Hydrogenation and hydrogenolysis of methylcyclohexane-1,4-dione on Pt and Pd catalysts

Relative hydrogenation reactivity of the two carbonyl groups of methylcyclohexane-1,4-dione can be estimated beside that of two related compounds, 2- and 3-methyl-cyclohexanones. The reaction is accompanied with hydrogenolysis on Pt and Pd. The less hindered 4-carbonyl group is selectively hydrogenolyzed.

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-1,4- , 2- 3-. Pt Pd. 4- .

     

Thermal studies on metal complexes of 5-nitroso-pyrimidine derivatives

Four Hg(II) complexes, containing as ligands 6-amino-5-nitrosouracil (AH), 6-amino-3-methyl-5-nitrosouracil (BH) or 6-amino-1-methyl-5-nitrosouracil (CH), have been synthesized and their thermal behaviour studied by TG and DSC techniques: Hg₃Cl₆(AH)₄, HgCl₂(BH)₂· 2 H₂O, HgCl₂(BH)₂ · H₂O and HgC₂· 2 H₂O.The dehydration processes take place in only one step, with enthalpies in the range 40.2–60.0 kJ · mole^–1 H₂O.Pyrolytic processes start between 200 and 250°, in all cases the thermal stability of the corresponding pyrimidine derivative being lower than that of the free ligand. These processes finish between 600 and 750°, with no residue.

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Zusammenfassung Vier 6-Amino-5-nitrosouracil (AH), 6-Amino-3-methyl-nitrosouracil (BH) oder 6-Amino-1-methyl-5-nitrosouracil (CH) als Liganden enthaltende Hg(II)-Komplexe wurden synthetisiert: Hg₃Cl₆(AH)₄, HgCl₂(BH)₂ · 2 H₂O, HgCl₂(BH)₂ · H₂O und HgC₂ · 2 H₂O. Die Dehydratisierungsprozesse verlaufen in nur einem Schritt mit Enthalpien im Bereich von 40.2–60.0 kJ pro mol H₂O. Pyrolitische Prozesse setzen zwischen 200 und 250° ein. In allen Fällen ist die thermische Stabilität der entsprechenden Pyrimidin-Derivate geringer als die der freien Liganden. Diese Prozesse sind zwischen 600 und 750° beendet, wobei kein Rückstand zurückbleibt.

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g₃l₆()₄, gl₂( )₂ · 2 ₂O, gl₂()₂ · ₂O, g₂ · 2 ₂, — 6- -5-, — 6--3- — 6---5- . , 40.2–60.0 · ^–1 ₂. 200 250° 600–750° - . , .

     

Rh and Pt catalysts activated by UHV treatments

The H₂–D₂ exchange activity of Rh powder was enhanced by high-temperature UHV treatment accompanied with an enrichment of the high-index faces of Rh. The thermal decomposition of H₂PtCl₆·6H₂O under UHV condition resulted in an extremely active Pt powder catalyst.

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H₂–D₂ Rh , - Rh - . H₂PtCl₆·6H₂O Pt .

     

Effect of Thermal Activation of Supported Catalysts Pt/MeOx,Where MOx = Al2O3, CeO2, La2O3, and ZrO2, for Complete Oxidation

A sharp increase in the atomic catalytic activity (ACA) of supported platinum catalysts in the model reaction of n-pentane complete oxidation is found on going from the preliminary calcination temperature of 500–600°C to a temperature of 700°C. ACA increases by an order of magnitude for the Pt/-Al₂O₃ system, 3 times for Pt/ZrO₂, and 1.5 times for Pt/CeO₂. The per-gram activities of all catalysts decrease because of a decrease in the dispersion of supported platinum with an increase in the temperature of preliminary calcination.     

Hydroxylation selectivity of phenol and O-protected phenols in the Fe(III)-catechol-H2O2 system

Oxidation selectivity of phenol and its O-protected derivatives in the Hamilton system has been studied. Modification of OH groups by acyl substituents and addition of -cyclodextrin markedly enhance paraselectivity of this reaction.

