Fast thermal desorption spectroscopy study of morphology and vaporization kinetics of polycrystalline ice films

Fast thermal desorption spectroscopy was used to investigate the vaporization kinetics of thin (50-100 nm) H(2)O(18) and HDO tracer layers from 2-5 microm thick polycrystalline H(2)O(16) ice films at temperatures ranging from -15 to -2 degrees C. The isothermal desorption spectra of tracer species demonstrate two distinct peaks, alpha and beta, which we attribute to the vaporization of H(2)O(18) initially trapped at or near the grain boundaries and in the crystallites of the polycrystalline ice, respectively. We show that the diffusive transport of the H(2)O(18) and HDO tracer molecules in the bulk of the H(2)O(16) film is slow as compared to the film vaporization. Thus, the two peaks in the isothermal spectra are due to unequal vaporization rates of H(2)O(18) from grain boundary grooves and from the crystallites and, therefore, can be used to determine independently the vaporization rate of the single crystal part of the film and rate of thermal etching of the film. Our analysis of the tracer vaporization kinetics demonstrates that the vaporization coefficient of single crystal ice is significantly greater than those predicted by the classical vaporization mechanism at temperatures near ice melting point. We discuss surface morphological dynamics and the bulk transport phenomena in single crystal and polycrystalline ice near 0 degrees C.     

Enhanced reducibility of Ce1-xTixO2 compared to that of CeO2 and higher redox catalytic activity of Ce1-x-yTixPtyO2-delta compared to that of Ce1-xPtxO2-delta

Nanocrystalline Ce(1)(-)(x)Ti(x)O(2) (0 < or = x < or = 0.4) and Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2)(-)(delta) (x = 0.15, y = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce(1)(-)(x)Ti(x)O(2) (x = 0.0-04) show complete reduction of Ti(4+) to Ti(3+) and reduction of approximately 20% Ce(4+) to Ce(3+) state compared to 8% Ce(4+) to Ce(3+) in the case of pure CeO(2) below 675 degrees C. The substitution of Ti ions in CeO(2) enhances the reducibility of CeO(2). Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 degrees C over Ce(0.84)Ti(0.15)Pt(0.01)O(2)(-)(delta) is 5 and that over Ce(0.99)Pt(0.01)O(2)(-)(delta) is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce(1-)(x)(-)(y)Ti(x)Pt(y)O(2) (x = 0.15, y = 0.01, 0.02) compared to Ce(1)(-)(x)Pt(x)O(2) (x = 0.01, 0.02). Synergistic involvement of Pt(2+)/Pt degrees and Ti(4+)/Ti(3+) redox couples in addition to Ce(4+)/Ce(3+) due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E(F) is shown to be responsible for improved redox property and higher catalytic activity.     

Visible absorption spectrum of the CH3CO radical

The visible absorption spectrum of the acetyl radical, CH(3)CO, was measured between 490 and 660 nm at 298 K using cavity ring-down spectroscopy. Gas-phase CH(3)CO radicals were produced using several methods including: (1) 248 nm pulsed laser photolysis of acetone (CH(3)C(O)CH(3)), methyl ethyl ketone (MEK, CH(3)C(O)CH(2)CH(3)), and biacetyl (CH(3)C(O)C(O)CH(3)), (2) Cl + CH(3)C(O)H --> CH(3)C(O) + HCl with Cl atoms produced via pulsed laser photolysis or in a discharge flow tube, and (3) OH + CH(3)C(O)H --> CH(3)CO + H(2)O with two different pulsed laser photolysis sources of OH radicals. The CH(3)CO absorption spectrum was assigned on the basis of the consistency of the spectra obtained from the different CH(3)CO sources and agreement of the measured rate coefficients for the reaction of the absorbing species with O(2) and O(3) with literature values for the CH(3)CO + O(2) + M and CH(3)CO + O(3) reactions. The CH(3)CO absorption spectrum between 490 and 660 nm has a broad peak centered near 535 nm and shows no discernible structure. The absorption cross section of CH(3)CO at 532 nm was measured to be (1.1 +/- 0.2) x 10(-19) cm(2) molecule(-1) (base e).     

Synthesis, crystal structure, and nonlinear optical behavior of beta-unsubstituted meso-meso E-vinylene-linked porphyrin dimers

[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.     

