Solid-phase catalytic reactions of tritium with carbohydrates: 3. Mechanism of isomerization of epimeric pentoses in the course of solid-phase catalytic hydrogenation with tritium

The mechanism of isomerization of ribose into arabinose in the solid phase under the action of spillover hydrogen in the course of solid-phase catalytic hydrogenation was studied. Isomerization of ribose was shown to occur by a complex mechanism similar to acid-catalyzed keto-enol tautomerization of epimeric sugars in solution; the active species in SCH of D-ribose with tritium is spillover hydrogen in the proton form.     

Solid-phase catalytic reactions of tritium with carbohydrates: 5. Mechanism of isomerization of epimeric disaccharides in the course of solid-phase catalytic hydrogenation with tritium

The influence exerted on the solid-phase catalytic hydrogenation of lactulose with tritium by temperature in the range 130–160°C, platinum group catalysts, solid phase composition, and support surface area was studied. Lactose was identified in the reaction products along with labeled lactulose. The mechanism of isomerization of sugars in the solid phase under the action of hydrogen spillover was suggested. Isomerization of sugars occurs by a complex mechanism similar to acid-catalyzed keto-enol tautomerization of epimeric sugars in solution, and the active species in SCH of sugars with tritium is spillover hydrogen in the form of proton.     

NMR lineshapes in solid tritium and deuterium-tritium

We report on the NMR free induction decays (FID) from solid tritium (T₂), deuterium-tritium (D-T), and H₂, HD and D₂ containing 2% tritium. The FID in pure components is typically a beat decay for spin concentrations >30%, an exponential for spin concentrations <10%, and a Gaussian or Gaussian-exponential for spin concentrations in between. In mixtures of DT or HD, the deuteron FID is a Gaussian-exponential while the T or H FID is typically a beat decay or Gaussian depending on temperature. The linewidth is shown to consist mostly of intermolecular and intramolecular contributions, with a small component resulting from unpaired hydrogen atoms created by the tritium radioactivity.     

Efficiency of isotope exchange between sodium 4-phenylbenzoate and activated tritium

The efficiency of the protium–tritium isotope exchange in the sodium 4-phenylbenzoate (PBNa) molecule on activating the reaction on a tungsten filament at 1940 K (target temperature 77 and 295 K) and on heating the substrate supported on 5% Pd/C in the presence of gaseous tritium is compared. It is shown that the reaction mechanism is laregly determined by the properties of the material on which this reaction occurs and not only by the method of generation of activated tritium species. In the reaction of tritium atom with PBNa deposited on glass walls of the reaction vessel, the isotope substitution of tritium for protium occurred by the radical mechanism, leading to the formation of [³H]PBNa and hydrogenation products. It is assumed that the spillover of tritium atom over the support (carbon) surface is accompanied by polarization of the electronic shell and formation of the cluster (^{3 +})(\(\bar e\)), which leads to changes in the composition of the reaction products. The combined treatment of PBNa on 5% Pd/C allows estimation of the concentration of clusters on the carbon surface, which reaches 10.9 particles per 100 nm² (9.2 nm² per cluster).     

The high-pressure equation of state of solid molecular tritium

We have calculated the equation of state for solid molecular tritium using a semiempirical approach, closely guided by experimental results for H₂ and D₂. TheT=0 K isotherm is calculated from a self-consistent phonon model of the free energy. Temperature effects are treated by the Mie-Grüneisen model. A tabulation of pressure, bulk modulus, and thermal expansion is given for a dense set of molar volumes, as a function of temperature up toP=22 kbar.     

Isotope exchange reactions of trans-zeatin with tritium

Isotope exchange of trans-zeatin with high-activity tritium water and with gaseous tritium in solution, and also the solid-phase catalytic hydrogenation of this compound were studied. The isotope exchange of trans-zeatin with gaseous tritium, both in solution and without a solvent at 160°C and higher temperatures, is accompanied by virtually complete hydrogenation of the starting compound with the formation of tritium-labeled dihydrozeatin. The isotope exchange of trans-zeatin with high-activity tritium water allows preparation of tritium-labeled zeatin in 67% yield and molar activity of 0.68 PBq mol^?1. When the solid-phase isotope exchange is performed at 150–155°C, the reaction products contain tritium-labeled trans-zeatin along with the hydrogenation product, dihydrozeatin. At 170°C, the only reaction product is dihydrozeatin. Thus, the selectivity of tritium labeling varies with the temperature of solid-phase catalytic hydrogenation. Below 160°C, the solid-phase reaction can be performed selectively, i.e., with the preservation of the double bond in the starting trans-zeatin. Above 170°C, the selectivity is lost, and the compound is virtually fully hydrogenated to dihydrozeatin.     

