Bidentate Configuration Research Articles

QM/MM Study of the Effect of Local Environment on Dissociative Adsorption in BaY Zeolites

Quantum chemical calculations were carried out to study the characteristics of dissociative adsorption on BaY zeolites, that is, adsorption of neutral HX versus H+X−. The calculations were performed using the QM/MM method ONIOM with the B3LYP functional, SDD basis set, and UFF force field. Key intermediates from the NOx reduction reaction were studied, including nitromethane, aci-nitromethane, acetic acid, CH2COOH, HNO3, HNO2, H2O, and O2NCH2COOH. Two factors were found to affect dissociative adsorption: the local environment in the zeolite and the acidity of the adsorbate molecule. The local environment can vary due to different aluminum arrangements around the active site. It was found that adsorbates in both neutral and dissociated forms are further stabilized as aluminum atoms are further apart because of the electrostatics. Generally, neutral intermediates adsorb with one oxygen atom coordinated to barium, whereas the dissociated intermediates adsorb with two oxygen atoms coordinated to barium in a bidentate configuration. For all of the intermediates studied except HNO3, the neutral form is favored over the dissociated form, and adsorbates with higher acidity have stronger tendencies to dissociate.

Adsorption of Phosphonic Acid at the TiO2 Anatase (101) and Rutile (110) Surfaces

The adsorption of phosphonic acid on the TiO2 anatase (101) and rutile (110) surfaces have been investigated by means of efficient density-functional-based tight-binding calculations. We studied the geometries and adsorption energies of several adsorption models to achieve clarification of the discrepancy in the experimental finding of a preferred binding state. In this paper we show that there are several adsorption structures likely to be present on the specific TiO2 surfaces. Those structures have exclusively a bidentate configuration. They have similar adsorption energies but different geometries. For the monodentate complexes, we find a strong trend of the adsorption geometry relaxing toward the bidentate coordination. Also, they have significantly smaller adsorption energies. Furthermore, we extensively demonstrate the reliability of the SCC-DFTB method for this chemical system, which opens the way for studies of adsorption on more complex titania materials.

Correlation between Bonding Geometry and Band Gap States at Organic−Inorganic Interfaces: Catechol on Rutile TiO2(110)

Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO(2) is presented. Using the organic catechol on rutile TiO(2)(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 x 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO(2).

Functionalization of β-Ga2O3 Nanoribbons: A Combined Computational and Infrared Spectroscopic Study

The adsorption of H 2O, alcohols (CH 3OH and 1-octanol), and carboxylic acids (formic, acetic, and pentanoic) on beta-Ga 2O 3 nanoribbons has been studied using infrared reflection-absorption spectroscopy (IRRAS) and/or ab initio computational modeling. Adsorption energies and geometries are sensitive to surface structure, and hydrogen bonding plays a significant role in stabilizing adsorbed species. On the more stable (100)-B surface, computation shows that the physisorption of H 2O or CH 3OH is weakly exothermic whereas chemisorption via O-H bond dissociation is weakly endothermic. Experiment finds that a large fraction of a saturation coverage of adsorbed 1-octanol is displaced by exposure to acetic acid vapor. This is consistent with computational results showing that acids adsorb more strongly than methanol on this surface. The remaining alcohol, not displaced by acetic acid, suggests the presence of defects and/or (100)-A regions because computation shows that this less-stable surface adsorbs methanol more strongly than does the (100)-B. The nu(C-H) modes of adsorbed 1-octanol are easily detected whereas no adsorbed H 2O is observed even though H 2O and CH 3OH exhibit similar adsorption energies. It is inferred from this that the failure to detect H 2O on the dominant (100)-B surface results from the orientation of the physisorbed H 2O essentially parallel to the surface. Computation shows that this configuration is stabilized by H bonding. For chemisorbed formic acid, computation shows that a bridging carboxylate structure is favored over a bidentate or monodentate configuration. Computation also shows that chemisorption is favored on the (100)-A surface but physisorption is favored on the more stable (100)-B. Analysis of IRRAS data for acetic and pentanoic acids finds evidence for both types of adsorption. The carboxylate resists displacement by H 2O vapor, which suggests that carboxylic acids may be useful for functionalizing beta-Ga 2O 3 surfaces. The results provide insight into the interplay between surface structure and reactivity on an oxide surface and about the importance of hydrogen bonding in determining adsorbate structure.