Stern-Gerlach experiments of one-dimensional metal-benzene sandwich clusters: Mn(C6H6)m (M = Al, Sc, Ti, and V)

A molecular beam of multilayer metal-benzene organometallic clusters Mn(C6H6)m (M = Al, Sc, Ti, and V) was produced by a laser vaporization synthesis method, and their magnetic deflections were measured. Multidecker sandwich clusters of transition-metal atoms and benzene Scn(C6H6)n+1 (n = 1, 2) and Vn(C6H6)n+1 (n = 1-4) possess magnetic moments that increase monotonously with n. The magnetic moments of Al(C6H6), Scn(C6H6)n+1, and Vn(C6H6)n+1 are smaller than that of their spin-only values as a result of intracluster spin relaxation, an effect that depends on the orbital angular momenta and bonding characters of the orbitals containing electron spin. While Ti(C6H6)2 was found to be nonmagnetic, Tin(C6H6)n+1 (n = 2, 3) possess nonzero magnetic moments. The mechanism of ferromagnetic spin ordering in M2(C6H6)3 (M = Sc, Ti, V) is discussed qualitatively in terms of molecular orbital analysis. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition-metal atoms are formally zerovalent.     

Underpotential deposition of hydrogen on benzene-modified Pt(111) in aqueous H2SO4

The Pt(111) electrode is modified by an overlayer of C6H6 (ads) upon its cycling in the 0.05-0.80 V range in aq H2SO4 + 1 mM C6H6. The C6H6 (ads) overlayer significantly changes the underpotential-deposited H (H(UPD)) and anion adsorption, and cyclic-voltammetry (CV) profiles show a sharp cathodic peak and an asymmetric anodic one in the 0.05-0.80 V potential range. The C6H6 (ads) layer blocks the (bi)sulfate adsorption but facilitates the adsorption of one monolayer of H(UPD). Cycling of the benzene-modified Pt(111) in benzene-free aq 0.05 H2SO4 from 0.05 to 0.80 V results in a partial desorption of C6H6 (ads) and in a partial recovery of the CV profile characteristic of an unmodified Pt(111). The peak potential of the cathodic and anodic feature is independent of the scan rate, s (10 < or = s < or = 100 mV s(-1)), and the peak current density increases linearly with an increase of the scan rate. Temperature variation modifies the peak potential and current density but does not affect the charge density of the cathodic or anodic feature. Temperature-dependent studies allow us to determine the thermodynamic state function for the H(UPD) adsorption and desorption. Delta G degrees(ads)(H(UPD))assumes values from -4 to -12 kJ mol(-1), while has values from 9 to 14 kJ mol(-1). The values of delta Delta G degrees (delta Delta G degrees = delat Delta G degrees(ads) + delta Delta D degrees(des)) decrease almost linearly from 6 kJ mol(-1) at theta(H(UPD) --> 0 to 0 kJ mol(-1) at theta(H(UPD) --> 1. The nonzero values of delta Delta G degrees testify that the adsorbing and desorbing H(UPD) adatoms interact with an energetically different substrate. The lateral interactions changed from repulsive (omega = 29 kJ mol(-1) at theta(H(UPD) --> 0) to attractive (omega = -28 kJ mol(-1) at theta(H(UPD) --> 1) as the H(UPD) coverage increases. The values of delta S degrees(ads)(H(UPD)) increase from 19 to 56 J K(-1) mol(-1), while those of delta S degrees(des)(H(UPD)) decrease from 45 to -30 J K(-1) mol(-1) with an increase of H(UPD) coverage. The values of delta H degrees(des)(H(UPD)) and delta H degrees(des)(H(UPD)) vary from 0 to 27 kJ mol(-1). The Pt(111)-H(UPD) surface bond energy at the benzene-modified Pt(111) electrode falls in the 191-218 kJ mol(-1) range and is weaker than in the case of the unmodified Pt(111) electrode in the same electrolyte.     

