Synthesis and Structure of the Layered Thorium Telluride CsTh(2)Te(6)

The compound CsTh(2)Te(6) has been synthesized at 800 degrees C by the reaction of Th with a Cs(2)Te(3)/Te melt as a reactive flux. The compound crystallizes in the space group -Cmcm of the orthorhombic system with two formula units in a cell of dimensions a = 4.367(2) ?, b = 25.119(10) ?, c = 6.140(3) ?, and V = 673.5(5) ?(3) at T = 113 K. The structure of CsTh(2)Te(6) has been determined from single-crystal X-ray data. The structure comprises infinite, two-dimensional double layers of ThTe(8)-bicapped trigonal prisms. The structural motif of the trigonal prisms resembles that found in UTe(2). Cs(+) cations, disordered equally over two crystallographically equivalent sites, separate the layers and are coordinated by eight Te atoms at the corners of a rectangular parallelepiped. Short Te-Te distances of 3.052(3) and 3.088(3) ? form linear, infinite, one-dimensional chains within the layers. Simple formalisms describe neither the Te-Te bonding in the chain nor the oxidation state of Th. The compound shows weak semiconducting behavior along the Th/Te layers perpendicular to the Te-Te chain.     

Asymmetric halo aldol reaction (AHA)

[reaction: see text] The asymmetric halo aldol reaction (AHA) using Evans oxazolidinones as chiral auxiliaries has been established for tandem I-C/C-C bond formations. The new asymmetric reaction provides a practical approach to a variety of halo aldols of a non-Evans type that cannot be easily synthesized by other methods. Excellent diastereoselectivity (>95%) and yields (80-93%) have been obtained for eight examples.     

Highly Z/E stereoselective approach to β-iodo aza Morita-Baylis-Hillman adducts

A multicomponent reaction between sulfonyl-protected imines, magnesium iodide, and acetylenic esters or ketones is described. The resulting β-iodo aza Morita-Baylis-Hillman adducts were obtained in good yields and excellent Z/E stereoselectivities. The reaction showed good tolerance for sulfonyl protecting groups, as well as for acetylenic ketones and esters. This work presents the first synthetic approach to β-iodo aza Morita-Baylis-Hillman adducts.     

Developments in analytical fourier-transform mass spectrometry

Development of a differentially-pumped, dual-cell geometry, coupled with the evolution of pulsed-laser desorption and Cs⁺-secondary-ion mass spectrometric (s.i.m.s.) desorption methods, has improved the analytical utility of Fourier-transform mass spectrometry (F.t.m.s.). A survey of applications and performances obtained in our laboratories is presented. Among the topics covered are ultra-high-resolution electron-impact and chemical-ionization mass spectra, gas chromatography/F.t.m.s. performance, pulsed Cs⁺-s.i.m.s./F.t.m.s. with cooled liquid matrices and solid samples, laser desorption/F.t.m.s. and accurate mass measurements taken under each of these modes of operation.     

Vibrational spectra of alkali hydrogen selenites,selenous acid,and their deuterated analogs

The infrared and Raman spectra of alkali hydrogen selenites [(MHSeO₃), where M = Li, Na, K, or Cs], selenous acid (H₂Seo₃), and their deuterated analogs have been recorded and interpreted. The internal mode frequencies observed in sodium hydrogen selenite, selenous acid, and sodium trihydrogen selenite are used to generate general valence force constants for HSeO^?₃ and H₂SeO₃ under the assumption of C_s geometry for each species. Calculations of observed deuterium frequency shifts are made to aid in the assignments of the various internal modes. A low-temperature proton-triggered phase transition is observed in CsHSeO₃ and is confirmed in KHSeO₃. A discussion of the effects of proton order and disorder upon the selenite frequencies is also presented.     

Improved detection limits for fast atom bombardment mass spectrometry: A study of time-dependent desorption using a model pulsed bombardment ionization method

Certain sample preparations for fast atom bombardment (FAB) yield an intense but brief pulse of sample ions at the onset of ionization. A model system is used to study this phenomenon. This system utilizes a conventional source of a constant flux of fast atoms and a probe that permits mechanical movement of the sample stage. This is used to simulate the effect of pulsing the atom beam. Experiments with model samples and selected ion monitoring detection show that, following bombardment initiation, sample ions are preferentially desorbed with respect to ions from the FAB matrix. Exhibition of preferential sample desorption depends upon the analyte behaving as a surfactant in the selected matrix. When used in conjunction with an array detector that permits data collection in a time-resolved manner, this system allows collection of useful mass spectra with significantly enhanced sensitivity compared to normal bombardment. When applied to the undecapeptide eledoisin (sequence pyro-EPSKDAFIGLM-NH₂, MW 1187.6 Da) this novel methodology allows an improvement in detection limit of at least three to four orders of magnitude over that observed when using conventional continuous FAB and a point detector.     

Novel imidazolination reaction of alkenes provides an easy access to new alpha,beta-differentiated 1,2-vicinal diamines

alpha,beta-Differentiated 1,2-vicinal diamines have been efficiently synthesized by using new electrophilic imidazolination reaction of alkenes. The hydrolysis of imidazolines was performed by treatment with 6 M HCl in THF at 70 degrees C without epimerization. Eight examples were examined to give good to excellent yields (87-96%).     

Energetic analysis of 24 C20 isomers

Twenty-four individual geometric structures of the 20-carbon system are investigated using hybrid Hartree–Fock/density functional theory (DFT-B3LYP) in combination with the 6-31G and 6-311G* basis sets. These computations are carried out in order to evaluate the relative energies of the various C₂₀ isomers utilizing both geometry optimization and single-point energy calculations. The relative energies of the three most widely studied C₂₀ isomers (ring, bowl, and cage) are compared to the energies of other cyclic, bowl-like, and miscellaneous isomers. Although the ring and the bowl isomers are predicted to be the two most stable species, we have identified three to four additional cyclic structures that are predicted to be energetically competitive with the fullerene structure.     

SPECTROPHOTOMETRIC IDENTITY OF THE ENERGY TRANSFER CHROMOPHORES IN RENILLA AND AEQUOREA GREEN-FLUORESCENT PROTEINS

Abstract— Spectral properties of guanidine-denaturated and pronase-digested green-fluorescent proteins (GFP) from two species of bioluminescent coelenterates have been investigated. Spectrophotometric titrations of Renilla and Aequorea GFP, following denaturation in 6 M guanidine HCl at elevated temperature, revealed identical absorption peaks in acid (383–384 nm) and in alkali (447–448 nm) and a single isosbestic point in the visible region at 405 nm. Both proteins exhibited a spectrophotometric pK. of 8.1 in guanidine -HCl. Pronase digestion of the heat-denaturated GFP's generated a methanol-soluble blue-fluorescent peptide with identical fluorescence emission spectra (λ_max= 430 nm, uncorrected; φ_f1= 0.003) for both coelenterate species. These data suggest that the large absorption differences between native Renilla and Aequorea GFP molecules result from unique protein environments imported to a common chromophore.