Solid-Phase Catalytic Reactions of Tritium with Carbohydrates: 1. Influence of Temperature,Catalysts, and Solid Phase Composition on Solid-Phase Catalytic Hydrogenation of <Emphasis Type="Italic">D</Emphasis>-Ribose with Tritium

Solid-phase catalytic hydrogenation of D-ribose with tritium was studied in relation to the temperature (varied in the range 90–130°C), platinum-group catalysts, and solid phase composition. Based on the results obtained, general approaches to synthesis of tritium-labeled ribose were formulated.     

Probability of the reaction at the first collision with polyethylene of tritium atoms generated by thermal activation

The influence of the experimental conditions on the probability of the reaction of tritium atoms generated in the gas phase by thermal dissociation of tritium on a tungsten filament (thermal activation) with hydrocarbons in a solid target was examined. Experiments were performed with polyethylene films placed in a special case with a calibrated slit in the center of the top lid. For tritium atoms reaching the target without energy loss, the probability of the reaction at the first collision with the hydrocarbon matrix cooled to 77 K decreases from 18 to 2.5% with a decrease in the atomizer temperature from 2000 to 1660 K. For tritium atoms preliminarily thermalized in the gas phase to 77 K, the probability of the reaction at the first collision is so low that it cannot be measured with our experimental technique. Nevertheless, even at 77 K tritium atoms can react with the target, yielding the compound labeled at the C-H bonds.     

A Comparative Study of the Reactions of Thermally Activated Tritium with Sugars and Diazines and of Solid-Phase Catalytic Hydrogenation of These Compounds with Tritium

The feasibility of the labeling procedure involving thermal activation (TA) of tritium was examined with the substrates that are commonly labeled by solid-phase catalytic hydrogenation (SCH) with tritium. Comparative characteristics of SCH and TA as procedures for tritium labeling of sugars and diazines were obtained. These two methods ensure comparable rates of tritium incorporation into purine and pyrimidine bases. The SCH allows preparation of tritium-labeled compounds with the maximum possible molar radioactivity. The molar radioactivity of the same compounds labeled using TA did not exceed 37 TBq mol⁻¹, because only a small fraction of the substrate could react with atomic tritium. Longer reaction times and increased amounts of tritium taken into the reaction resulted in stronger degradation of the substrates. On the assumption that the reactive tritium atoms penetrate into the target to a depth of 0.5 nm, the actual specific radioactivity of the labeled compound in the zone accessible for atomic tritium reaches 0.2–2 PBq mol⁻¹. Ways are suggested to increase the molar radioactivity of compounds labeled using thermal activation of tritium.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 284–288.Original Russian Text Copyright © 2005 by Sidorov, Badun, Baitova, Baitov, Platoshina, Myasoedov, Fedoseev.     

Low-temperature synthesis of Li4SiO4 pebbles as tritium breeding material by SiO2 coating approach

Li₄SiO₄ pebbles are widely selected as attractive ceramic tritium breeding materials for the test blanket modules (TBM) of international thermonuclear experimental reactor (ITER). In this work, SiO₂ coating approach was first employed to synthesize precursor powders for fabricating the Li₄SiO₄ pebbles. Lithium source and silicon source were mixed by hydrolyzing tetraethyl orthosilicate (TEOS) in the Li₂CO₃ solution. Compared with the traditional solid state method, the precursor powders synthesized by SiO₂ coating approach displayed distinct coating structure, which was able to effectively lower the phase formation temperature and the sintering temperature of Li₄SiO₄ pebbles, and then decrease the grain size of Li₄SiO₄ pebbles. Thermogravimetry and different scanning calorimetry (TG-DSC) and X-ray diffraction (XRD) analyses indicate that the phase formation temperature of Li₄SiO₄ synthesized by SiO₂ coating approach is far below that of the conventional solid state reaction. Then, wet method was employed to prepare green spheres with the as-prepared precursor powders. Finally, Li₄SiO₄ pebbles with small grain size (average value 1.12?µm), high phase purity, good sphericity, and uniform microstructure could be obtained by sintering the green spheres at a low temperature of 625°C, which are expected to show favorable tritium release behavior and be used as tritium breeding materials for blankets.     

Specific features of deuterium and tritium labeling of SB258585

Samples of SB258 585 labeled with hydrogen isotopes were synthesized. [²H]SB258 585 containing one ²H atom per molecule and [³H]SB258585 with the molar radioactivity of 15 Ci mmol^?1 were obtained in preparative amounts. From 0.18 to 1.5 ²H atoms were incorporated, on the average, into an SB 258 585 molecule depending on the reaction conditions. The isotope exchange efficiency strongly depends not only on the catalyst-substrate ratio and on the reaction temperature, but also on processes occurring on the support surface. The isotope effects strongly influence the degree of deuterium or tritium incorporation into the samples in cases when the organic compound largely decomposes in the course of the reaction.     

