Three-valence-particle fission product 51 135Sb84

By analysis of fission product γ-ray data measured at Eurogam II using a ₂₄₈Cm source, yrast levels up to about 2 MeV in the N=84 three-particle nucleus ₁₃₅Sb have been identified. These levels are interpreted as π g^7/2ν f^7/2 ₂ and π g^7/2ν f^7/2h^9/2 states with the help of shell model calculations using empirical nucleon-nucleon interactions. Received: 30 July 1998     

A Definitive Example of Aluminum-27 Chemical Shielding Anisotropy

Solid-state²⁷Al NMR spectra have been obtained for a crystalline 1:1 complex of AlCl₃and OPCl₃. Aluminum chloride phosphoryl chloride, AlCl₃· OPCl₃(1), is unusual in that the Al–O–P bond angle is close to 180°. From analysis of the²⁷Al MAS NMR spectra, it was determined that the²⁷Al nuclear quadrupole coupling constant is 6.0(1) MHz, the asymmetry in the electric field gradient (efg) tensor is 0.15(2), and the isotropic chemical shift, δ_iso(²⁷Al), is 88(1) ppm. Solid-state²⁷Al NMR of a stationary sample reveals a line shape affected by a combination of anisotropic chemical shielding and second-order quadrupolar interactions. Analysis of this spectrum yields a chemical shift anisotropy of 60(1) ppm and orientations of the chemical shift and electric field gradient tensors in the molecular frame. Experimental results are compared with those calculated usingab initioHartree–Fock and density functional theory.     

The S-wave pion–nucleon scattering lengths from pionic atoms using effective field theory

The pion–deuteron scattering length is computed to next-to-next-to-leading order in baryon chiral perturbation theory. A modified power-counting is then formulated which properly accounts for infrared enhancements engendered by the large size of the deuteron, as compared to the pion Compton wavelength. We use the precise experimental value of the real part of the pion–deuteron scattering length determined from the decay of pionic deuterium, together with constraints on pion–nucleon scattering lengths from the decay of pionic hydrogen, to extract the isovector and isoscalar S-wave pion–nucleon scattering lengths, a⁻ and a⁺, respectively. We find a⁻=[0.0936(±0.0011)]M_π⁻¹ and a⁺=[−0.0029(±0.0009)]M_π⁻¹.     

Matroids and quotients of spheres

For any linear quotient of a sphere, where is an elementary abelian p–group, there is a corresponding representable matroid which only depends on the isometry class of X. When p is 2 or 3 this correspondence induces a bijection between isometry classes of linear quotients of spheres by elementary abelian p–groups, and matroids representable over Not only do the matroids give a great deal of information about the geometry and topology of the quotient spaces, but the topology of the quotient spaces point to new insights into some familiar matroid invariants. These include a generalization of the Crapo–Rota critical problem inequality and an unexpected relationship between and whether or not the matroid is affine. Received: 7 February 2001; in final form: 30 October 2001/ Published online: 29 April 2002     

The Influence of Solvent Viscosity on the Fluorescence Decay and Time-Resolved Anisotropy of Green Fluorescent Protein

This report describes fluorescence decay and time-resolved anisotropy studies of green fluorescent protein (GFP) in various environments. The addition of glucose and fructose, NaCl, or polyethylene glycol changes the viscosity of the medium surrounding the GFP. Both the time-resolved anisotropy and the fluorescence decay of GFP are measured and it is shown that only the time-resolved anisotropy of GFP is affected by the viscosity, but not its fluorescence decay.     

Intramolecular cyclization of 4-aryl-1,4-dihydropyridine acetals

The synthesis of 4-aryl-1,4-dihydropyridines possessing a protected aldehyde functionality at the ortho position is described. These compounds undergo iminium ion mediated cyclization when treated with titanium tetrachloride or gaseous hydrogen chloride. Mechanistically, cyclization utilizes attack of C-3 of the dihydropyridine on the electrophilic species generated from complexation of the acetal. This methodology highlights opportunities for the construction of novel conformationally constrained dihydropyridine analogs.     

A low noise receiver for millimeter and submillimeter wavelengths

A broadband, low noise heterodyne receiver, suitable for astronomical use, has been built using a Pb alloy superconducting tunnel junction (SIS). The RF coupling is quasioptical via a bowtie antenna on a quartz lens and is accomplished without any tuning elements. In this preliminary version the double sideband receiver noise temperature rises from 205 K at 116 GHz to 375 K at 349 Ghz, and to 815 K at 466 GHz. This is the most versatile and sensitive receiver yet reported for sub-mm wavelengths.     

Topology of covalent non-crystalline solids III: Kinetic model of the glass transition

The temperature dependence of the viscosity of glass-forming materials is derived from the assumption of constant configurational specific heat in the supercooled liquid by using rate theory and a specific structural model. It is argued that the model is less restrictive than appearances suggest. It may be qualitatively corrent for many glass-forming systems, although it has been designed specifically for chalcogenide alloy glasses.