Absolute Gas-phase Research Articles

A theoretical approach to the corrosion inhibition of iron in acidic solution by a green formulation derived from Nigella sativa L seeds oil

A theoretical approach combining density functional theory (DFT) computation, Monte Carlo (MC) methods, and molecular dynamics (MD) simulations was used to investigate and validate the iron corrosion inhibition performances in the acidic medium of a green formulation based on the oil extracted from Nigella sativa L. DFT calculations allowed to determine the energetic, global, and local chemical reactivity parameters. The most stable adsorption configuration of the fatty acids components of the Nigella sativa L. formulation was determined by the Monte Carlo methods with the adsorption locator module and by molecular dynamics simulations. Results indicated that all the compounds are absorbed with a totally flat orientation on the first layer of the metal surface.The active sites of molecules were effectively established by using DFT through the analysis of frontier molecular orbitals (FMO), the Hirshfeld population analysis (HPA), the electrostatic potential (ESP), molecular electron density analysis (MED), and Fukui indices (FI). The solvent effect was investigated by using the conductor-like polarizable continuum model (CPCM). Protonation effects were also analysed through the protonation energy (PE), the proton affinity (PA), and the absolute gas phase basicity (GB). The formation of a coating on the iron surface validated the good protective action of the green formulation derived from Nigella sativa L.The computational and theoretical approaches substantiate our experimental electrochemical and spectroscopic studies [1].

Direct Constraints on the Extremely Metal-poor Massive Stars Underlying Nebular C iv Emission from Ultra-deep HST/COS Ultraviolet Spectroscopy

Metal-poor nearby galaxies hosting massive stars have a fundamental role to play in our understanding of both high-redshift galaxies and low-metallicity stellar populations. But while much attention has been focused on their bright nebular gas emission, the massive stars that power it remain challenging to constrain. Here we present exceptionally deep Hubble Space Telescope ultraviolet spectra targeting six local (z < 0.02) galaxies that power strong nebular C iv emission approaching that encountered at z > 6. We find that the strength and spectral profile of the nebular C iv in these new spectra follow a sequence evocative of resonant scattering models, indicating that the hot circumgalactic medium likely plays a key role in regulating C iv escape locally. We constrain the metallicity of the massive stars in each galaxy by fitting the forest of photospheric absorption lines, reporting measurements driven by iron that lie uniformly below 10% solar. Comparison with the gas-phase oxygen abundances reveals evidence for enhancement in O/Fe 2–4 times above solar across the sample, robust to assumptions about the absolute gas-phase metallicity scale. This supports the idea that these local systems are more chemically similar to their primordial high-redshift counterparts than to the bulk of nearby galaxies. Finally, we find significant tension between the strong stellar wind profiles observed and our population synthesis models constrained by the photospheric forest in our highest-quality spectra. This reinforces the need for caution in interpreting wind lines in isolation at high redshift, but also suggests a unique path toward validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.

Direct Far-infrared Metal Abundances (FIRA). I. M101

Abstract Accurately determining gas-phase metal abundances within galaxies is critical as metals strongly affect the physics of the interstellar medium. To date, the vast majority of widely used gas-phase abundance indicators rely on emission from bright optical lines, whose emissivities are highly sensitive to the electron temperature. Alternatively, direct-abundance methods exist that measure the temperature of the emitting gas directly, though these methods usually require challenging observations of highly excited auroral lines. Low-lying far-infrared (FIR) fine structure lines are largely insensitive to electron temperature and thus provide an attractive alternative to optically derived abundances. Here, we introduce the far-infrared abundance (FIRA) project, which employs these FIR transitions, together with both radio free–free emission and hydrogen recombination lines, to derive direct, absolute gas-phase oxygen abundances. Our first target is M101, a nearby spiral galaxy with a relatively steep abundance gradient. Our results are consistent with the O++ electron temperatures and absolute oxygen abundances derived using optical direct-abundance methods by the CHemical Abundance Of Spirals (CHAOS) program, with a small difference (∼1.5σ) in the radial abundance gradients derived by the FIR/free–free-normalized versus CHAOS/direct-abundance techniques. This initial result demonstrates the validity of the FIRA methodology—with the promise of determining absolute metal abundances within dusty star-forming galaxies, both locally and at high redshift.

