Does ionized diacetylene have a positive proton affinity?

Singly and doubly charged C₄H₃^+/2+ ions generated upon electron ionization (EI) of the neutral precursors 1,3-butadiene, benzene, and exo-methylene cyclopropane, respectively, are examined by sector-field mass-spectrometry. Charge stripping of the mass-selected monocations affords the corresponding dications and charge exchange of the C₄H₃²⁺ dications allows for the reverse redox process. Refined analysis and additional MS/MS studies suggest that the monocations are mixtures of isomeric ions formed upon ionization, whereas only a single type of dication seems to be formed. As an average of energy-resolved measurements, a vertical ionization energy of IE_v(C₄H₃⁺)=16.5±0.4 eV is derived. In addition to the experimental work, density functional theory is used for a computational exploration of the mono- and dicationic species. The best theoretical estimates are IE_a(C₄H₃⁺)=16.33 eV and IE_v(C₄H₃⁺)=16.49 eV for the most stable isomer H₂C=C---CCH⁺. Combination of the experimental and theoretical findings leads to the conclusion that the diacetylene cation C₄H₂⁺ has indeed a positive proton affinity of PA(C₄H₂⁺)=1.50±0.42 eV.     

Calculated optical spectrum of model oxyheme complex

The optical spectrum of a model oxyheme complex has been calculated using a new intermediate neglect of differential overlap (INDO-SCF-CI ) method that allows for the inclusion of configuration interaction and transition metals. In addition to the porphyrin π→π* transitions common to all heme proteins, four weak x,y polarized transitions observed only in oxyheme complexes have been calculated and assigned to excitations involving the lowest-empty highly delocalized (Oπ, dπ) orbital. Two broad z-polarized bands observed in the single-crystal polarized absorption spectra of oxymyoglobin and hemoglobin have also been calculated. Controversy exists over the assignment of these transitions and, in particular, over the extent of involvement of the oxygen ligand. Our calculations assign the weaker near-IR visible band mainly to the d σ dπ→ dπ* excitations and the more intense UV band mainly to a_2u → dσ* excitations. While significant participation (25%) of the highly delocalized (Oπ, dπ) virtual orbital is also found, these z-polarized transitions need not be totally unique to oxyheme complexes, in keeping with experimental observation.     

Pattern transfer and photoluminescence damage assessment of deep-submicrometer features etched by photon-induced cryoetching

We present a study of pattern-transfer and etch-induced damage in photon-induced cryoetching. Features with effective radii as small as 100 nm have been formed in both bulk and layered GaAs/AlGaAs materials. A measurement of the photoluminescence of etch-defined deep- submicrometer structures material suggests that this form of etching results in minimal process-induced damage. Modeling of the luminescence vs feature size for these features shows that the luminescence is limited only by carrier diffusion and non-radiative surface recombination.     

Bioorthogonal organic chemistry in living cells: novel strategies for labeling biomolecules

The chemical labeling of biomolecules continues to be an important tool for the study of their function and cellular fate. Attention is increasingly focused on labeling of biomolecules in living cells, since cell lysis introduces many artefacts. In addition, with the advances in biocompatible synthetic organic chemistry, a whole new field of opportunity has opened up, affording high diversity in the nature of the label as well as a choice of ligation reactions. In recent years, several different two-step labeling strategies have emerged. These rely on the introduction of a bioorthogonal attachment site into a biomolecule, then ligation of a reporter molecule to this site using bioorthogonal organic chemistry. This Perspective focuses on these techniques, their implications and future directions.     

A potentiometric study of the water-soluble silver(I) complexes of n,n'-bis(2-hydroxyethyl)dithiooxamide : Part III. Determination of the different species

Silver(I) and N,N'-bis(2-hydroxyethyl)dithiooxamide (H₂X) in acidic solutions, form a series of water-soluble polynuclear complexes. These complexes can be represented by two general formulae: H_2qAg_pX_q and H_2q-1Ag_pX_q with q=1,2,3 and p=q, q-1.