Introduction of hydrogen isotopes into Win 55212 and CP 55940, selective agonists of cannabinoid receptors

Selective agonists of cannabinoid receptors Win 55 212 and CP 55 940, labeled with hydrogen isotopes, were prepared. The content of isotopomers in deuterium-labeled Win 55 212 and CP 55 940, and also the deuterium distribution in fragments of the [²H]Win 55212 molecule were determined by mass spectrometry. [³H]Win 55 212 and [³H]CP 55 940 with molar radioactivities of 55 and 70 Ci mmol⁻¹, respectively, were prepared by the reaction with gaseous tritium. The efficiency of isotope exchange with activated hydrogen species under the conditions of primary and secondary hydrogen spillover is discussed.     

Nonequilibrium processes in reactions of hot tritium atoms with cooled solid targets. Influence of the atomizer temperature on formation of labeled substances

A system consisting of a cold target and “hot” atoms generated by dissociation of tritium on a tungsten wire was studied with the aim to determine conditions for preparing tritium-labeled organic compounds with the maximal radiochemical yield. The influence of the atomizer temperature on the result of the reaction of tritium atoms with amino acids and tetraalkylammonium bromides was studied; homological series of the substrates were examined with the aim to evaluate the contributions of functional groups and hydrocarbon tail to the processes occurring in the target. The dependence of the yield of the labeled parent compound on the atomizer temperature varied in the range 1600–2000 K was determined. The rates of decarboxylation and deamination sharply grew with increasing temperature of the tungsten wire. The highest yield of labeled amino acids was attained at an atomizer temperature of 1800–1900 K, and at higher temperature their yield decreased. The difference between the activation energies of the elimination of the carboxy and amino groups and of the isotope exchange of hydrogen for tritium in the C-H bond appeared to be 93 and 59 kJ mol^?1, respectively. For alkyltrimethylammonium bromides with the alkyl radicals C₁₂H₂₅, C₁₄H₂₉, and C₁₆H₃₃, the yield of the labeled parent compound reached 80–90% and was virtually independent of the atomizer temperature. The capability of tritium atoms to penetrate into the targets was evaluated. For the exponential model of the attenuation of the flow of tritium atoms inside the target, the attenuation factor for freeze-dried amino acids and alkyltrimethylammonium bromides as targets was 1.8 nm^?1.     

Influence of Tritium Spillover on the Efficiency of Labeling of Organic Compounds

Various mechanisms of hydrogen spillover are discussed. A number of evidences are given that tritium incorporation by isotope exchange occurs with the participation of both tritium cations and atomic tritium. The molar radioactivity of the labeled products prepared at temperatures of up to 180°C is, apparently, mainly determined by reactions of the organic compound with tritium cations, and the degree of isotope exchange depends chiefly on the efficiency of tritium spillover to the bulk of the organic substance supported on the catalyst.     

Nonequilibrium processes in reactions of hot tritium atoms with cooled solid targets. Attenuation of the flow of tritium atoms in adsorption layers of alkyltrimethylammonium bromides

The permeability for atomic tritium of adsorption layers of aqueous solutio ns of alkyltrimethylammonium bromides RN(CH₃)₃Br differing in the length of the hydrocarbon radical R (C₁₂H₂₅, C₁₄H₂₉, C₁₆H₃₃) was studied. The change in the radioactivity of alanine present in the examined solutions was used as the measure of the attenuation of the flow of reactive tritium atoms. The formation of an adsorption monolayer of surfactant molecules on the liquid-gas phase boundary caused the radioactivity of alanine to decrease by a factor of 6.5 (R = C₁₂H₂₅) to 8 (R = C₁₆H₃₃) relative to the experiment with a straight alanine solution. A correlation between the structure of the adsorbed layers of alkyltrimethylammonium bromides and their permeability to atomic tritium is discussed.     

Polymer films as indicator of hydrogen spillover through the gas phase

Films of poly-ε-caproamide (PA), polyethylene (PE), and polyethylene terephthalate (PET) were used for detection of hydrogen spillover through the gas phase. The hydrogen used in the experiments contained tritium activated by two procedures (W wire, 2000 K; 5% Pd/C, 335 K). The radioactivity of the films was recorded by classical and digital autoradiography and by liquid scintillation counting. Under the action of “hot” atoms generated on a W wire, the maximal specific radioactivity of the films, equal to 420, 415, and 330 mCi cm^–2 for PA, PE, and PET, respectively, was reached in 100 s. Preliminary thermalization of the atoms to a temperature of 77–335 K influenced the decrease in the film radioactivity differently. The effective activation energy of the reaction in the range 298–318 K was 21, 30, and 12.5 kJ mol–1 for PA. PE, and PET, respectively. Under the conditions of heating 5% Pd/C to 335 K for 25 min, the radioactivity of PA, PE, and PET was 1.6, 0.05, and 0.15 μCi cm^–2, respectively. The revealed difference in the radioactivity of the films suggests different mechanisms of the interaction of tritium with organic molecules at different activation methods.     

