffusion.

Insights into the mechanism of acetic acid hydrogenation to ethanol on Cu(111) surface

Density functional theory (DFT) calculations were employed to theoretically explain the reaction mechanism of acetic acid hydrogenation to ethanol on Cu catalyst. The activation barriers of key elementary steps and the adsorption configurations of key intermediates involved in acetic acid hydrogenation on Cu(111) surface were investigated. The results indicated that the direct dissociation of acetic acid to acetyl (CH3COOH→CH3CO+OH) is the rate-determined step. The activation barrier of acetic acid scission to acetyl and the adsorption energy of acetic acid are two descriptors which could determine the conversion of acetic acid. The descriptors might have effects on the ethanol selectivity including: the adsorption energy of acetaldehyde and the activation barriers for OH bond formation of C2-oxygenates (CH3CO+H→CH3COH, CH3CHO+H→CH3CHOH and CH3CH2O+H→CH3CH2OH). These proposed descriptors could be used as references to design new Cu-based catalysts that have excellent catalytic performance.

Investigation of the mechanism of electron capture and electron transfer dissociation of peptides with a covalently attached free radical hydrogen atom scavenger

The mechanisms of electron capture and electron transfer dissociation (ECD and ETD) are investigated by covalently attaching a free-radical hydrogen atom scavenger to a peptide. The 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) radical was chosen as the scavenger due to its high hydrogen atom affinity (ca. 280kJ/mol) and low electron affinity (ca. 0.45eV), and was derivatized to the model peptide, FQXTEMPOEEQQQTEDELQDK. The XTEMPO residue represents a cysteinyl residue derivatized with an acetamido-TEMPO group. The acetamide group without TEMPO was also examined as a control. The gas phase proton affinity (882kJ/mol) of TEMPO is similar to backbone amide carbonyls (889kJ/mol), minimizing perturbation to internal solvation and sites of protonation of the derivatized peptides. Collision-induced dissociation (CID) of the TEMPO-tagged peptide dication generated stable odd-electron b and y type ions without indication of any TEMPO radical induced fragmentation initiated by hydrogen abstraction. The type and abundance of fragment ions observed in the CID spectra of the TEMPO and acetamide tagged peptides are very similar. However, ECD of the TEMPO-labeled peptide dication yielded no backbone cleavage. We propose that a labile hydrogen atom in the charge reduced radical ions is scavenged by the TEMPO radical moiety, resulting in inhibition of NCα backbone cleavage processes. Supplemental activation after electron attachment (ETcaD) and CID of the charge-reduced precursor ion generated by electron transfer of the TEMPO-tagged peptide dication produced a series of b+H (bH) and y+H (yH) ions along with some c ions having suppressed intensities, consistent with stable OH bond formation at the TEMPO group. In summary, the results indicate that ECD and ETD backbone cleavage processes are inhibited by scavenging of a labile hydrogen atom by the localized TEMPO radical moiety. This observation supports the conjecture that ECD and ETD processes involve long-lived intermediates formed by electron capture/transfer in which a labile hydrogen atom is present and plays a key role with low energy processes leading to c and z ion formation. Ab initio and density functional calculations are performed to support our conclusion, which depends most importantly on the proton affinity, electron affinity and hydrogen atom affinity of the TEMPO moiety.

Reduction mechanism for Eu ions in Al2O3-containing glasses by heat treatment in H2 gas.

Superior functional glasses doped with rare-earth ions have been prepared by controlling the valence states of rare-earth ions. However, recent work has revealed unresolved questions about the controlling mechanism of rare-earth ions' valence states. To address these questions, oxide glasses with and without Al2O3 and doped with Eu(3+) ions were prepared by a melting process; then, the valence states of Eu(3+) ions were investigated during heating under a hydrogen environment. The Eu(3+) ions were reduced to Eu(2+) only in the glass containing Al(3+) ions; the reduction occurred in the center of the glass over a short heating period. It was discovered that the reduction of Eu(3+) ions concurrently occurred with the formation of OH bonds which were bound with Al(3+) ions. Considering this and the data for the H2 gas diffusion through the glass, we conclude that diffusing H2 gas molecules react with Al-O(-) bonds surrounding Eu(3+) ions to form AlOH bonds and reduce Eu(3+) ions to Eu(2+) via the extracted electrons. When H2 reacts with a glass structure, that hydrogen has transformed into -OH bonds and the hydrogen concentration in the glass decreases. In order to make up the lost hydrogen, more hydrogen molecules can enter into the glass, resulting in the fast reduction of Eu(3+) ions in the center of the glass.

