Conformational analysis of 1,1,2,2-tetraaryl-disilanes

A study of the static stereochemistry of 1,1,2,2-tetraphenyldisilane (1) and 1,1,2,2-tetramesityldisilane (2) by empirical force field calculations, X-ray diffraction, and ¹H NMR reveals that the preference for the anti conformation, exhibited by the unclamped 1,1,2,2-tetraarylethanes, is sustained in the analogous disilanes, although in somewhat attenuated form. This anti preference stands in contrast to the gauche preference of 1,1,2,2-tetraalkyldisilanes. Examination of ³J_HH coupling constants for R₂HSiSiHR₂; (R = phenyl, mesityl, 2,6-dimethyl-phenyl, t-butyl, cyclohexyl) suggests the existence of a Karplus relation for H-Si-Si-H systems.     

CELL MEMBRANE PHOTOMODIFICATION: RELATIVE EFFECTIVENESS OF HALOGENATED FLUORESCEINS FOR PHOTOHEMOLYSIS

Abstract— Relative potency of 10 fluorescein derivatives as sensitizers of delayed photohemolysis of human erythrocytes has been assessed. Dilute suspensions of washed cells were illuminated in the presence of sensitizer at different concentrations for 1 h and analyzed for percent hemolysis following 23 h of dark incubation. Plots of percent hemolysis versus concentration showed a steep dependence on concentration for all sensitizers. Additional measurements of octanol/water partition coefficients, photon absorption in octanol and in saline and photobleach rates were made. After correction for absorption cross section, the effectiveness values ranged over more than three orders of magnitude with fluorescein being the least potent and rose bengal the most. A reasonable prediction of potency is obtained by taking the product of partition coefficient, relative absorption in octanol versus water and molecular weight of the substituents added to the fluorescein skeleton. The results suggest that the influence of halogen substitution on sensitizing potency is exerted by four factors; (1) distribution of sensitizer into a low polarity region of the cell membrane, (2) absorption efficiency in a low polarity region, (3) triplet quantum yield, and (4) photobleach rate.     

Solvent effects on substitution reactions at complexes of the [Fe(CN)5L]3- type in binary aqueous mixtures

Summary The kinetics of replacement of 4,4-bipyridine, (4,4-bipy), and 4-cyanopyridine, (4-CNpy), by cyanide in [Fe(CN)₅-(4,4-bipy)]^3-. at 298 K have been studied in binary aqueous mixtures containing different amounts of t-butanol, methanol, glycerol, ethyleneglycol and sucrose. The plots of logarithms of the limiting rate constantsversus the mole fraction of water are linear over the entire composition range studied, showing the importance of solvation phenomena. A different straight line of log (k_L/s^–1)versus X_H2O is obtained for each mixture, indicating the influence of other solvent parameters on the reaction rate. A multiparameter regression of G _exp with A (acidity vector), B (basicity vector) and G^E is used for both reactions; plots of G _calc versus G _exp are linear with slopes of near unity.     

Reactions of furazano[3,4-b]quinoxalines with phosphorus ylides and an unusual oxidative transformation of the transylidation product

Reactions of furazano[3,4-b]quinoxalines 1 with phosphorus ylides 2 afford the transylidation product 3 and/or 4,9-dihydrofurazano[3,4-b]quinoxalines 4. Oxidation of 3 with phenyliodide bis-trifluoroacetate gave the fused furan derivative 13 in high yield.     

Variational bounds on the radius of convergence of the born series

The radius of convergence of the Born series for potential scattering problems is discussed in terms of the eigen-values of the scattering integral equation. Variational procedures yielding upper and lower bounds for these eigen-values are introduced. These bounds axe calculated using various trial functions and compared to the exact values for the square well, exponential repulsive, and Yukawa potentials.     

Quantitation of agmatine by liquid chromatography with laser-induced fluorescence detection

A high performance liquid chromatography (HPLC) method is described for the determination of agmatine, an endogenous neuromodulator. The method involves pre-column derivatization of the sample with a fluorescent tagging reagent, 7-fluoro-4-nitrobenzoxadiazole (NBD-F). The resulting agmatine derivative is stable and can be readily extracted into ethyl acetate at pH 8.5. The extraction enhances the quantification of low level agmatine because it eliminates chromatographic peaks caused by endogenous amino acids. The HPLC separation is carried out on a C₈ reversed phase column and completed in less than 10 min. With laser-induced fluorescence (LIF) detection, the detection limit is 5×10⁻⁹ M agmatine. Method precision (coefficient of variation) is 5% for agmatine in human plasma at the sub-μM level. This method has been validated by determination of agmatine in biological samples including human plasma and rat brain and stomach tissues.     

N‐(4‐Nitrobenzoyl)‐N′‐phenylhydrazine: a three‐dimensional hydrogen‐bonded framework

In the title compound (systematic name: N‐anilino‐4‐nitrobenzamide), C₁₃H₁₁N₃O₃, the molecules are linked into a complex three‐dimensional framework structure by a combination of two‐centre N—H...O and C—H...O hydrogen bonds and a three‐centre N—H...(O,N) hydrogen bond.     

Fe(II) in different macrocycles: electronic structures and properties

A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), phthalocyanine (Pc), porphycene (Pn), dibenzoporphycene (DBPn), and hemiporphyrazine (HPz) with iron (Fe) has been carried out using a density functional theory (DFT) method. The difference in the core size and shape of the macrocycle has a substantial effect on the electronic structure and properties of the overall system. The ground states of FeP and FePc were identified to be the 3A2g [(d(xy))2(d(z)2)2(d(pi))2] state, followed by 3E(g) [(d(xy))2(d(z)2)1(d(pi))3]. For FePz, however, the 3E(g)-3A2g energy gap of 0.02 eV may be too small to distinguish between the ground and excited states. When the symmetry of the macrocycle is reduced from D4h to D2h, the degeneracy of the d(pi) (d(xz), d(yz)) orbitals is removed, and the ground state becomes 3B2g [(d(xy))2(d(z)2)1(d(yz))2(d(xz))1] or 3B3g [...(d(yz))1(d(xz))2] for FePn, FeDBPn, and FeHPz. The calculations also show how the change of the macrocycle can influence the axial ligand coordination of pyridine (Py) and CO to the Fe(II) complexes. Finally, the electronic structures of the mono- and dipositive and -negative ions for all the unligated and ligated iron macrocycles were elucidated, which is important for understanding the redox properties of these compounds. The differences in the observed electrochemical (oxidation and reduction) properties between metal porphycenes (MPn) and metal porphyrins (MP) can be accounted for by the calculated results (orbital energy level diagrams, ionization potentials, and electron affinities).     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

The vibrational relaxation of H2. I. Experimental measurements of the rate of relaxation by H2, He,Ne, Ar,and Kr

The vibrational relaxation of gaseous H₂ in mixtures with He, Ne, Ar, and Kr was studied by the laser Schlieren technique in incidents shock waves at 1350–3000 K. From the results of 155 experiments the following standard relaxation times for self-relaxation of H₂ and relaxation of H₂ by He, Ne, Ar and Kr were obtained:

pτ is in atm s, and the qouted uncertainties are standard deviations. The results for H₂/H₂ and H₂/Ar are in very good agreement with previous results of Kiefer and Lutz, and the extrapolated for H₂/H₂, H₂/He and H₂/Ar agree very well with low temperature data Ducuing.The linear mixture rule for a additivity of relaxation rates was found to hold, to within experimental accuracy, for the mixtures studied in the present work.