Ag+ reduction and silver nanoparticle synthesis at the plasma–liquid interface by an RF driven atmospheric pressure plasma jet: Mechanisms and the effect of surfactant

The involvement of plasma produced species in the reduction of silver ions at the plasma–liquid interface is investigated using a well-characterized radio-frequency driven atmospheric pressure plasma jet. The absolute gas phase H density was measured using two photon absorption laser induced fluorescence in the free jet. Broadband absorption and transmission electron microscopy were used to study the synthesis of silver nanoparticles (AgNPs). It is shown that fructose, an often used surfactant/stabilizer for AgNP synthesis, also acts as a reducing agent after plasma exposure. Nonetheless, surfactant free AgNP synthesis is observed. Several experimental findings indicate that H plays an important role in the reduction of silver ions for the plasma conditions in this study. Vacuum ultraviolet photons generated by the plasma are able to reduce silver ions in the presence of fructose. Adding H2 to the argon feed gas leads to the production of a large amount of AgNPs having a particle size distribution with a maximum at a diameter of 2–3 nm, which is not observed for argon plasmas. This finding is consistent with a smaller concentration of reducing species at the plasma–liquid interface for Ar with the H2 admixture plasma. The smaller flux of reactive species to the liquid is in this case due to a less strong interaction of the plasma with the liquid. The formation of the nanoparticles was observed even at a distance of 6–7 mm below the tip of the plasma plume, conditions not favoring the injection of electrons.

Antioxidative stress effect of epicatechin and catechin induced by Aβ25–35 in rats and use of the electrostatic potential and the Fukui function as a tool to elucidate specific sites of interaction

Alzheimer's disease (AD) is a neurodegenerative disorder caused by the aggregation of the amyloid-beta peptide (Aβ) in senile plaques and cerebral vasculature. The Aβ25–35 fraction has shown the most toxicity; its neurotoxic mechanisms are associated with the generation of oxidative stress and reactive astrogliosis that induce neuronal death and memory impairment. Studies indicate that pharmacological treatment with flavonoids reduces the rate of AD, in particular, it has been shown that antioxidants are compounds that could interact with this peptide due to their antioxidant proprieties. In this study, experimental and computational tools were used to calculate the molecular electrostatic potential and the Fukui function with the Gaussian 09 computational program, to predict the most reactive parts of these molecules and make the complex between Aβ25–35 and two flavonoids (catechin and epicatechin) in the absolute gas-phase, where a possible interaction between them was observed. This is important for understanding the Aβ25–35–Flavonoid (A–F) interaction as a therapeutic strategy to inhibit the neurotoxic effects that this peptide causes in AD, which currently is still considered an ambiguous process.

Direct quantification of chemical warfare agents and related compounds at low ppt levels: comparing active capillary dielectric barrier discharge plasma ionization and secondary electrospray ionization mass spectrometry.

A novel active capillary dielectric barrier discharge plasma ionization (DBDI) technique for mass spectrometry is applied to the direct detection of 13 chemical warfare related compounds, including sarin, and compared to secondary electrospray ionization (SESI) in terms of selectivity and sensitivity. The investigated compounds include an intact chemical warfare agent and structurally related molecules, hydrolysis products and/or precursors of highly toxic nerve agents (G-series, V-series, and "new" nerve agents), and blistering and incapacitating warfare agents. Well-defined analyte gas phase concentrations were generated by a pressure-assisted nanospray with consecutive thermal evaporation and dilution. Identification was achieved by selected reaction monitoring (SRM). The most abundant fragment ion intensity of each compound was used for quantification. For DBDI and SESI, absolute gas phase detection limits in the low ppt range (in MS/MS mode) were achieved for all compounds investigated. Although the sensitivity of both methods was comparable, the active capillary DBDI sensitivity was found to be dependent on the applied AC voltage, thus enabling direct tuning of the sensitivity and the in-source fragmentation, which may become a key feature in terms of field applicability. Our findings underline the applicability of DBDI and SESI for the direct, sensitive detection and quantification of several CWA types and their degradation products. Furthermore, they suggest the use of DBDI in combination with hand-held instruments for CWAs on-site monitoring.