Hydrogen implantation in silicates: The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space

AbstractHydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H+ implantation of amorphous SiO2 and olivine in ultrahigh vacuum. The samples were implanted with 2–10 keV H+, in the range of solar wind energies, and the OH absorption band at ~2.8 µm measured by transmission Fourier transform infrared spectroscopy. For 2 keV protons in SiO2, the OH band depth saturated at fluences F ~5 × 1016 H+/cm2 to a maximum 0.0032 absorption band depth, corresponding to a column density ηs = 1.1 × 1016 OH/cm2. The corresponding values for 5 keV protons in olivine are &gt;2 × 1017/cm2, 0.0067, and 4.0 × 1016 OH/cm2. The initial conversion rate of implanted H+ into hydroxyl species was found to be ~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.

Reaction mechanisms of CO2 electrochemical reduction on Cu(1 1 1) determined with density functional theory

Density functional theory (DFT) was used to determine the potential-dependent reaction free energies and activation barriers for several reaction paths of carbon dioxide (CO2) electrochemical reduction on the Cu(111) surface. The role of water solvation on CO2 reduction paths was explored by evaluating water-assisted surface hydrogenation and proton (H) shuttling with various solvation models. Electrochemical OH bond formation reactions occur through water-assisted H-shuttling, whereas CH bond formation occurs with negligible H2O involvement via direct reaction with adsorbed H* on the Cu(111) surface. The DFT-computed kinetic path shows that the experimentally observed production of methane and ethylene on Cu(111) catalysts occurs through the reduction of carbon monoxide (CO*) to a hydroxymethylidyne (COH*) intermediate. Methane is produced from the reduction of the COH* to C* and then sequential hydrogenation. Ethylene production shares the COH* path with methane production, where the methane to ethylene selectivity depends on CH2∗ and H* coverages. The reported potential-dependent activation barriers provide kinetics consistent with observed experimental reduction overpotentials and selectivity to methane and ethylene over methanol for the electroreduction of CO2 on Cu catalysts.

Hydrogen-incorporated ZnO nanowire films: stable and high electrical conductivity

Post-growth hydrogen annealing treatment of highly oriented ZnO nanowire (NW) films (ZnO : H) results in high electrical conductivity (3.7 × 103 S m−1) and fully suppressed defect emission at room temperature. The formation of hydrogen-related vacancy complexes is responsible for the suppression of vacancies ( and ), leading to a reduction in defect-based emission. ZnO : H NW films show five orders larger stable electrical conductance with a four-fold increment in carrier mobility (7–28 cm2 V−1 s−1). As compared with pristine NWs, the carrier concentration in ZnO : H NW films increases from 1015 to 1019 cm−3, which is in the range of commercial transparent conducting oxides. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analyses reveal stable OH bond formation, which strongly supports the prediction of H doping. These films offer a promising conducting oxide platform for photovoltaic applications.

Effect of dopant nature on structures and lattice dynamics of proton-conducting BaZrO3

The influence of the acceptor-dopants on the local and long range structures and lattice vibrations of barium zirconate has been studied with Density Functional Theory (DFT) based periodic approach. Structural details of BaZr1 − xMxHxO3 (M = In and Sc, and x = 0.125, 0.25 and 0.375) with protonic defects in three different oxygen configurations are presented and discussed in relation with the results for Y-doped barium zirconates, obtained previously with the same methodology. The oxide symmetry and the short and long range structural arrangements in the M-rich and Zr-rich regions were established nearly independent on the dopant chemical nature, concentration and arrangement in the cell. The OH bond formation is however strongly influenced by the dopant nature, which is associated with the differences in dopant–oxygen bond character. The computed lattice vibrations in zirconates with Y, In and Sc dopants showed that the OH vibrations vary by 250–1500 cm− 1 depending on the acceptor-dopant.

