Crystal and molecular structures of the complexes of cobalt dichloride with 1-isopropenylimidazole and 1-vinylimidazole

The crystal and molecular structures of bis(1-isopropenylimidazole)dichlorocobalt (C₁₂H₁₆Cl₂·N₄Co) [R 0.036 (R _W 0.089) for 3229 unique reflections with I > 2σ(I)] and tetra(1-vinylimidazole)dichlorocobalt (C₂₀H₂₄Cl₂N₈Co) [R 0.031 (R _W 0.072) for 1863 unique reflections with I > 2σ(I)] were determined. In these molecular complexes, the monodentate terminal 1-alkenylimidazole ligands coordinate to the metal via a “pyridine” nitrogen atom. In C₁₂H₁₆Cl₂N₄Co, the Co atom has a distorted tetrahedral 2N,2Cl coordination. The coordination polyhedron of cobalt in C₂₀H₂₄Cl₂N₈Co is a strongly elongated 4N,2Cl octahedron. The Co-N and Co-Cl bonds [Co-N 2.015(2) and 2.032(4) Å; Co-Cl 2.229(2) Å] in the tetrahedral complex C₁₂H₁₆Cl₂N₄Co are shorter than those in the octahedral complex C₂₀H₂₄Cl₂N₈Co [Co-N 2.134(2) and 2.157(2) Å; Co-Cl 2.518(1) Å]. In the structures of both complexes there are short contacts involving the Cl atoms.     

Bayesian analysis and inference from QSAR predictive model results

QSAR models have been under development for decades but acceptance and utilization of model results have been slow, in part, because there is no widely accepted metric for assessing their reliability. We reapply a method commonly used in quantitative epidemiology and medical decision-making for evaluating the results of screening tests to assess reliability of a QSAR model. It quantifies the accuracy (expressed as sensitivity and specificity) of QSAR models as conditional probabilities of correct and incorrect classification of chemical characteristic, given a true characteristic. Using Bayes formula, these conditional probabilities are combined with prior information to generate a posterior distribution to determine the probability a specific chemical has a particular characteristic, given a model prediction. As an example, we apply this approach to evaluate the predictive reliability of a CATABOL model and base on it a "ready" and "not ready" biodegradability classification. Finally, we show how predictive capability of the model can be improved by sequential use of two models, the first one with high sensitivity and the second with high specificity.     

Kinetics of the hydroxylation of N-picryl-4-[4-(dimethylamino)styryl]pyridinium salts by molecular oxygen in acetonitrile

The reactions of N-picryl-4-[4-(dimethylamino)styryl]pyridinium salts with molecular oxygen in the presence of 4-[4-(dimethylamino)styryl]pyridine (DASP) in acetonitrile proceeds with the formation of DASP picrate. The reactions are first-order relative to the salt cation, molecular oxygen, and the catalyst (DASP). L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R, Lyuksemburg ul., Donetsk 340114, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 5, pp. 286–289, September–October, 1998.     

Nitropyrazoles

The double Vilsmeier formylation of theC-methyl group in pyrazole derivatives has been shown to occur when a nitro-group is in the adjacent position of the ring. A method for the synthesis ofC-(diformylmethyl)nitropyrazoles based on this reaction has been developed.For the previous communication, see Ref. 1; for Part 2, see Ref.2 Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1273–1276, July, 1993.     

Singlet–triplet splittings as obtained from the Xα-scattered wave method: A theoretical analysis

The theoretical determination of singlet–triplet splittings of the excited states of closed-shell molecules is discussed with emphasis on the use of the Xα–scattered wave method. With this method, splittings can be computed in two independent ways. Both will have corrections due to orbital relaxation effects. The singlet–triplet splittings obtained with the Xα–scattered wave method for several large organic molecules are reviewed. It is seen that the results using the two splitting formulas differ by a large amount; in fact, they differ by the magnitude of the computed splitting. We provide a criterion for choosing the results obtained with one method over the other by showing that they both give upper bounds to the best result obtainable within the framework of the model. However, because of the large differences between the two methods, the quantitative value of the splitting should be viewed with caution.     

Exact spin-pairing energies at the crossovers in octahedral d 4, d 5, d 6, and d 7 transition metal complexes

Exact spin-pairing energies are calculated by direct diagonalization of the relevant ligand field plus interelectronic repulsion matrices for the configurations d ⁴, d ⁵, d ⁶, and d ⁷ of octahedral transition metal ions. The results are presented in terms of /B as function of = C/B for the range of values =3.0 to 8.0. Comparison with the quantity resulting from a simplified approach in which configuration interaction is neglected or considered on an approximate basis only reveals significant differences. Useful estimates of spin-pairing energies are provided, in addition, on the basis of empirical magnetic and electronic spectral data.