Absolute electron total ionization cross-sections: molecular analogues of DNA and RNA nucleobase and sugar constituents.

Accurate ionization cross-sections for DNA and RNA constituents in the condensed or aqueous phase are important parameters for models simulating radiation damage to genetic material in living cells. In this work, absolute gas-phase electron total ionization cross-sections (TICSs) have been measured for a series of six aromatic and eight non-aromatic cyclic species that can be considered as prototype functional group analogues for the nucleobases and sugar backbone constituents of DNA and RNA. TICSs for water, hexane, and ethylacetamide (a peptide bond analogue) are also reported. The experimental apparatus utilizes a cylindrical ion collector that surrounds the ionization region, providing essentially unit detection efficiency. Two theoretical models, the polarizability-correlation method and binary-encounter Bethe theory, are able to reproduce the measured maximum TICS well for all species studied. An empirical energy-dependent correction is found to yield improvement in the agreement between experimental energy-dependent cross sections and the predictions of the BEB model. Having characterised and optimised the performance of both models, they are then used to predict TICSs for gas-phase DNA and RNA nucleobases and sugars. Direct experimental determinations of TICSs for these species are difficult because of their low volatility, which makes it difficult to prepare suitable gas-phase samples for measurement.

Understanding the Nucleophilic Character and Stability of the Carbanions and Alkoxides of 1-(9-Anthryl)ethanol and Derivatives

The nucleophilic character and stability of the carbanions vs. alkoxides derived from 2,2,2-trifluoro-1-(9-anthryl)ethanol and 1-(9-anthryl)ethanol containing X electron-releasing and X electron-acceptor substituents attached to C-10, have been studied at the B3LYP/6-31+G(d,p) level of theory. Results analyzed in terms of the absolute gas-phase acidity, Fukui function, the local hard and soft acids and bases principle, and the molecular electrostatic potential, show that the central ring of the 9-anthryl group confers an ambident nucleophilic character and stabilizes the conjugated carbanion by electron-acceptor delocalization.

Quantum chemical investigation on the influence of amino substitution on proton affinity of oxazolidin-2-one

The scenarios of preferred protonation sites and the absolute gas-phase proton affinities of C5- and N4-amino derivatives of oxazolidinone (OXA) molecules possessing two oxygen and two nitrogen atoms, are studied to investigate the effect of substitution of amino group on geometry, electronic structure, and proton affinities of these molecules. The natural bond orbital analysis is invoked to obtain the second-order delocalization energies, occupations of lone pairs, charge distribution, and bond orders to rationalize the obtained results. Our findings reveal a strong nucleophilicity of O1 site in C5-amino and N4-amino-substituted OXA isomers just as in un-substituted OXA. The substituent nitrogen in N4-amino-substituted OXA has comparable electrophilicity to O1 site while lesser than acyl oxygen and higher than nitrogen of OXA ring in C5-amino-substituted OXA. The PA values of C5- and N4-amino-substituted OXA isomers span in the range 172.06–205.77 kcal mol−1 (at CBS-Q). The PA values for the potential sites increase in the range 1.96–27.08 kcal mol−1 as a result of the amino substitution at C5 and N4 in orientation (b) while exceptionally they decrease by 0.57–2.95 kcal mol−1 as a result of the amino substitution at N4 in orientation (a). The results for the order of PA values of potential sites have been supported by molecular electrostatic potential maps. Our findings indicate that the factors such as geometrical rearrangements, variations in atomic charge densities and electron delocalization, effect of substituent, intramolecular hydrogen bonding, and electronic changes direct the relative stabilities and proton affinities of N, C5-substituted amino OXA isomers.