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O- . , OH- - - .

     

Isotope exchange of nitrogen on magnesium, calcium and lanthanum nitrides

It has been found that calcium nitride possesses a high catalytic activity in the isotope exchange of nitrogen. The catalytic activity of nitrides is shown to be independent of the heat of their formation. The effect of H₂ and O₂ on the isotope exchange of nitrogen is studied.

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. . H₂ O₂ .

     

Thermochemical studies on the systems ACl/CeCl3 (A=Na-Cs)

The pseudobinary systems ACl/CeCl₃ (A=Na-Cs) were reinvestigated by means of DTA. The following compounds were found and identified through their X-ray patterns (primarily detected compounds in italics):NaCe _1.67 Cl ₆; K₃CeCl₆, K₂CeCl₅,KCe _1.67 Cl ₆;Rb ₃ CeCl ₆, Rb₂CeCl₅,RbCe ₂ Cl ₇; Cs₃CeCl₆, Cs₂CeCl₅,CsCe ₂ Cl ₇. The compounds A₂CeCl₅ crystallize with the K₂PrCl₅ structure. The high-temperature modifications of the compounds A₃CeCl₆ have the cubic elpasolite structure. The hexagonal unit cell of KCe_1.67Cl₆ is related to the CeCl₃ structure: 0.33 Ce³⁺ are substituted by one K^s+. The structure of CsCe₂Cl₇ can be described with a hexagonal subcellZ=4,a=9.72 andc=14.94 Å; however, small superstructure reflexions reduce the symmetry.The thermodynamic functions for the reactionsnACl+CeCl₃=A _n CeCl _n+3(A=K, Rb) were determined by means of e.m.f. measurements.Measurements of the e.m.f.E vs. T yielded the Gibbs enthalpiesG ^r . The temperature-dependence was found to be linear.The most important result is the formation of the elpasolites (A₃CeCl₆) and ACe₂Cl₇.

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Zusammenfassung Die pseudobinären Systeme ACl/CeCl₃ (A=Na-Cs) wurden mittels Differenzthermoanalyse (DTA) neu untersucht. Folgende Verbindungen wurden gefunden und durch ihre Röntgenbeugungsmuster charakterisiert (erstmalig nachgewiesene Verbindungen sind kursiv):NaCe _1.67 Cl ₆; K₃CeCl₆, K₂CeCl₅,KCe _1.67 Cl ₆;Rb ₃ CeCl ₆, Rb₂CeCl₅,RbCe ₂ Cl ₇; Cs₃CeCl₆, Cs₂CeCl₅,CsCe ₂ Cl ₇. Die Verbindungen A₂CeCl₅ kristallisieren im K₂PrCl₅-Typ. Die Hochtemperaturmodifikationen der Verbindungen A₃CeCl₆ liegen in der kubischen Elpasolith-Struktur vor. Die hexagonale Elementarzelle des KCe_1.67Cl₆ ist mit der CeCl₃-Struktur verwandt: 0,33 Ce³⁺ sind durch ein K⁺ ersetzt. Die Struktur des CsCe₂Cl₇ kann mit einer hexagonalen Pseudozelle beschrieben werden:a=9,72;c=14,94Å;Z=4; schwache Überstrukturreflexe reduzieren jedoch die Symmetrie. Die thermodynamischen Funktionen für die Reaktionen:n ACl + CeCl₃=A _n CeCl _n+3 (A=K, Rb) wurden durch EMK-Messungen bestimmt. Messungen der Abhängigkeit von e. m. f.E gegenT ergaben die Gibbs-EnthalpienG ^r . Es wurde eine lineare Temperaturabhängigkeit gefunden. Als wichtigstes Resultat ist die Bildung von Elpasoliten (A₃CeCl₆) und von Verbindungen (ACe₂Cl₇) anzusehen.