The vaporization rate of ice at temperatures near its melting point

The first study of free vaporization kinetics of ice at temperatures near its melting point is reported. The experimental approach employed is based on a unique combination of thermal desorption spectroscopy, microcalorimetry, and time-of-flight mass spectrometry, making it possible to overcome challenges associated with the introduction of volatile solids into a high vacuum environment. Measurements of the vaporization rate of polycrystalline ice demonstrate that the vaporization kinetics deviate dramatically from those predicted by a simple mobile precursor mechanism. The vaporization rate follows Arrhenius behavior from -40 to 0 degrees C with an effective activation energy of 50+/-4 kJ/mol, which is significantly higher than the value predicted by the simple mobile precursor mechanism. Extrapolation of earlier measurements conducted below -40 degrees C yields a value of approximately 0.02 at 0 degrees C for the vaporization coefficient alphav. In contrast, experimentally determined vaporization coefficient is found to be 0.7+/-0.3 and shows a weak dependence on temperature up to the bulk melting point. The role of possible surface phase transitions in the mechanisms of release and uptake of H2O and other chemical species by ice surfaces is discussed.     

New challenges and insights in the detection and spectral identification of organic explosives by laser induced breakdown spectroscopy

With the objective of detection and identification of explosives, different organic compounds, including aromatic nitrocompounds, RDX, anthracene, 2,4-diaminotoluene (DAT), 4-methyl-3-nitroaniline (MNA) and pentaerythritol (PENT) have been analyzed by laser induced breakdown spectroscopy (LIBS). To avoid the secondary ionization and to discriminate between the spectral contribution due to air from that of the compound in the plasma generated in air, the emission signatures from atomic lines (C at 247.9 nm, H at 656.3 nm, N at 746.8 nm and O at 777.2 nm) and molecular bands (CN at 388.3 nm and C₂ at 516.5 nm) have been investigated in plasmas generated in air and in helium. The different possible pathways leading to the observation of molecular emissions have been studied, together with a discussion of the most useful tools for the explosives discrimination. Moreover, the effect of the laser fluence on the atomic and molecular emissions and their relationship with the oxygen balance of an organic explosive is presented.     

Oxidation of phenol in aqueous acid: characterization and reactions of radical cations vis-à-vis the phenoxyl radical

Aqueous sulfuric acid containing up to approximately 14 M acid (H0 > or = -7.0) was used as solvent in pulse radiolytic redox studies to characterize cationic transients of phenol (C6H5OH) and map their reactions. The primary radical yields were first measured to correlate the variation in various radical concentrations as a function of increasing acid fraction in the solvent. Compared to their respective values at pH 2, the G(Ox*) increased with almost a linear slope of approximately 0.024 micromol J(-1) for H0(-1) (or pH(-1)) up to H0 -6.0 (Ox* = *OH + SO4*-), whereas G(H*) increased with a slope of approximately 0.033 micromol J(-1) for H0(-1) (or pH(-1)) up to H0 -5.0. In the presence of > 10 M acid (H0 < -5.0), phenol was oxidized to its radical cation, C6H5OH*+, which further reacted with phenol and generated the secondary, dimeric radical cation, (C6H5OH)2*+, following an equilibrium reaction C6H5OH*+ + C6H5OH <==> (C6H5OH)2*+, with K(eq) = 315 +/- 15 M(-1). The two cationic radicals were characterized from their individual UV-vis absorption spectra and acidity. The C6H5OH*+ absorption peaks are centered at 276 and 419 nm, and it was found to be more acidic (pKa = -2.75 +/- 0.05) than (C6H5OH)2*+ (pKa = -1.98 +/- 0.02), having its major peak at 410 nm. On the other hand, in the presence of < 6.5 M acid the C6H5O* reactions followed the radical dimerization route, independent of the parent phenol concentration.     

Hydroxyl radical self-recombination reaction and absorption spectrum in water up to 350 degrees C

The rate constant for the self-recombination of hydroxyl radicals (*OH) in aqueous solution giving H2O2 product has been measured from 150 to 350 degrees C by direct measurement of the *OH radical transient optical absorption at 250 nm. The values of the rate constant are smaller than previously predicted by extrapolation to the 200-350 degrees C range and show virtually no change in this range. In combining these measurements with previous results, the non-Arrhenius behavior can be well described in terms of the Noyes equation kobs-1 = kact-1+ kdiff-1, using the diffusion-limited rate constant kdiff estimated from the Smoluchowski equation and an activated barrier rate kact nearly equal to the gas-phase high-pressure limiting rate constant for this reaction. The aqueous *OH radical spectrum between 230 and 320 nm is reported up to 350 degrees C. A weak band at 310 nm grows in at higher temperature, while the stronger band at 230 nm decreases. An isosbestic point appears near 305 nm. We assign the 230 nm band to hydrogen-bonded *OH radical, and the 310 nm band is assigned to "free" *OH. On the basis of the spectrum change relative to room temperature, over half of the *OH radicals are in the latter form at 350 degrees C.     