First Experimental Determination of the Absolute Gas-Phase Rate Coefficient for the Reaction of OH with 4-Hydroxy-2-Butanone (4H2B) at 294 K by Vapor Pressure Measurements of 4H2B

The reaction of the OH radicals with 4-hydroxy-2-butanone was investigated in the gas phase using an absolute rate method at room temperature and over the pressure range 10-330 Torr in He and air as diluent gases. The rate coefficients were measured using pulsed laser photolysis (PLP) of H(2)O(2) to produce OH and laser induced fluorescence (LIF) to measure the OH temporal profile. An average value of (4.8 ± 1.2) × 10(-12) cm(3) molecule(-1) s(-1) was obtained. The OH quantum yield following the 266 nm pulsed laser photolysis of 4-hydroxy-2-butanone was measured for the first time and found to be about 0.3%. The investigated kinetic study required accurate measurements of the vapor pressure of 4-hydroxy-2-butanone, which was measured using a static apparatus. The vapor pressure was found to range from 0.056 to 7.11 Torr between 254 and 323 K. This work provides the first absolute rate coefficients for the reaction of 4-hydroxy-2-butanone with OH and the first experimental saturated vapor pressures of the studied compound below 311 K. The obtained results are compared to those of the literature and the effects of the experimental conditions on the reactivity are examined. The calculated tropospheric lifetime obtained in this work suggests that once emitted into the atmosphere, 4H2B may contribute to the photochemical pollution in a local or regional scale.

Gas-phase acidities of lysine homologues and proline analogs from the extended kinetic method

The absolute gas-phase acidities (ΔHacid) for three lysine homologues (ornithine (Orn), 2,4-diaminobutanoic acid (Daba) and 2,3-diaminopropanoic acid (Dapa)) and 2 proline analogs (azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip)) were determined using the extended kinetic method in an electrospray ionization–triple quadrupole instrument. The gas-phase acidities of the three lysine homologues (1415±10, 1419±7, and 1418±8kJ/mol for Orn, Daba, and Dapa, respectively) are the same as that of lysine (1416±7kJ/mol) obtained from earlier studies within error limits. The two proline analogs are less acidic (1425±13 and 1432±11kJ/mol for Aze and Pip) than the lysine analogs, but have the same acidity as proline (1430±7kJ/mol). Experimental acidities are supported by density functional theory calculations at the B3LYP/6-311++G**//B3LYP/6-31+G* level, which give predictions for the acidities from an isodesmic reaction with acetic acid as the reference bases. Agreement between theory is excellent (within 5kJ/mol) for Daba, Dapa, Aze, and Pip. The computed acidity for Orn is 9kJ/mol higher than the measured acidity, but is still within the error limits. As the difference in acidities within the sets of analogs is smaller than the absolute error bars, relative acidities (ΔGacid) were obtained using kinetic method ratios with 3-OH-benzoic acid as the reference acid. Relative acidity (ΔGacid) orderings of Dapa>Lys>Daba>Orn and Pro>Pip>Aze were obtained.

Influence of Fluorine Atoms and Aromatic Rings on the Acidity of Ethanol

Absolute gas-phase acidities Delta(acid)G(0)(OH) and Delta(acid)G(0)(CH) were calculated at the B3LYP and MP2 levels using six different standard basis sets for the OH and CH heterolytic bond cleavage of ethanol and twelve derivatives of the type CH(3-n)F(n)CHX(r)OH, where n ranges from zero to three and represents the number of fluorine atoms and r represents hydrogen and the type of aromatic ring, namely: X(0) = hydrogen, X(1) = phenyl, X(2) = 1-naphthyl, and X(3) = 9-anthryl. The similarity between calculated and experimental Delta(acid)G(0)(OH) values for ethanol (1a), 2-fluoroethanol (1b), 2,2-difluoroethanol (1c), 2,2,2-trifluoroethanol (1d), and 1-phenylethanol (2a) was used to validate the right theoretical method for this study. Substituent partial contributions to hydroxyl-, methylene-, and methine-hydrogen acidities were evaluated by linear combination. Good parameter fittings of the primary and secondary alcohols were obtained and interpreted as additive contribution of the substituent effects. The nonlinear contributions were identified. Calculations prove that fluoroalcohols exhibit C-H acidity, which is usually lower than O-H acidity. In principle, the inversion of this acidity order is possible by the introduction of a large aromatic ring instead to increase the number of fluorine atoms.

