Crystal and molecular structure of the photocatalyst tris(2,2′-bipyrazine)ruthenium(II) hexafluorophosphate

The X-ray structure determination of the complex tris(2,2-bipyrazine)ruthenium(II) hexafluorophosphate, [Ru(C₈N₄H₆)₃](PF₆)₂, has shown that the compound crystallizes in the monoclinic space group P2₁/c witha=13.459(5),b=23.486(8),c=17.913(10) Å,=139.85(1)°, andZ=4. Each asymmetric unit also contains one dimethylformamide and one water molecule. Least-squares refinement led to an R factor of 0.074 based on 3463 reflections for which I>2(I). The [Ru(C₈N₄H₆)₃]²⁺ cation has no crystallographic symmetry, but the requirements for point symmetry 32 are nearly met. The six Ru-N bond lengths agree within experimental error and have an average length of 2.05(1) Å.     

Relationship between aerodynamic measures of glottal efficiency and stroboscopic findings in asymptomatic singing students

Singing requires exquisite coordination between the respiratory and phonatory systems to efficiently control glottal airflow. Asymptomatic singing students underwent pulmonary function testing (PFT), videostrobolaryngoscopic examination, and measures of glottal efficiency (maximum phonation time [MPT], glottal flow rate [GFR], and phonation quotient [PQ]) performed in both a sung and spoken tone. Pulmonary function and glottal efficiency values were within reported normative data for professional singers. However, sung tones were made with significantly higher GFR and PQ and lower PQ than spoken tones. The mean GFR was not related to the degree of glottal closure (by videostrobolaryngoscopy) or underlying pulmonary support.     

Dilute solution properties of asymmetric six-arm star polystyrenes

Six-arm star polystyrenes having varying numbers of short and long arms attached to the same molecule have been synthesized by anionic polymerization. The molecules have been characterized by high resolution size exclusion chromatography using multiangle light scattering and viscosity detectors. This technique has allowed the radii of gyration and intrinsic viscosities to be measured for stars with each possible combination of arms. The branching parameters g and g′ are computed and compared with theoretical expectations. It is found that short arms add preferentially to the stars, because of reduced steric effects. The molecule with one long and five short arms exhibits behavior closest to that of a linear chain (largest branching ratios). The effect of arm polydispersity on solution properties of stars is discussed. © 1995 John Wiley & Sons, Inc.     

Strong F-regularity in images of regular rings

We characterize strong F-regularity, a property associated with tight closure, in a large class of rings. A special case of our results is a workable criterion in complete intersection rings.

     

Benzimidazo[1,2-c]quinazoline dimers as potential antitumor agents

The 6-substituted benzimidazo[1,2-c]quinazoline 1 is a lead structure from our DNA intercalator program and is cytotoxic to the human colon cancer tumor line HT-29 with an inhibitory concentration 50, IC₅₀ of 4.00 μM. In order to try and improve the limited cytotoxicity of this class of compound we prepared a series consisting of two benzimidazo[1,2-c]quinazoline moieties linked by a polyalkylamino bridge, of different length and substitution. The compound with the -NH-(CH₂)₃-N(CH₃)-(CH₂)₃-NH-bridge had an inhibitory concentration 50, IC₅₀ of 0.5 μM. When tested in vivo, however, no clear anti-tumor activity was produced in the human breast cancer tumor line MX-1 or the human melanoma tumor line LOX, human tumor xenografts models.     

Using correlations to compare additions to alkenes: homogeneous hydrogenation by using Wilkinson's catalyst

Plots of the logarithms of relative rates of homogeneous catalytic hydrogenation of alkenes (log k(rel) values) by using Wilkinson's catalyst versus their ionization potentials (IPs) and versus their lowest unoccupied molecular orbital energy levels (LUMOs) display good-to-excellent correlations. The correlations indicate that the rate-determining step of this reaction is a nucleophilic addition to the alkene double bond, which is dependent upon both electronic effects and steric effects. This conclusion is in agreement with only two of three previously proposed mechanisms for the reaction, effectively ruling out one in which the rate-determining step involves electrophilic addition to the alkene. Characteristics of the analysis using these correlations are compared and contrasted with other additions to alkenes, such as the Wacker oxidation, to probe patterns in transition state characteristics.     

Why are BINOL-based monophosphites such efficient ligands in Rh-catalyzed asymmetric olefin hydrogenation?

Whereas recent synthetic studies concerning Rh-catalyzed olefin hydrogenation based on BINOL-derived monodentate phosphites have resulted in an efficient and economically attractive preparative method, very little is known concerning the source of the unexpectedly high levels of enantioselectivity (ee often 90-99%). The present mechanistic study, which includes the NMR characterization of the precatalysts, kinetic measurements with focus on nonlinear effects, and DFT calculations, constitutes a first step in understanding this hydrogenation system. The two most important features which have emerged from these efforts are the following: (1) two monodentate P-ligands are attached to rhodium, and (2) the lock-and-key mechanism holds, in which the thermodynamics of Rh/olefin complexation with formation of the major and minor diastereomeric intermediates dictates the stereochemical outcome. The major diastereomer leads to the favored enantiomeric product, which is opposite to the state of affairs in classical Rh-catalyzed olefin hydrogenation based on chiral chelating diphosphines (anti lock-and-key mechanism as proposed by Halpern).     

Effect of variability in level on forward masking and on increment detection

In the first of four experiments, all with the same four subjects, varying the level of a forward masker from interval to interval in a two-interval forced-choice (2IFC) adaptive procedure had little effect on threshold. In the second experiment, the signal level was fixed and performance was measured in units of d'. Varying the level of the forward masker again had little effect. Analyses of trial-by-trial data indicated that subjects did not vote for the interval with the higher-level masker, as would an energy detector. Performance was better on trials where the masker level in the interval with the signal was lower and was relatively independent of masker level in the nonsignal interval. In the third experiment, these results were replicated for a wider range of masker variability and with maskers lower in frequency than the signal. In the fourth experiment, the same range of variability from interval to interval was imposed on the level of the pedestal in an increment-detection task. Results were similar to those observed in forward masking. The results suggest that decision processes involved in both forward masking and increment detection are similar and that neither is based on energy detection. Template matching remains a viable alternative.