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ACl/CeCl₃, A . - ( ):NaCe _1.67 Cl ₆; K₃CeCl₆, K₂CeCl₅,KCe _1.67 Cl ₆;Rb ₃ CeCl ₆, Rb₂CeCl₅,RbCe ₂ Cl ₇; Cs₃CeCl₆, Cs₂CeCl₅ CsCe ₂Cl₇. A₂CeCl₅ K₂PrCl₅. A₃CeCl₆ . KCe_1.67Cl₆ CeCl₃, 0,33 Ce³⁺ . CsCe₂Cl₇ a=9,72 Å,c=14,94 Å Z=4. , . n ACl + CeCl₃=A _n CeCl _n+3(A=K, Rb) . . . . . .E , G ^r . ACe₂Cl₇.

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Presented as a poster at the 8th ICTA, Bratislava, 1985.

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This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.     

51V-NMR spectra of vanadates and oxosulfato-vanadates of alkali metals

A relation between the type of vanadium environment and the chemical shift anisotropy in the⁵¹V-NMR spectra has been established. The results obtained are compared with spectra of real catalysts.

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⁵¹V , .

     

Pressure dependence ofT x2113;ℓx2113;ℓx2113;ℓ

Summary For simplified analysis of PVT data to locateT and other secondary transitions, we demonstrate that series expansion of the term, lne(1 +P/b), in the Tait equation yields a number of approximations of varyine accuracy but all linear inP. The most useful ones are: (1 – V/V_o)=(C'/b)PforP/b < 0.3;P ¹ (1 - V/V_o) = (C/b) (1 – (P/2b)) forP/b < 0.6 but > 0.2 and the compressibility, /gz = (C/b)(1 -P/b) which requires a calculation of point-to-point derivatives, V,^–1 (/gDV//gDP), and hence is less satisfactory. C' andb are parameters in the Tait equation. We use C' = 0.0894.b = C' /-K, where /-K, is bulk modulus, increases with decreasing temperature but also has a step increase at each pressure-induced isothermal transition.b has a value of 1000 – 1500 bars nearT .Isothermal plots of (1 - V/V_o) againstP consist of two or more relatively straight line plots whose intersections give pressures,P ⁱ _n corresponding to each successive transition.T is usually the first transition,P ⁱ _l P ⁱ _n,moves to higher values as temperature increases.T starts at or near 1.2Tg and increases at a rate,dT /dP, which can be estimated. For iso PMMA data of Quach, Wilson and Simha, we estimateT = 110 °C atP = 0,dT /dP = 70 K/kbar. The original authors reportdTg/dP = 21.1 K/kbar.The same simplified equations apply even better to the glassy state relaxation for whichb is about 4100 bars at 26°C. We find that iso PMMA does indeed have adT at 10 °C (contrary to some authors) and adTß/dP of 70 K/kbar. These simple approximations do not work at isotherms nearTg whereb is changing; not near two close transitions for which there are insufficient data points to resolve them.Using values ofdT /dP previously determined for other polymers, and now for iso PMMA, we show thatdT /dP increases with the expansivity in free volume, /gDa, and hence, via Simha-Boyer, linearly inTg ¹.dT /dP 28 × 103/Tg (K).Finally, we present evidence which suggests that for weak secondary transitions each intersection,P ⁱ _n , gives a new origin with the new value of bn so that the effective pressure is P-Pⁱ _n-1 and (P-P ⁱ _n-1 )/b _n always remains less than about 0.5.