DFT study of carboxylic acids modes of adsorption on rutile TiO 2(0 1 1) surfaces

Plane wave density functional theory (PWDFT) has been employed to study the energy and structure of formic and acetic acids dissociative adsorption on rutile TiO 2(0 1 1) surfaces. Both the bulk terminated (1 × 1) and reconstructed (2 × 1) surfaces were considered. On both surfaces the bridging (bidentate) configuration was found more stable than the monodentate configuration for formate species by 25–40 kJ mol −1. The difference in stability between the bridging and monodentate species is more pronounced in the case of acetates on the 2 × 1 reconstructed surface where the former adsorption was found equal to 162 kJ mol −1 while the latter dropped to 100 kJ mol −1. The decrease in stability of the monodentate configuration in the case of acetate species is attributed to the considerable repulsion between the methyl group and terminal Ti O groups. Analyses of the density of states indicated that while the dissociated adsorption of formic acid on the 2 × 1 reconstructed surface appears complete with a clear formation of surface hydroxyl that on the 1 × 1 surface may still involve electron sharing between surface hydroxyls and the formate species. The chelating mode, whereby the two oxygen atoms of the carboxylate species are in close proximity with one surface Ti atoms was found unstable for both acids and on both surfaces. Comparison with experimental TPD results of carboxylic acids over the rutile TiO 2 (0 1 1) surface is presented and discussed.

Adsorption and Protonation of CO2 on Partially Hydroxylated γ-Al2O3 Surfaces: A Density Functional Theory Study

Adsorption and protonation of CO2 on the (110) and (100) surfaces of gamma-Al2O3 have been studied using density functional theory slab calculations. On the dry (110) and (100) surfaces, the O-Al bridge sites were found to be energetically favorable for CO2 adsorption. The adsorbed CO2 was bound in a bidentate configuration across the O-Al bridge sites, forming a carbonate species. The strongest binding with an adsorption energy of 0.80 eV occurs at the O3c-Al5c bridge site of the (100) surface. Dissociation of water across the O-Al bridge sites resulted in partially hydroxylated surfaces, and the dissociation is energetically favorable on both surfaces. Water dissociation on the (110) surface has a barrier of 0.42 eV, but the same process on the (100) surface has no barrier with respect to the isolated water molecule. On the partially hydroxylated gamma-Al2O3 surfaces, a bicarbonate species was formed by protonating the carbonate species with the protons from neighboring hydroxyl groups. The energy difference between the bicarbonate species and the coadsorbed bidentate carbonate species and hydroxyls is only 0.04 eV on the (110) surface, but the difference reaches 0.97 eV on the (100) surface. The activation barrier for forming the bicarbonate species on the (100) surface, 0.42 eV, is also lower than that on the (110) surface (0.53 eV).

Synthesis of magnetite nanoparticle aqueous dispersions in an ionic liquid containing acrylic acid anion

Stable magnetite nanoparticle aqueous dispersions were prepared by precipitating ferrous ions in the presence of an ionic liquid containing acrylic acid anion, viz., N-methylimidazolium acrylic acid (MImAA) in air at high pH. The size of the as-prepared magnetite nanoparticles could be tuned from 12 nm to 20 nm depending on the concentration of MImAA in the medium. X-ray diffractions showed the presence of only Fe 3O 4 phase in the nanoparticles. The ionic liquid with some dimers was demonstrated to be an effective stabilizer for magnetite nanoparticle dispersions. The FTIR and TGA analysis confirmed the attachment of the acrylic acid anion in ionic liquid predominantly on the particle surface in the chelating bidentate configuration.

Molecular Structure of [Cu2(MeCN)2(μ-tpy)2][BPh4]2: A Helical Di-Cuprous Terpyridine Complex

AbstractThe molecular structure of [Cu 2 (MeCN) 2 (μ-tpy) 2 ][BPh 4 ] 2 has been determined. The two terpyridine ligands coordinate to the same Cu(I) center in a bidentate configuration, in which the copper adopted a distorted tetrahedral geometry with the four nitrogen donors. The third ring of each terpyridine ligand forms a bridge with a second Cu(I) center, whose tetrahedral coordination sphere is completed by two acetonitrile ligands. The resulting structure represents a section of a double helix. Crystal data: space group C2/c, a = 27.202(5), b = 12.995(3), c = 23.409(5) A, β = 123.13(3)°; V = 6930(2) A 3 , Z = 4, R = 0.0613, wR 2 = 0.0946.Graphical AbstractThe molecular structure of [Cu 2 (MeCN) 2 (μ-tpy) 2 ][BPh 4 ] 2 has been determined. The terpyridine ligands coordinate in a bidentate fashion to the same Cu center and monodentate to a second Cu center resulting in a double helical structural motif.[IMAGE]

Spectroscopic and structural studies of di-ureasils doped with lithium perchlorate