In situ parametric study of alkali release in pulverized coal combustion: Effects of operating conditions and gas composition

This work concerns a parametric study of alkali release in a lab-scale, pulverized coal combustor (drop tube reactor) at atmospheric pressure. Measurements were made at steady reactor conditions using excimer laser fragmentation fluorescence (ELIF) and with direct optical access to the flue gas pipe. In this way, absolute gas-phase alkali species could be determined in situ, continuously, with sub-ppb sensitivity, directly in the flue gas. A hard coal was fired in the range 1000–1300 °C, for residence times in the range 3–5 s and for air numbers λ (air/fuel ratios) from 1.15 to 1.50. In addition, the amount of chlorine, water vapor and sulfur, respectively, was increased in known amounts by controlled dosing of HCl, H 2O and SO 2 into the combustion gas to determine effects of these components on release or capture of the alkali species. The experimental results are also compared with values calculated using ash/fuel analyses and sequential extraction to obtain a fuller picture of alkali release in pulverized fuel combustion.

Ultraviolet photochemistry of trichlorovinylsilane and allyltrichlorosilane: vinyl radical (HCCH2) and allyl radical (H2CCHCH2) production in 193 nm photolysis

The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A''<-- X[combining tilde](1)A'' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.

A Comparison of Data Analysis Methods for Determining Gas Phase Stabilities by CID: Alkali Metal Complexes of Polyether Ionophore Antibiotics

The gas phase stabilities of Group I metal complexes of the polyether ionophore antibiotics lasalocid and monensin were investigated by collision induced dissociation mass spectrometry. Electrospray ionization was used with a triple quadrupole mass spectrometer for the determination of threshold dissociation energies upon application of increasing collision energies. Various data analysis techniques for the determination of dissociation energies are discussed to assess the most suitable method for determining the stabilities of the ionophore-metal complexes studied here. In all cases only the relative stabilities of different complexes may be obtained by the method presented in this study, which does not assess absolute gas phase dissociation energies. Correction factors have been applied, however, to account for the energy conversion during collisions of different metal complexes and the varying degrees of freedom of different sized ligands, allowing for the comparison of the stabilities of different ionophores with like-metals. The measured threshold dissociation energies were compared with respect to the ionic radius of the metal cation, revealing a maximum stability for the K+ complexes of both lasalocid and monensin. A striking decrease in the stabilities of the Rb+ and Cs+ complexes was observed and is believed to be related to a decreasing degree of coordination that the ionophores can accomplish with the larger metals.

Gas-phase acidity of D-glucose. A density functional theory study.

The gas-phase acidity of D-glucopyranose was studied by means of B3LYP calculations combined with 6-31G(d,p) or 6-31+G(d,p) standard basis sets. For each anomer, deprotonation of the various primary and secondary hydroxyl groups was considered. As in solution, the anomeric hydroxyl is found to be the most acidic for both anomers, but only when the 6-31+G(d,p) basis set is used for geometry optimization. Deprotonation of the anomeric hydroxyl induces an important C(1)--O endocyclic bond elongation and subsequently promotes an energetically favored ring-opening process as attested by the very small calculated activation barriers. The results also suggest that interconversion between the various deprotonated alpha- and beta-anomers may easily occur under slightly energetic conditions. B3LYP/6-311+G(2df,2p) calculations led to the an absolute gas-phase acidity of deltaacidGo(298)(alpha-D-glucose) = 1398 kJ mol(-1). This estimate matches well the only experimental value available to date. Finally, this study again confirms that the use of diffuse functions on heavy atoms is necessary to describe anionic systems properly and to achieve good relative and absolute gas-phase acidities.