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Zusammenfassung Die Flüssig/Flüssig-UmwandlungT und andere sekundäre Umwandlungstemperaturen können aus PVT-Daten über eine Reihe geschlossener Ausdrücke erhalten werden, die alle durch Reihenentwicklung des Ausdrucks In (1 +P/b) der Tait-Gleichung gewinnbar sind. Die folgenden Ansätze erwiesen sich als besonders nützlich: (1 – V/V_o) = (C/b)P für Plb < 0,3 und P-(1 - V/V_o) = (Clb) (1 - (Pl2b)) fürP/b zwischen 0,2 und 0,6 wobeiC undb Parameter in der Tait-Gleichung sind. Die Verwendung der Kompressibilität ist weniger zufriedenstellend, da sie Punkt für Punkt über den Ausdruck V- (/gDVI /gDP) berechnet werden muß, bevor sie in den ebenfalls abgeleiteten Ausdruck = (C/b)(1 -P/b) eingesetzt werden kann. Gute Resultate werden mit C = 0,0894 undb = C /-Kerhalten, wobei /-K der Kompressionsmodul ist. Der Kompressionsmodul nimmt mit abnehmender Temperatur zu, weist aber auch eine stufenweise Zunahme bei jeder druckinduzierten isothermen Umwandlung auf. b hat in der Nähe vonT Werte von ca. 1000–1500 bar.Die Auftragung von (1-V/V_o) gegenP für konstante Temperatur liefert zwei oder mehr ziemlich gerade Linien, deren Schnittpunkte für jede entsprechende Umwandlung die DrückeP ⁱ _n angeben. Der erste UmwandlungsdruckP ⁱ _n, entspricht gewöhnlich der UmwandlungstemperaturT . Eine Temperaturzunahme führt zu höheren P _n ⁱ '-Werten.T weist bei Normaldruck etwa den Wert 1,2 Tg auf. Die ZunahmeT dP mit dem Druck kann aus Literaturdaten abgeschätzt werden. Aus den Daten von Quach, Wilson und Simha für isotaktisches PMMA erhalten wirT = 110 °C für P = 0 unddT /dP = 70 K/kbar. Diese Autoren berichtendTg/dP = 21,1 K/kbar.Die gleichen vereinfachten Gleichungen können sogar noch besser für die Analyse der ß-Relaxationen im Glaszustand angewendet werden. Die Konstanteb beträgt hier ca. 4100 bar bei 26 °C. Im Gegensatz zu anderen Autoren finden wir für isotaktisches PMMA eine-Umwandlung mitTß = 10 °C und dTßldP = 70 K/kbar. Diese Approximationen gelten jedoch nicht in der Nähe der Glasübergangstemperatur.Tg, da hier die Größeb nicht konstant ist. Sie können ebenfalls nicht angewendet werden, wenn zwei Übergangstemperaturen so nahe beieinander liegen, daß die Zahl der Meßpunkte nicht für eine Analyse ausreicht.Mit den zuvor für andere Polymere und nunmehr auch für isotaktisches PMMA erhaltenen Werten fürdT /dP kann gezeigt werden, daßdT /dP mit dem Unterschied /gDa der Ausdehnungskoeffizienten ansteigt. Nach der SimhaBoyer-Regel mußdT /dP daher auch linear mitTg ^-1 zunehmen. Für "dT /dP finden wir so 28 000/Tg(K).Es werden ferner Hinweise gefunden, daß die Drücke Pⁱ _n bei jeder schwachen sekundären Umwandlung einen neuen Ursprung mit einem neuen Wert vonb in der Weise geben, daß der effektive Druck (P – Pn _i ^n–1 ) beträgt und daß (P-P ⁱ _n ¹/b _n immer niedriger als ca. 0,5 bleibt.

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With 6 figures and 1 table

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Dedicated to Prof. Dr. G. Rehage on the occasion of his 60th bithday     

On theC v toC p conversion for solid linear macromolecules

TheC _v toC _p conversion for solid linear macromolecules via the Nernst-Lindemann equationC _p -C _v =A ₀,C _p ² T/T_m is discussed on hand of data for 10 crystals and seven glasses. An average value ofA ₀=(5.11±2.41) · 10^–3 mol K J^–1 was calculated if the mole is assumed to refer to heavy atoms only. ThisA ₀ is numerically equal to the original Nernst—Lindemann constant.