Synthesis of [3H]Sulfobromophthalein and Related Problems of Tritium Labeling by Solid-Phase Isotope Exchange

Tritium-labeled sulfobromophthalein with a molar radioactivity of 0.5-0.6 PBq mol^-1 was prepared. Various aspects of tritium labeling of organic compounds by solid-phase catalytic isotope exchange are considered. A number of arguments are given in favor of the hypothesis that the degree of isotope exchange mainly depends on the efficiency of tritium spillover in the bulk of the organic substances applied onto the catalyst surface. At present, it can be considered as a reliably proved fact that at temperatures up to 180-200°C the solid-phase isotope exchange mainly occurs via reaction with tritium cations. Apparently, the contribution of the reactions with atomic tritium to labeling is significant only if there is no spillover of tritium cations to the bulk of the organic compound and the substrate withstands heating to 280-300°C.     

Quantum Tunneling of D(H) Atoms and 2 – in Solid Hydrogen

This paper contains three results. First, the rate constants for the tunneling reaction HD + D H + D ₂ in solid HD increase steeply with increasing temperature above 5 K, while they are almost independent of temperature below 5 K. A mechanism of a vacancy–assisted tunneling reaction is proposed to account for this temperature dependence. Second, a hydrogen atom and a hydrogen molecule form a van der Waals complex in the Ar matrix at 20 K, where the tunneling reaction HD + D H + D ₂ takes place in this complex. The analysis of well–resolved ESR spectra of the complex determined the distance between a hydrogen atom and a hydrogen molecule as 2.3 – 2.5 Å. Third, the decay rate constants of anions in solid parahydrogen decrease with decreasing temperature below 6.6 K, attain the minimum value at 5 K, and then increase with decreasing temperature in the range of 5 2.7 K. The abnormal temperature dependence of the decay rate constants below 5 K is ascribed to a phonon–scattering process of quantum diffusion.     

Study of hydrogenated nanoamorphous silicon(na-Si:H) thin film prepared by RF magnetron sputtering for graded optical band gap (<Emphasis Type="Italic">E</Emphasis><Superscript>opt</Superscript><Subscript>g</Subscript>)

The schematic of the energy band gap figure of the graded optical band gap (E_g^opt) in p-i-n layer in na-Si:H solar cells was given in the paper. The intrinsic hydrogenated nanoamorphous silicon(na-Si:H) thin films with the graded band gap as a function of depth through the films were prepared by varying the processing power, gas pressure, gas composition, and etc., We have carried out a investigation of the relationships between the E_g^opt with the crystallization ratio (Xc) and the E_g^opt with the nanocrystalline grain size (D) in na-Si:H thin films grown by PECVD on glass substrates through XRD, Raman scattering, transmission. The E_g^opt increase with the decreases of the crystallization ratio (Xc) and the nanocrystalline grain size (D). The hydrogen dilution ratio is found to increase basically both the crystallization ratio (Xc) and the nanocrystalline grain size (D). Two relationships in na-Si:H are discussed by the etching effect of atomic hydrogen in the framework of the growth mechanism and the quantum size effect (QSE).     

Phase separation innH2-nD2 alloys

Using powder x-ray technique we have studied the structure of solid mixtures of normal hydrogen and deuteriumnH₂-nD₂, the samples being prepared by deposition from the gas onto a substrate at a temperature near 5 K. The content ofnD₂ in the alloys was varied from 0 to 100%. Over the range 20 to 85%, the diffraction patterns contain two sets of hep reflections, which correspond to phases rich in hydrogen and deuterium. As the temperature was raised to 12–14 K (depending on the composition), the separated alloys with comparatively fine crystallites started to show indications of growth and, simultaneously, homogenization, sometimes to completion. These states remained stable upon subsequent decreases in temperature. Analysis of the data supports the claim that phase separation occurs innH₂-nD₂ alloys during sample preparation. This result confirms the finding by Kogan, Lazarev, and Bulatova. The characteristic points (kinks on the molar volume vs. composition, the position of the boundary of the homogenization area, etc.) are treated as belonging to lability rather than equilibrium lines. Proceeding from these arguments we reconstruct a hypothetical phase diagram which has the maximum separation temperatureT _c within the range 8–11 K. As shown using a model, which accounts for the electrical quadrupole (EQQ) interaction betweenJ=1 species, theT _c is substantially higher than predicted by Mullin and Sarin. We suggest some explanation for the striking difference between the results of different studies carried out to search for phase separation in solid mixtures of deuterium and hydrogen on samples prepared by different methods.