Di-urea cross-linked POE/siloxane hybrid ormolytes (di-ureasils) doped with a wide concentration range of lithium perchlorate trihydrate (LiClO 4·3H 2O) (200 ≥ n ≥ 0.5, where n expresses the salt content in terms of the number of ether oxygen atoms per Li + ion) have been analysed by Fourier transform infrared and Raman (FT-IR and FT-Raman, respectively) spectroscopies and X-ray diffraction (XRD). The results obtained lead us to conclude that the xerogels with n ≥ 5 are totally amorphous. At n ≤ 1 free salt is observed. “Free” ClO 4 − ions appear to be the main charge carriers at the conductivity maximum located within the 25 ≤ n ≤ 8 composition range of this family of ormolytes. At n = 15 ClO 4 − ions coordinated in mono/tridentate (C 3v symmetry) and bidentate (C 2v symmetry) configurations were detected. In salt-rich samples with n < 15 there is a marked tendency for ionic association. The resulting decrease that occurs in the concentration of “free” ions is consistent with the observed significant decrease of the ionic conductivity. The analysis of the “amide I” and “amide II” regions provided solid proof that the Li + ions bond to the urea carbonyl oxygen atoms over the entire range of salt concentration studied.

Adsorption of phenylalanine on single crystal rutile TiO 2(1 1 0) surface

The adsorption of the aromatic amino acid, phenylalanine on a TiO 2 rutile (1 1 0) single crystal surface has been investigated with photoemission and NEXAFS (near edge X-ray absorption fine structure) spectroscopy. The results indicate initial adsorption via the carboxylate group in a bidentate configuration with the phenyl ring oriented at approximately 25° to the surface normal. The amino group remains as NH 2. Subsequent layers of phenylalanine appear to adsorb as neutral molecules with H-bonding between NH 2 and C O groups.

Effect of H2O on the Adsorption of NO2 on γ-Al2O3: an in Situ FTIR/MS Study

The effect of water on the adsorption of NO2 onto a γ-Al2O3 catalyst support surface was investigated using Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS). Upon room-temperature exposure of the alumina surface to small amounts of NO2, nitrites and nitrates are formed, and at higher NO2 doses only nitrates are observed. The surface nitrates formed were of bridging monodentate, bridging bidentate, and monodentate configuration. At elevated NO2 pressures, the surface hydroxyls were consumed in their reaction with NO2 giving primarily bridge-bound nitrates. A significant amount of weakly adsorbed N2O3 was seen as well. Exposure of the NO2-saturated γ-Al2O3 surface to H2O resulted in the desorption of some NO2 + NO as H2O interacted with the weakly held N2O3, while the bridging monodentate surface nitrates converted into monodentate nitrates. The conversion of these oxide-bound nitrates to water-solvated nitrates was observed at high water doses when the presence of liquid-like water is expected on the surface. The addition of H2O to the NO2-saturated γ-Al2O3 did not affect the amount of NOx strongly adsorbed on the support surface. In particular, no NOx desorption was observed when the NO2-saturated sample was heated to 573 K prior to room-temperature H2O exposure. The effect of water is completely reversible; i.e., during temperature-programmed desorption (TPD) experiments following NO2 and H2O coadsorption, the same IR spectra were observed at temperatures above that required for H2O desorption as seen for NO2 adsorption only experiments.

Structure and Bonding of the Multifunctional Amino Acid l-DOPA on Au(110)

In investigations of the proteins which are responsible for the surface adhesion of the blue mussel Mytilus edulis, an unusually frequent appearance of the otherwise rare amino acid 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) has been observed. This amino acid is thought to play a major role in the mechanism of mussel adhesion. Here we report a detailed structural and spectroscopic investigation of the interface between L-DOPA and a single-crystalline Au(110) model surface, with the aim of understanding fundamentals about the surface bonding of this amino acid and its role in mussel adhesion. Molecular layers are deposited by organic molecular beam deposition (OMBD) in an ultrahigh-vacuum environment. The following experimental techniques have been applied: ex situ Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). Vibrational spectra of isolated L-DOPA molecules and the zwitterionic bulk have been calculated using density functional theory (DFT). The predicted modes are assigned to observed spectra, allowing conclusions regarding the molecule-substrate and molecule-molecule interactions at the L-DOPA/Au(110) interface. We find that zwitterionic L-DOPA forms a monochiral, one-domain commensurate monolayer on Au(110), with the catechol rings on top of [110] gold rows, oriented parallel to the surface. The (2 x 1)-Au(110) surface reconstruction is not lifted. The carboxylate group is found in a bidentate or bridging configuration, the amino group is tilted out of the surface plane, and the hydroxyl groups do not dehydrogenate on Au(110). Similar to the case for the bulk, molecules form dimers on Au(110). However, the number of hydrogen bridge bonds between L-DOPA molecules is reduced as compared to the bulk. Thicker layers which are deposited onto the commensurate interface do not order in the bulk structure. In conclusion, our study shows that the aromatic ring system of L-DOPA functions as a surface anchor. Since it is also known that the hydroxyl groups support cross-link reactions between L-DOPA residues in the mussel glue protein, we can conclude that the catechol ring supports surface adhesion of mussel proteins via two independent functions.