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Zusammenfassung An Hand von sich auf 10 Kristalle und 7 Gläser beziehenden Daten wird die Umrechnung vonC _v -inC _p -Werte für feste lineare Makromoleküle mittels der Nernst-Lindemann-GleichungC _p -C _v =A ₀ C _p ² T/T _m diskutiert. Ein Durchschnittswert vonA ₀=(5.11±2.41) · 10^–3 mol K J^–1 wurde unter der Annahme berechnet, daß sich das Mol nur auf schwere Atome bezieht. DieserA ₀-Wert ist numerisch gleich der ursprünglichen Nerns-Lindemann-Konstanten.

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10 , - C _v C _p , —C _p –C _v = _{0 p} ² / . , , ₀ (5.11±2.41) · 10^–3 · ^–1. ₀ — .

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Support by the National Science Foundation, Polymers Program (Grant No. DMR 83-17097) is gratefully acknowledged.     

A note on diffusion stabilized Pt/C oxidation catalysts: Absence of gas-liquid boundary deactivation results in relatively high efficiency (Pt/C oxidation catalysts; Part 6)

Deactivation of Pt/C aqueous phase oxidation catalysts may occur at the gas-liquid boundary. This feature explains the relatively high and stable activity of diffusion stabilized Pt/C catalysts in the oxidation of substrates with a low reactivity.

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, Pt/C : . Pt/C, , .

     

Hreels studies of Hads effect on NO adsorption on Pt(111)

HREELS studies of NO molecular adsorption on clean and hydrogen covered Pt(111) have revealed that a reversible transition between bridge (₁) and on-top (₂) states of NO_ads is realized in the (NO+H₂)/Pt(111) system. In the presence of H_ads the adsorption of NO in the ₁ state is inhibited and the main state of NO_ads is ₂. Upon heating the layer (₂-NO_ads+H_ads) hydrogen desorption is accompanied by on-top to bridge state conversion.

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NO Pt(III). , (NO+H₂)/Pt(III) (₁) (₂) NO. H NO ₁ , , ₂ NO. (₂-NO+H) NO .

     

The Study of Nitric Oxide Adsorption and the Mechanism of Surface “Explosions” in the Reaction of CO + NO on Pt(100) and Pd(110) Single Crystal Surfaces

High resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) were used to study NO adsorption and the reactivity of CO_ads and NO_ads molecules on Pd(110) and Pt(100) single crystal surfaces. Compared to the Pt(100)-(1 × 1) surface, the unreconstructed Pt(100)-hex surface is chemically inert toward NO dissociation into N_ads and O_ads atoms. When a mixed adsorbed CO_ads + NO_ads layer is heated, a so-called surface explosion is observed when the reaction products (N₂, CO₂, and N₂O) synchronously desorb in the form of sharp peaks with a half-width of 7-20 K. The shape specificity of TPR spectra suggests that the vacancy mechanism consists of the autocatalytic character of the reaction initiated by the formation an initial concentration of active sites due to partial desorption of molecules from the CO_ads + NO_ads layer upon heating to high temperatures. Kinetic experiments carried out on the Pd(110) surface at a constant reaction pressure and a linear increase in the temperature confirm the explosive mechanism of the reaction NO + CO.     

Transients of the Open-Circuit Potential during Carbon Monoxide Interaction with Oxygen Preliminarily Adsorbed on Smooth and Platinized Platinum Electrodes

Variations of potential E in time , observed during the carbon monoxide interaction with preliminarily-adsorbed oxygen O_ads on smooth and platinized platinum electrodes under open-circuit conditions (supporting electrolyte 0.5 M H₂SO₄), are measured. The potential decay rate on smooth Pt is more than ten times that on Pt/Pt; there are some differences in the transients as well. The obtained data suggest that CO interacts with O_ads on smooth Pt and Pt/Pt via different mechanisms. Two models for the process on smooth platinum are considered. In one model, the interaction of O_ads with CO from solution is accepted as the rate-determining step; in the other, the interaction of O_ads with CO_ads. A comparison of theoretical E vs. dependences with experimental data using the MathCad program suggests that CO interacts with O_ads via both mechanisms.     