Structural properties of Y-doped BaZrO3 as a function of dopant concentration and position: A density functional study

Relative stabilities, structural details and hydrogen binding sites in models of Y-doped barium zirconates with 12.5, 25 and 37.5% yttrium have been studied using Density Functional Theory (DFT) based periodic approach. Two stable crystal structures were obtained for the possible Zr substitutions in the 25 and 37.5% Y-containing BaZrO3. Tetragonal space group symmetries were found for Y≥25% and a volume reduction is obtained with increasing the acceptor dopant percentage. Structural and electronic properties remain very similar in all the considered models. A well established charge difference is noticed only for oxygen sites in the three possible ZrOZr, ZrOY and YOY configurations. Examination of H binding strength in various OH local environments indicated a general tendency to stabilize the OH bonds in the ZrOHY configuration(s) compared to the ZrOHZr ones. A clear tendency of increasing the OH stabilization with increasing Y content from 12.5 to 25% is not obtained. The strongest OH bonds are formed in the 37.5% Y-doped BaZrO3 model. Formation of OH bonds in YOHY configurations was not found that suggest a smaller probability for H-uptake in Y-doped zirconates with high YOY concentrations.

Dynamics of Hydration in Vanadia–Titania Catalysts at Low Loading: A Theoretical and Experimental Study

The hydration process of dehydrated vanadia–titania catalysts at low loading is investigated using periodic DFT calculations. We focus on the early stages of the hydration process of the vanadia–titania in order to shed light onto the structural and dynamical changes occurring at the molecular level. Hydration is modeled by addition of successive water molecules to the dehydrated models. Special attention is paid to the V–OH bond formation and the transformation between different surface species. It is found that at low vanadia coverage the predominant surface species are OV(OH)O2 monomers with high affinity for water. Interestingly, OVO3 pyramids are stable only under severe dehydrating conditions, and hydroxylated species are expected to be present even at low water content. While low water content leads to water dissociation and supports hydration, higher content leads to a dynamic equilibrium between hydrated vanadia surface species. Interconversion between different surface species is fast and depends on the water coverage, through a fast hydrogen transfer mechanism. Leaching of OV(OH)3 species is observed in the case of high water content. The number of adsorbed water molecules depends on temperature, but even at high temperature, water adsorption is preferred, which is relevant to the state of titania-supported catalysts during reaction conditions in which water is fed or generated during reaction. Calculated harmonic vibrations are provided for the most stable surface species; their redshift upon coordination with water and their blueshift upon progressive dehydration are experimentally confirmed by in situ Raman spectra in dry and humid air at increasing temperatures.

Hydroxyl radical production and storage in analogues of amorphous interstellar silicates: a possible “wet” accretion phase for inner telluric planets

Context. Interstellar silicate grains are thought to be amorphized by interaction with high- and low-energy particle interactions in astrophysical environments. In addition, low energy (a few keV) particles will implant atoms within the grains.Aims. In this paper we experimentally investigate the consequence of the implantation of H+ at low irradiation energies into analogues of interstellar silicate grains, and look for the formation of hydroxyl radicals within the silicate matrix.Methods. Thin amorphous silicate films (~100 nm) were sequentially irradiated with H+ ions at low energies (3.5, 2.5 and then 1.5 keV) ensuring an implantation of the ions through the full depth of the films. The fluences used, 3 × 1016 , 1017 and 3 × 1017 H+ /cm2 , are compatible with those expected in shocks in the interstellar medium. We used infrared spectroscopy to monitor and quantify the OH band evolution after irradiation. In order to distinguish the newly formed OH groups from those originating from unavoidable atmospheric contamination, the D/H depth ratios were measured with a NanoSIMS ion microprobe.Results. An increase in the OH band strength in the infrared spectra after irradiation reveals the formation of OH bonds within the irradiated silicate thin films. NanoSIMS measurements of the D/H signature in the region of ion implantation show that the newly-formed OH groups make up about 40% of the observed OH band in the IR, the rest are due to an atmospheric hydroxylation of the sample. Only about 2% of the incident ions lead to OH bond formation and, at most, the irradiated silicates retain about 3% of the incident protons as OH groups within their structure. Conclusions. Our laboratory experimental simulations show a possible production and storage of hydroxyl radicals in amorphous laboratory silicates. In the astrophysical context, such OH radicals, strongly bonded to pre-accretion material, could constitute a non negligible reservoir of -OH, thus water. These experimental results allow us to revisit and reinstate the hypothesis of a possible “wet” accretion of the telluric planets early in the history of the formation of the Solar System.