O–H Bond dissociation enthalpies in hydroxyphenols. A time-resolved photoacoustic calorimetry and quantum chemistry study

Time-resolved photoacoustic calorimetry (TR-PAC) was used to investigate the energetics of O–H bonds of phenol, catechol, pyrogallol, and phloroglucinol. Values of −27.1 ± 3.9, −44.1 ± 4.4 and −1.6 ± 3.8 kJ mol−1, respectively, were obtained for the solution-phase (acetonitrile) O–H bond dissociation enthalpies of the last three compounds relative to the O–H bond dissociation enthalpy in phenol, ΔDHosln(ArO–H) = DHosln(ArO–H) − DHosln(PhO–H). A value of 388.7 ± 3.7 kJ mol−1 was determined for the PhO–H bond dissociation enthalpy in acetonitrile. Density functional theory (MPW1PW91/aug-cc-pVDZ) calculations and complete basis set (CBS-4M) calculations were carried out to analyse intramolecular hydrogen bonding and to predict gas-phase O–H bond dissociation enthalpies, DHo(ArO–H). A microsolvation model, based on the DFT calculations, was used to study the differential solvation of the phenols and their radicals in acetonitrile and to bridge solution- and gas-phase data. The results strongly suggest that ΔDHosln(ArO–H) ≈ ΔDHo(ArO–H). Hence, to calculate absolute gas-phase O–H bond dissociation enthalpies in substituted phenols from the corresponding solution-phase values, the solvation enthalpies of the substituted phenols and their radicals are not required.

Gas-phase acidities and sites of deprotonation of 2-ketones and structures of the corresponding enolates

The gas-phase acidities of seven different 2-alkanone molecules have been investigated using pulsed ionization high pressure mass spectrometry (HPMS). From the temperature dependence of the equilibrium constant for proton-exchange reactions, the enthalpy and entropy changes for these reactions were determined. These experimental values agreed well with the G3(MP2) calculated values. From the thermochemistry of the proton-exchange reactions a gas-phase acidity scale, relative to acetone, was constructed for 2-ketones up to and including 2-decanone. Furthermore, an absolute gas-phase acidity scale is presented which is anchored to the gas-phase acidity value for acetone determined by Bartmess et al. [J. Am. Chem. Soc. 101 (1979) 6046]. The entropy changes associated with the proton-exchange reactions were all found to be very close to zero which suggests that there is no intramolecular solvation occurring in any of the enolate ions over the temperature range studied. The G3(MP2) calculations predict that the primary and secondary enolates of butanone, formed by deprotonation at either C1 or C3, respectively, have nearly identical basicities. However, as the 2-ketone chain length increases, the secondary enolate becomes more stable, with respect to the primary enolate. These theoretical predictions are in agreement with the stability ordering for enolates (2°≥1°⪢3°) found experimentally by Chyall et al. [J. Am. Chem. Soc. 116 (1994) 8681].

The temperature dependence of absolute gas phase acidities

Using both ab initio and semiempirical calculations, the temperature dependence of the thermochemical cycle relating gas phase enthalpies of acidity to electron affinities and bond dissociation energies is examined. In almost all cases, the effect of temperature is less than the uncertainties in the thermochemical quantities themselves.

Protonation of thymine, cytosine, adenine, and guanine DNA nucleic acid bases: Theoretical investigation into the framework of density functional theory

Gradient-corrected density functional computations with triple-zeta-type basis sets were performed to determine the preferred protonation site and the absolute gas-phase proton affinities of the most stable tautomer of the DNA bases thymine (T), cytosine (C), adenine (A), and guanine (G). Charge distribution, bond orders, and molecular electrostatic potentials were considered to rationalize the obtained results. The vibrational frequencies and the contribution of the zero-point energies were also computed. Significant geometrical changes in bond lengths and angles near the protonation sites were found. At 298 K, proton affinities values of 208.8 (T), 229.1 (C), 225.8 (A), and 230.3 (G) kcal/mol were obtained in agreement with experimental results. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 989–1000, 1998