SCF Xα DW nuclear magnetic screening tensor calculations

Conclusions Calculations have been performed on the nuclear magnetic screening tensors for molecules via perturbation theory and the X DW method, and a comparison is made with results from other methods. Although the absolute errors in the for the molecules are appreciable, the relative errors in determining the NR chemical shifts are much less (see for example, the shift for CO relative to CH₄), which is a hopeful factor leading one to expect that X DW can be widely used in analyzing NMR spectra. At the same time, X DW can closely reproduces the anisotropy, which is particularly important for compounds such as V₂O₅ or its derivatives, which show very large anisotropy in the magnetic screening tensors.Catalysis Institute, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 5, pp. 32–36, September–October, 1988.     

Triterpene Glycosides of Astragalus and Their Genins. LXI. Occurrence of Cycloartane 6-O-Monodesmosides

Cyclosiversigenin 6-O--L-rhamnopyranoside and 6-O--D-glucopyranoside were isolated fromAstragalus coluteocarpusBoiss. (Leguminosae) andAstragalus dissectusB. Fedtsch. et N. Ivanova, respectively. Cyclosiversigenin 5-O--L-rhamnopyranoside was shown to be an artifact forAstragalus coluteocarpus.Thus, the cyclosiversigenin 6-O--D-glucopyranoside that was isolated from certainAstragalusspecies is hypothesized also to be an artifact. Glycosylation of the 6 -hydroxyl group of cycloartanes by D-glucose and D-xylose, in contrast with other substituents, does not change the low-field position of the PMR signal of the 4-CH ₃ group (1.65 2.01 ppm) that is caused by the influence of deuteropyridine directly on the 6 -hydroxyl. Obviously one of the hydroxyls of the -D-glucopyranoside or -D-xylopyranoside residues has the same effect in this instance.     

Triterpene glycosides and their genins fromThalictrum foetidum. V. Structure of cyclofoetoside B

A new glycoside (cyclofoetoside B) (I) has been isolated from the epigeal part of the plantThalictrum foetidum L. (Ranunculaceae). On the basis of chemical transformations and with the aid of physicochemical characteristics it has been established that cyclofoetoside B is 24S-cycloartane-3, 16, 24, 25, 29-pentaol 3-O--L-arabinopyranoside 16-O-[O--L-rhamnopyranoside-(1 6)--D-glucopyranoside], C₄₇-H₈₀O₁₇, mp 194–197°C (methanol); [] _D ²⁴ +15.7 ± 2° (c 0.88; pyridine). The enzymatic hydrolysis of (I) has yielded cyclofoetigenin B (III), 24S-cycloartane-3,16,24,25,29-pentaol 16-O--D-glucopyranoside, (IV), C₃₆H₆₂O₁₀, mp 223–225°C (acetone), [] _D ²⁴ +37 ± 2° (c 0.97; methanol) and 24S-cycloartane-3,16,24,25,29-pentaol 16-O-[O--L-rhamnopyranosyl-(1 6)--D-glucopyranoside, C₄₂H₇₂O₁₄, mp 229–231°C (methanol), [] _D ³⁰ +41 ± 2° (c 0.7; methanol). Details of the IR and¹H and¹³C NMR spectra of the compounds are given.Irkutsk Institute of Organic Chemistry, Academy of Sciences of the USSR. Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Trashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 341–345, May–June, 1986.     

Differential thermal analysis studies on the crystallization of strontium tungstate from sodium tungstate melt. Nucleation mechanism and induction periods

The nucleation mechanism, kinetics and induction periods in strontium tungstate crystallization from sodium tungstate melt in platinum crucibles were investigated by differential thermal analysis. Heterogeneous nuclei first formed on the metal platinate particles within the solution during the induction periods (¯t); the main crystal growth started only after ¯t and few new nuclei were then formed. At any crystallization temperature, ¯t varied inversely with the cooling rate (R _T), and with the rate (R_C) of development of excess solute concentration according to the relation ¯t = 1/(k ₁ R _c ), wherek ₁ and are constants. The critical temperature (¯T), critical supersaturation(¯ S), k ₁ and were evaluated.

