Calculation of gauche coupling constants from substituent electronegativities

The calculation of gauche coupling constants from substituent electronegativities by a method which takes into account the orientation of the substituents is described. This correlation has been derived from the coupling constants of disubstituted ethanes and has been applied to the calculation of coupling constants in cyclohexanes and 6-membered heterocycles. The detection of angular distortion in 6-membered rings from the difference between calculated and observed coupling constants is also described.     

Universality in the critical two-dimensionalφ 4-model

The equilibrium properties of the discreteφ ⁴-model ind=2 dimensions are investigated at critically using the hybrid Monte Carlo algorithm and the Ferrenberg-Swendsen extrapolation scheme. By combining the extrapolation scheme with a finite-size scaling analysis, the critical line of the model could be located very accurately. The universality of the model along the critical line is then explicity demonstrated both with respect to the universal nature of the probability distribution function of the global order parameter and by direct comparison with the critical two-dimensional Ising model.     

Identification of quinoline nitration products by NOE

The structures of the products from the nitration of 2- and 4-oxodihydroquinolines were assigned based on Nuclear Overhauser Effects of their nmr spectra and confirmed by unambiguous synthesis.     

Massenspektroskopische Fragmentierung der Trimethylsilyl-Derivate von 6-Sulfons?ure- und Phosphors?ure-substituierten Pteridinen

Ohne ZusammenfassungWir danken der Robert A. Welch Foundation für die Literaturhinweise, ebenso der Rice-Universität, an der die wesentlichen Versuche durchgeführt werden konnten. Das hochauflösende Massenspektrometer Modell 21-110B der Consolidated Electrodynamics Corporation wurde von der National Science Foundation und dem National Institute of Health zur Verfügung gestellt. Das Manuskript wurde geschrieben, während einer der Autoren (P. H.) als Alexander von Humboldt-Stipendiat an der Universität Göttingen war.     

Rearrangements and stereomutations of metallacycles derived from allenes and imidozirconium complexes

The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing beta-hydrogen atoms racemize at room temperature by reversible beta-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95 degrees C; racemization of these metallacycles is also catalyzed by mild oxidants.     

Carbon-13 magnetic resonance: γ-anti substituent effects

γ-anti Substituent effects of the first row elements (CH₃, NH₂, OH, F) and the higher-row halogens on ¹³C chemical shifts are shown to have separate linear relationships with element electronegativity. The halogens generally show an increasing upfield shift with increasing electronegativity, but the response of the effect of the first-row elements to changing electronegativity is dependent upon the substitution pattern of the atoms involved. In molecules with the substituent at a bridgehead, increasing electronegativity causes increasing downfield shift of the γ-anti carbon, whereas in other systems which have the substituent at a secondary carbon, increasing electronegativity causes an increasing upfield shift. Examples are cited to show that the γ-anti effect of a hydroxyl group is more shielding (by about 2 ppm) when a 1,3-diaxial interaction between an α and a γ hydrogen is present, than when this interaction is removed by replacement of either of the hydrogens. This correlation is taken as evidence of this hydrogen-hydrogen interaction as one pathway for the transmission of the γ-anti substituent effects of first-row elements.     

Significant Reduction of UVB Caused by Smoke from Biomass Burning in Brazil

Abstract— Solar UVB radiation, total ozone and the aerosol optical thickness (AOT) of the atmosphere were measured at various sites in Brazil during the burning season of 1995. Smoke from biomass burning caused very significant AOT and up to an 81% reduction in UVB at Cuiabd, hundreds of kilometers from the most widespread burning. Little or no smoke was apparent upwind of the major burning regions at and near Manaus, and UVB was close to the expected values. There is an increased incidence of respiratory, cardiopulmonary and other diseases associated with severe air pollution, but the responsible biological mechanisms are unknown. The bactericidal effects of solar UVB are well known, and significantly reduced UVB resulting from severe air pollution in regions where UVB levels are ordinarily high might enhance the survivability of pathogenic organisms in air and water and on surfaces exposed to sunlight.