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Zusammenfassung Exakte Spinpaarungsenergien für die Konfigurationen d ⁴, d ⁵, d ⁶ und d ⁷ oktaedrischer Übergangsmetallionen werden durch direkte Diagonalisierung der entsprechenden Matrizen des Ligandenfeldes sowie der Elektronenwechselwirkung berechnet. Die Ergebnisse für /B werden in Abhängigkeit von = C/B für den Wertebereich =3.0 bis 8.0 angegeben. Ein Vergleich mit der Größe , die bei einer vereinfachten Behandlung unter Vernachlässigung oder näherungsweiser Berücksichtigung der Konfigurationswechselwirkung erhalten wird, zeigt auffallende Unterschiede. Nützliche Abschätzungen der Spinpaarungsenergie werden außerdem unter Benutzung empirischer magnetischer und elektronenspektroskopischer Daten erhalten.

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Résumé Les énergies exactes de couplage de spin sont calculées par diagonalisation directe du champ de ligand correspondant en plus des matrices de répulsion électronique pour les configurations d ⁴, d ⁵, d ⁶ et d ⁷ des ions octaédriques des métaux de transition. Les résultats sont présentés en termes de /B en fonction de = C/B dan l'intervalle =3,0 à 8,0. On trouve des différences significatives par comparaison de avec les valeurs résultant d'une approche simplifiée sans interaction de configuration ou avec interaction de configuration approchée. De plus, des estimations des énergies de couplage de spin sont obtenues à partir de données empiriques magnétiques et spectrales.

     

THE EFFECTS OF HOST-CELL REACTIVATION ON ASSAY OF U.V.-IRRADIATED HAEMOPHILUS INFLUENZAE TRANSFORMING DNA

Abstract— The host cell reactivation (HCR) mechanism in Haemophilus influenzae cells is inhibited by sub-microgram concentrations of acriflavine (as is already known to be true for Escherichia coli ). Exposure of these cells to similar concentrations of the drug during bacterial transformation increases the apparent ultraviolet light (u.v.) sensitivity of previously irradiated transforming DNA, indicating a repair of this DNA after uptake by the cells under normal conditions. Repair is inhibited by applying acriflavine at any time up to one hour after competent cells contact the irradiated transforming DNA. The fraction of the u.v. damage repaired by HCR is very different for different genetic markers. Those markers which are most u.v. sensitive when assayed in the absence of acriflavine are most poorly repaired, suggesting that this is the reason for their higher sensitivity. For all markers the fraction of the damage repairable by in vitro photoreactivation is approximately constant, and strongly overlaps the damage repairable by HCR. The degree of HCR achieved can be altered by experimental treatment of the H. influenzae DNA prior to transformation. Thus treatment of irradiated DNA with an enzyme from Micrococcus lysodeikticus –known to attack u.v. damaged, but not undamaged DNA–prevents subsequent intracellular repair of the same u.v. lesions whose repair is inhibited by acriflavine. Similarly, partial replacement of the thymine in transforming DNA by 5-bromouracil (BU) strongly inhibits repair of subsequent u.v. damage. As in bacteriophage, the BU effect is relieved if the u.v. exposure occurs in the presence of cysteamine. It is clear that intracellular repair must be considered in interpreting experiments with u.v.-irradiated transforming DNA.     

Electrochemical synthesis of coatings in the system Ti-Si-B from molten salts

Powder and coatings of metal-like refractory compounds (MLRC) can be produced by electrochemical synthesis from molten salts. The stoichiometry of the deposited MLRC was found to correlate with the molar ratio of MLRC component ions in the melts. The system Ti-Si-B is of particular interest in terms of electrochemical synthesis since the titanium, silicon and boron potentials in the melt are close together. The electrochemical synthesis has been investigated in the system NaCl-KCl-NaF-K₂TiF₆-K₂SiF₆-KBF₄ at 700°C. The opportunity of deposition of new ternary compound in the system Ti-Si-B is shown by the electrochemical synthesis from molten salts.     

Platinum(II) Mono-N1-(2"-Tetrahydrofuranyl)-5-Fluorouracilate Complexes with Ammonia

The reaction between cis-[Pt(NH₃)₂Cl₂], N¹-(2"-tetrahydrofuranyl)-5-fluorouracil (HL), and NaOH (taken in a molar ratio of 1 : 1 : 1) or between cis-[Pt(NH₃)₂Cl₂], HL, and Ag₂O (taken in the molar ratio of 1 : 1 : 0.5) was used to synthesize complexes Ia (in the case of NaOH) or Ib (in the case of Ag₂O) with composition Pt(NH₃)₂LCl. The model complex [Pt(NH₃)₃L]NO₃ (II) was synthesized by the reaction between [Pt(NH₃)₃Cl]Cl, HL, AgNO₃, and Ag₂O (1 : 1 : 1 : 0.5). The obtained compounds were characterized by chemical analysis, chromatography, thermogravimetry, conductometry, potentiometry, IR, electronic, and NMR spectroscopies. Complexes Ia and Ib were found to have both similar and different properties. The structures of the compounds and the type of L^– coordination to platinum(II) were suggested. Cytotoxic properties of Iaand Ib were studied.     

k0-measurements and related nuclear data compilation for (n, γ) reactor neutron activation analysis

Recommended k₀-factors and related nuclear data for use in (n, γ) activation analysis are given for 72 isotopes. In addition the basic nuclear constants and experimental parameters needed in the k₀ standardization method are reviewed. For convenient data reduction, computer programs were developed.