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Zusammenfassung Der Keimbildungsmechanismus, sowie die Kinetik und die Induktionsperioden wurden bei der Kristallisation von Strontiumwolframat aus Natriumwolframatschmelzen in Platintiegeln durch Differentialthermoanalyse untersucht. Die heterogenen Kristallkeime wurden zuerst während der Induktionsperiode () an den Metallplatinat-Teilchen in der Lösung gebildet: das Hauptkristallwachstum begann erst nach und nachfolgend wurden einige neue Keime gebildet. Bei jeder Kristallisationstemperatur änderte sich umgekehrt proportional zu den Abkühlungsgeschwindigkeiten (R _T) und den Geschwindigkeiten (R _c) der Bildung überschüssig gelösten Materials, entsprechend der Gleichung = 1/(k ₁ R _c , wobeik ₁ und Konstanten sind. Die kritische Temperatur (¯T), die kritische Übersättigung (¯S),k ₁ und wurden bewertet.

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Résumé On a étudié par analyse thermique différentielle le mécanisme de nucléation ainsi que la cinétique et les périodes d'induction de la cristallisation du tungstate de strontium, à partir des bains fondus de tungstate de sodium, dans des creusets de platine. Des noyaux hétérogènes se forment d'abord sur les particules de platinate métallique dans la solution, au cours des périodes d'induction (): la croissance principale des cristaux ne commence qu'après et il se forme alors quelques noyaux nouveaux. A une température quelconque de cristallisation, varie de façon inverse avec les vitesses de refroidissement (R _T) et les vitesses de développement (R _c ) de concentrations de sursaturation de la substance en solution, selon la relation = 1/(k ₁ R _c , oùk ₁ et sont des constantes. On a évalué la température critique (¯T), la sursaturation critiques (¯S),k ₁ et.

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, . - (¯ t). ¯t . - ,¯t (R _T), (R _c) ¯t=1/(K₁ R _c ), ₁ — . (¯), (_-S), K₁ .

     

Carbon monoxide adsorption on Pt/Pt electrodes modified with silver adatoms

Adsorption of carbon monoxide on the Pt/Pt surface in the presence of preliminarily accumulated different amounts of UPD silver (_Ag) is studied by using voltammetry, electrode washings, and analytical techniques. It is found that carbon monoxide is adsorbed only on the sites unoccupied by Ag_ad and rather than displacing the latter atoms to the noticeable extent. The fraction of carbon monoxide chemisorbed in the linear form increases with _Ag. The shape of anodic voltammetric curves and the number of peaks in the region where the mixed layer CO_ads + Ag_ad is desorbed strongly depend on the CO_ads : Ag_ad ratio; at small and medium _Ag, three and more peaks are observed. Electroanalytic detection of silver passed to solution in various potential ranges in the course of the mixed-layer oxidation allowed us to identify the processes corresponding to different peaks.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 241–246.Original Russian Text Copyright © 2005 by Maksimov, Podlovchenko.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Couplage vibronique et anisotropie paramagnétique dans un complexe cubique 2 T 2 soumis à un champ trigonal

The influence of vibronic coupling on the average paramagnetism and the paramagnetic anisotropy of a cubic complex, the electronic ground state ²T₂ of which is perturbed by a trigonal field, is investigated. It is necessary to introduce the following parameters: the spin-orbit coupling coefficient , the vibronic coupling coefficient x, the frequency _g3 of the E modes of vibration, the splitting of the ² T ₂ level in the trigonal field and the covalence parameter k.For given x and , the influence of the vibronic coupling is more important if in the trigonal field the electronic ground state of the complex is ²E than if it is ²A. For given x and v (=/), the smaller ¦¦, the greater the influence of vibronic coupling. The respective effects of vibronic coupling and covalence are compared. Finally, the case of the first row transition-metal complexes is briefly discussed.