In-Situ Infrared Study of the Adsorption and Oxidation of Oxalic Acid at Single-Crystal and Thin-Film Gold Electrodes: A Combined External Reflection Infrared and ATR−SEIRAS Approach

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR-SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm(-)(1) that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the nu(s)(O-C-O) and nu(C-OH) + delta(C-O-H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK(a) for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.

Rietveld structure refinement of NH4-exchanged natural chabazite

In this work the results of the X-ray Rietveld structure refinement of a natural and the corresponding NH 4 -exchanged chabazite from Nova Scotia (Canada) are exposed. Experimental data were collected using a laboratory powder diffractometer equipped with copper tube and graphite crystal monochromator. The outcome of this crystal-structure study is useful for (i) understanding the structure modifications induced by the NH 4 + exchange; (ii) understanding the physical-chemical and technological properties of NH 4 -chabazite, a zeolite vastly used for industrial applications; (iii) understanding the mechanism of proton conductivity of the NH 4 -exchanged zeolites, precursors of catalytically active H + forms. The position and orientation of the NH 4 + ion were initially refined using the rigid body model and later with the aid of soft constraints. The coordination number of the ammonium ion is 9 with two equally possible and mutually exclusive configurations. One coordination environment includes 3 oxygen atoms O3, 3 oxygen atoms O4, and 3 H 2 O molecules W2a. The other environment includes 3 oxygen atoms O3, 3 oxygen atoms O4, and 3 H 2 O molecules W3. As already shown for other NH 4 -zeolites, the N-O distances are larger than the N-H 2 O distances. The local geometry of the ammonium ion points to a monodentate configuration. A bidentate configuration of the hydrogen bonds for NH 4 + is also possible if the long H2…O3 separation is considered to be at bond distance. For industrial and technological applications, knowledge of the local environment of NH 4 + in the cavities of zeolites is important. Weak hydrogen bonds with framework oxygen atoms implies that the ammonium molecule can be easily exchanged or desorbed. This property is attractive for agronomy, horticulture and soil remediation where zeolite can be added to chemical fertilizers to improve the soil9s chemical and physical properties for plant growth, to increase fertilizer efficiency and to reduce the leaching of nutrients, to reduce the dissolution rate of a soluble fertilizer via ion exchange or combination of mineral dissolution and ion exchange, and to act as remediation agents in soils.

Tribochemistry of Fluorinated Fluids Hydroxyl Groups on Steel and Aluminum Surfaces

This paper describes products of tribochemical reactions of Z-Dol 2500 and 1H,1H,2H,2H-tetra-fluorooctanol, produced on steel and aluminum surfaces. The fluorooctanol was used as a model compound to study the reactions of hydroxyl groups. Tribochemical reactions were initiated by friction in the steel/steel and steel/Al contacts, using an Optimol SRV (Schwingung Reibung Verschleiβ) reciprocating sliding tester. The products of these reactions, deposited in the wear scars on steel and Al discs, were studied using reflective FTIR spectrometry (microscopy and grazing angle) and X-Ray photoelectron spectroscopy (XPS). Under boundary lubrication conditions on metal surfaces, fluorinated compounds containing hydroxyl groups formed carboxylic and alkoxyl salts. FTIR spectra indicate that Fe and Al carboxylic salts occur mainly in bidentate configuration. Presented at the STLE Annual Meeting in Las Vegas, Nevada May 15-19, 2005 Review led by Tom Karis

Nature and thermal stability of adsorbed intermediates formed during the reaction of diesel soot with nitrogen dioxide

The reactivity of a diesel exhaust soot sample towards 1000ppm NO2/balance Ar was investigated at moderate temperature using thermogravimetric analysis (TGA), temperature-programmed desorption (TPD-MS) and diffuse reflectance infrared spectroscopy (DRIFTS) with the aim to evidence the nature and thermal stability of adsorbed intermediates formed during the reaction. In the range 25–200°C, the reaction pathways are influenced by the temperature dependency of the reaction regimes. While weak adsorption of NO2 prevails at temperatures under ca. 60°C, it is found that associative and dissociative chemisorption occurs on the whole range of temperatures investigated, creating, respectively, nitrogenated (RNOx) and oxygenated (ROx) adsorbed intermediates on soot surface. DRIFTS and TPD experiments reveal that major surface species include on the one hand alkyl-nitrite and -nitrate esters adopting presumably a chelated or bidentate configuration on soot surface and on the other hand more stable acidic oxygenated surface complexes formed by reduction of NO2.