Spectral analysis of laser-induced ultraweak delayed luminescence in cultured normal and tumor human cells: temperature dependence

The emission spectrum of ultraviolet A laser induced ultraweak delayed luminescence in cell cultures of mammalian cells depended on the temperature during irradiation and photonic measurements. A new method using a sophisticated photomultiplier system was developed in order to find differences between normal and tumor cells. The maximal peak of the emitted light for cultures measured at low temperature of 10 degrees C was near 510 nm in the green visible region while following irradiation at 32 degrees C this maximum was shifted to yellow-orange at 570 nm both in normal and melanoma cells. Overall, this ultraweak photonic model of cultured cells provides to be a new and powerful non-invasive tool for developing new strategies in skin cancer detection.     

Real-time monitoring of Yb vapor density using an extended cavity violet diode laser

Based on laser absorption spectroscopy (LAS), we developed a vapor density monitor for controlling the vaporization rate of Yb using a tunable diode laser. The laser source consisted of an extended cavity violet diode laser which has an emission wavelength of 398.8 nm coincident with the Yb absorption transition line, 6s(2) 1S(0)-6s6p 1P(1). The light emitted from the diode laser was transmitted across an atomic vapor column generated by heating the Yb metal, while the laser frequency was scanned across the atomic transition line. By comparing the amount of incident light to the amount of light transmitted after the light passed through the vapor column, the vapor density was determined using the Beer's law. From the experimental results, we demonstrated that the diode-laser-based LAS operated successfully for the real-time monitoring of the Yb vapor density.     

Study of benzylperoxy radical using laser photolysis: ultraviolet spectrum, self-reaction, and reaction with HO2 kinetics

The ultraviolet absorption spectrum of benzylperoxy radical and the kinetics of the reactions 2C(6)H(5)CH(2)O(2) --> products (I) and C(6)H(5)CH(2)O(2) + HO(2) --> products (II) are studied. Experiments are carried out using the laser photolysis technique with time-resolved UV-visible absorption spectroscopy over the temperature range 298-353 K and the pressure range 50-200 Torr. The UV spectrum is determined relative to the known cross section of the ethylperoxy radical C(2)H(5)O(2) at 250 nm. Using factor analysis, the spectrum obtained is refined and the concentrations of the main absorbing species are extracted. The kinetic parameters are determined by analyzing and simulating the temporal profiles of the species concentrations and the experimental optical densities in the spectral region 220-280 nm. These are obtained using the recent UV spectra of the absorbing species existing in our mechanism. The Arrhenius expressions for reactions I and II are (cm(3).molecule(-1).s(-1)) k(I) = 2.50 x 10(-14)e(1562/)(T) and k(II) = 5.70 x 10(-14)e(1649/)(T). Our results are discussed and compared to literature data.     

注射成型掺酞菁铟反饱和吸收材料的研究

反饱和吸收是指介质的吸收系数随输入光强的增加而增大的一种现象。反饱和吸收材料在全光开关、光计算与通信以及光限幅和光稳幅等光电子领域中都有着广泛的应用前景。酞菁类化合物以其较大的非共振三阶极化率、内在的快速响应和良好的化学和热稳定性倍受关注。目前所研究的反饱和吸收材料多是溶液和薄膜样品，在实用化方面存在较大困难，并且一些反饱和吸收性能好的酞菁化合物在溶剂中易于分解，给薄膜制备和溶液的稳定带来了困难。本文采用注射成型方法将酞菁铟掺杂到聚甲基丙烯酸甲酯（PMMA）中，研究了样品的反饱和吸收性能，国内外鲜见相关报道。     

Generation and characterization of [(P)M-(X)-Co(TMPA)]n+ assemblies; P = Porphyrinate, M = FeIII and CoIII, X = O2-, OH-, O2(2-), and TMPA = tris(2-pyridylmethyl)amine

With the established chemistry of bridged [(porphyrinate)FeIII-X-CuII(ligand)]n+ [X = O2- (oxo), OH- (hydroxo), O22- (peroxo)] complexes, we investigated the effect of cobalt ion substitution for copper or copper and iron. Thus, in this report, the generation and characterization of new mu-oxo, micro-hydroxo, and micro-peroxo (micro-X) assemblies of [(porphyrinate)MIII-X-CoII/III(TMPA)]n+ assemblies is described, where M = FeIII or CoIII and TMPA = tris(2-pyridylmethyl)amine. The mu-oxo complex [(F8TPP)FeIII-O-CoII(TMPA)]+ (1, F8TPP = tetrakis(2,6-difluorphenyl)porphyrinate) was isolated by an acid-base self-assembly reaction of a 1:1 mixture of (F8TPP)FeIII-OH and [CoII(TMPA)(MeCN)]2+ upon addition of triethylamine. The crystal structure of 1.2C4H10O proved the presence of an unsupported Fe-O-Co moiety; angleFe-O-Co = 171.6 degrees and d(Fe...Co) = 3.58 A. Complex 1 was further characterized by UV-vis (lambdamax = 437 (Soret) and 557 nm), 1H NMR [delta 40.6 (pyrrole-H), 8.8 and 8.7 (m-phenyl-H), 8.0 (p-phenyl-H), 4.4 (PY-4H), 2.6 (PY-3H), 1.0 (PY-5H), -1.1 (PY-6H), and -2.7 (TMPA-CH2-) ppm], electrospray ionization (ESI) and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometric methods, Evans method NMR (microeff = 3.1), and superconducting quantum interference device (SQUID) susceptometry (J = -114 cm-1, S = 1). The micro-hydroxo analogue [(F8TPP)FeIII-(OH)-CoII(TMPA)]+ (2) [UV-vis lambdamax = 567 nm; delta 78 ppm (pyrrole-H); Evans NMR microeff = 3.7] was generated by addition of 1 equiv of triflic acid to 1. The protonation is completely reversible, and 1 is regenerated from 2 by addition of triethylamine. While (F8TPP)FeII/[CoII(TMPA)(MeCN)]2+/O2 chemistry does not lead to a stable micro-peroxo species, a dicobalt micro-peroxo complex [(TPP)CoIII-(O22-)-CoIII(TMPA)]2+ (3, TPP = meso-tetraphenylporphyrinate) forms from a reaction of O2 with a 1:1 mixture of the CoII precursor components at -80 degrees C [UV-vis lambdamax = 435 (Soret), 548, and 583 (weak) nm; silent EPR spectrum; diamagnetic NMR spectrum]. The oxygenation/deoxygenation equilibrium is reversible; warming solutions of 3 releases approximately 1 equiv of O2 and the reduced complexes are reformed.     

Photoionization thresholds of melanins obtained from free electron laser-photoelectron emission microscopy, femtosecond transient absorption spectroscopy and electron paramagnetic resonance measurements of oxygen photoconsumption

Free electron laser-photoelectron emission microscopy (FEL-PEEM), femtosecond absorption spectroscopy and electron paramagnetic resonance (EPR) measurements of oxygen photoconsumption were used to probe the threshold potential for ionization of eumelanosomes and pheomelanosomes isolated from human hair. FEL-PEEM data show that both pigments are characterized by an ionization threshold at 282 nm. However, pheomelanosomes exhibit a second ionization threshold at 326 nm, which is interpreted to be reflective of the benzothiazine structural motif present in pheomelanin and absent in eumelanin. The lower ionization threshold for pheomelanin is supported by femtosecond transient absorption spectroscopy. Unlike photolysis at 350 nm, following excitation of solubalized synthetic pheomelanin at 303 nm, the transient spectrum observed between 500 and 700 nm matches that for the solvated electron, indicating the photoionization threshold for the solubalized pigment is between 350 and 303 nm. For the same synthetic pheomelanin, EPR oximetry experiments reveal an increased rate of oxygen uptake between 338 nm and 323 nm, narrowing the threshold for photoionization to sit between these two wavelengths. These results on the solubalized synthetic pigment are consistent with the FEL-PEEM results on the human melanosomes. The lower ionization potential observed for pheomelanin could be an important part of the explanation for the greater incidence rate of UV-induced skin cancers in red-haired individuals.     