CO2 Sorption on MgO and CaO Surfaces: A Comparative Quantum Chemical Cluster Study

A comparative quantum chemical study of CO2 adsorption on MgO and CaO has been carried out. Theoretical infrared (IR) frequencies are calculated and compared with IR experiments from the literature. The results show that CO2 adsorbs as monodentate on edge sites and bidentate on corner sites on MgO. The former assignment contradicts the common assumption of adsorption of CO2 in a bidentate configuration. On CaO, CO2 adsorbs as monodentate on both edge and corner sites, which is a reinterpretation of earlier experimental work. On terrace (100) sites, none of the adsorption modes on MgO or CaO possess calculated frequencies in agreement with the experimental IR spectra. These experimental bands were tentatively assigned to some slightly perturbed double negatively charged carbonate ions at the surface, rather than the monodentate structure suggested in the literature.

Vibrational EELS and DFT study of propionic acid and pyruvic acid on Ni(100): Effects of keto group substitution on room-temperature adsorption and thermal chemistry

Abstract The room-temperature adsorption and thermally induced processes of propionic acid and pyruvic acid on Ni(1 0 0) have been investigated by electron energy loss spectroscopy (EELS). Computational vibrational analysis of the optimized bidentate structures for acid–Ni model complexes (involving the organic acid and a Ni atom) has been performed by using the two-layer ONIOM method with the Density Functional Theory and used to interpret the vibrational EELS data. Dehydrogenation of the hydroxyl group is found to result in bonding of the carboxylate group in the propionate and pyruvate adspecies to either a single Ni surface atom in a bidentate configuration or two neighbouring Ni atoms in a bridge configuration. Given the similarities in the total energies and related vibrational frequencies obtained by the calculations in the case of pyruvate adspecies, it is difficult to differentiate the alternate adsorption structure, in which the keto O and hydroxyl O atoms are bonded to a Ni atom in a five-member chelate ring configuration. Furthermore, temperature-dependent EELS studies show that both the propionate and pyruvate adspecies could decompose upon annealing to above 400 K and further dissociate to CO adspecies above 550 K and to C and/or O above 600 K.

Structural and Spectroelectrochemical Study of Carbonate and Bicarbonate Adsorbed on Pt(111) and Pd/Pt(111) Electrodes

Spectroelectrochemical experiments and density functional theory (DFT) based calculations are combined to elucidate the nature of the species adsorbed when a perchloric acid solution saturated with carbon dioxide is in contact with well-defined electrode surfaces. Previous results reported for the Pt(111) electrode are revised and completed with new data for a palladium monolayer deposited on the Pt(111) electrode. From the DFT calculations and from the potential and pH dependent behavior of the main adsorbate vibrational bands, it is suggested that, for potentials below 0.65 V, bicarbonate species are adsorbed at the Pt(111) electrode surface in a short-bridge bidentate configuration. At higher potentials, the adsorbate adlayer is completed by the formation of adsorbed carbonate also in a short-bridge configuration. The ratio between the surface coverages of adsorbed bicarbonate and carbonate depends on both the electrode potential and the solution pH. Adsorbed bicarbonate prevails at the palladium monol...

FTIR Study of Adsorption and Photoassisted Oxygen Isotopic Exchange of Carbon Monoxide, Carbon Dioxide, Carbonate, and Formate on TiO2

In-situ FTIR has been employed to investigate adsorption and photoassisted oxygen scrambling of CO, CO2, CO3, and HCOO on powdered TiO2 with 18O2. As clean TiO2 is exposed to CO at 35 °C, the IR frequencies of adsorbed CO shows that CO is bonded to Ti ions with an oxidation state +4 or less. The CO molecules that are bonded to two metal centers are more stable than those to only one center. In the case of CO2 adsorption, it produces adsorbed CO3 and CO2 with Ti−O−C−O bonding. Adsorbed CO does not exchange oxygen with 18O2, while adsorbed CO2, CO3, and HCOO do under UV irradiation. In the case of CO3 with bidentate configuration, only the oxygen bonded to the carbon with carbonyl character is involved in the exchange. Possible mechanisms for the oxygen scrambling of CO2, CO3, and HCOO are discussed.