Crystal growth, thermal and optical studies on a semiorganic nonlinear optical material for blue-green laser generation

The semiorganic nonlinear optical material l-histidine bromide (l-HB) has been synthesized in aqueous solution and characterized by FT-IR, FT-Raman and FT-NMR spectroscopic techniques. The single crystals with dimensions 9mm x 4mm x 3mm were grown by slow evaporation techniques. The grown crystals were subjected to single crystal X-ray diffraction to determine the unit cell dimensions. The thermal stability of the grown crystal was analyzed by thermogravimetric (TG), differential thermal (DT) and differential scanning calorimetric (DSC) analyses. The UV-vis transmittance spectrum shows that it has a good optical transmittance in the entire visible region with the lower cutoff wavelength at 220 nm. The SHG conversion efficiency and laser damage threshold were measured using a Nd:YAG laser (1064 nm). The optical birefringence was measured in the visible region as a function of temperature in the range 30-150 degrees C by interference technique.     

Rapid determination of hydrogen peroxide in whitening patches for teeth using a new portable near-infrared spectrometer

A nondestructive determination method of hydrogen peroxide in whitening patches for teeth was developed by using a new portable near-infrared (NIR) spectrometer. Development of the portable NIR spectrometer was based on microchip technologies with photodiode arrays. By using the portable NIR spectrometer, the new determination method is very rapid; it requires less than 1 s. The conventional method for the determination of hydrogen peroxide, redox titration, requires about 2 h of analysis, including the sample extraction time from a sample matrix. The conventional method also uses hazardous and harmful solvents and, furthermore, its samples cannot be used after titration. To find the peak due to the O–H bond vibration of hydrogen peroxide under the existence of water which shows huge absorption O–H absorption around 1450 nm, the NIR spectra of a hydrogen peroxide aqueous solution were investigated. A clear variation of absorption based on the concentration of the hydrogen peroxide due to the O–H bond vibration was found in the standard deviation plot around 1400 nm. In this study, two kinds of whitening patch products, A and B, were used for samples. A partial least squares (PLS) regression was used for calibration and validation in the 1100 to 1720 nm spectral range. For validation results, the standard error of prediction (SEP) was 0.38% for Patch A and 0.37% for Patch B. This study shows the feasibility of using the portable NIR spectrometer with photodiode arrays for the rapid and safe determination of hydrogen peroxide in whitening patches.     

In situ surface oxidation study of a planar Co/SiO2/Si(100) model catalyst with nanosized cobalt crystallites under model Fischer-Tropsch synthesis conditions

The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study a planar Co/SiO(2)/Si(100) model catalyst with well-defined cobalt crystallites, close to the threshold value reported for oxidation in the literature (4-10 nm), was prepared by the spin coating method. The planar Co/SiO(2)/Si(100) model catalyst was characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering. The surface oxidation behavior of the nanosized metallic cobalt crystallites of 4-5 nm was studied using in situ near-edge X-ray absorption fine structure under model FTS conditions, i.e., H(2)/H(2)O = 1, P(Total) = 0.4 mbar, and 150-450 degrees C. No surface oxidation of metallic cobalt was observed under these model FTS conditions over a wide temperature range, i.e., 150-400 degrees C.     

Density functional computation of 49Ti NMR chemical shifts

49Ti chemical shifts of TiX4 (X = Cl, Br, F), TiClnMe(4-n) (n = 0-3), Ti(C5H5)2X2 (X = F, Cl, Br) and Ti(CO)6(2-) were computed, using geometries optimized with the gradient-corrected BP86 density functional, at the GIAO (gauge-including atomic orbitals)-Hartree-Fock, BPW91, and B3LYP levels. For this set of compounds, substituent effects on delta(49Ti) are reasonably well described with all methods considered; judged from mean absolute deviations from experiment, B3LYP performs best. Zero-point corrections to the delta(49Ti) values, evaluated from a perturbational approach based on vibrationally averaged effective geometries, turn out to be fairly small. Electric field gradients computed with the B3LYP functional do not correlate with trends in 49Ti NMR linewidths. Attempts are reported to correlate the delta(49Ti) values of Ti[YC(O)CHC(O)Y]2Cl2 (Y = H, Me, CF3, CN, F, Cl and Br) with the rate-limiting propagation barrier for ethylene polymerization using catalysts derived from these precursors.