One photon double ionization of helium: threshold behaviour

In order to study the dynamics of double photoionization of helium, we report new coincidence measurements between low energy electrons and doubly charged ions, from 78 to 95 eV photon energy. We show that the range of validity of the Wannier theory depends upon the observable. For the exponentn of the threshold law, this range amounts to some 3 eV above onset, thus confirming previous published experimental work. In contrast, the energy distribution of the two outgoing electrons is found flat, within 20%, in agreement with the theoretical predictions, but in a 15 eV energy range above threshold.     

BENZOPHENONE PHOTOSENSITIZATION OF 2'-DEOXYGUANOSINE: CHARACTERIZATION OF THE 2R AND 2s DIASTEREOISOMERS OF 1-(2-DEOXY-β-D-erythoW-PENTOFURANOSYL)-2-METHOXY-4,5-IMIDAZOLIDINEDIONE. A MODEL SYSTEM FOR THE INVESTIGATION OF PHOTOSENSITIZED FORMATION OF DNA-PROTEIN CROSSLINKS

Abstract Benzophenone-mediated photosensitization of 2'-deoxyguanosine and its 3',5'-di-O-acetyl derivative, used as DNA model compounds, in oxygen-saturated water-methanol (1:1) solution results in the nucleophilic addition of methanol to the guanine base. The resulting modified nucleosides have been isolated by reverse-phase high-performance liquid chromatography and characterized by extensive spectroscopic measurements including ¹³C and ^IH nuclear magnetic resonance, fast atom bombardment mass spectrometry and circular dichroism as the 2R and 2s diastereoisomers of 1-(2-deoxy-˜-o-erythro-pentofuranosyl)-2-methoxy-4,5-imidazolinedione and their related 3',5'-di-O-acetyl derivates. Information concerning the absolute configuration of the two pairs of diastereoisomers was inferred from detailed nuclear Overhauser effect experiments. A reaction mechanism, involving guanine radical intermediates, is proposed to explain the generation of these new guanine photoproducts.     

Surface reactions of 3-butenenitrile on the Si(001)-2 x 1 surface at room temperature

Using a combination of local -- scanning tunneling microscopy -- and spatially integrated, but chemically sensitive probes -- X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy -- we have examined how 3-butenenitrile reacts with the Si(001)-2 x 1 surface at room temperature. Electron spectroscopies indicate three different nitrogen chemical bonds: a Si-C=N-Si bond, a C=C=N cumulative double bond, and a CN moiety datively bonded to a silicon atom. All molecular imprints detected by scanning tunneling microscopy (STM) involve two adjacent silicon dimers in the same row. The three geometries we propose -- a double di-sigma bonding via the CN and the C=C, a cumulative double bond formation associated with alphaC-H bond dissociation, and a di-sigma vinyl bonding plus a CN datively bonded to a silicon atom -- are all compatible with electron spectroscopies and data. Real-time Auger yield kinetic measurements show that the double di-sigma bonding geometry is unstable when exposed to a continuous flux of 3-butenenitrile molecules, as the Si-C=N-Si unit transforms into a CN moiety. A model is proposed to explain this observation.     

Parallel supported synthesis of polyamine-imidazole conjugates

[reaction: see text] A small collection of nine polyamine-imidazole conjugates, potentially acting as RNases A mimics, has been synthesized on SynPhase lanterns using amino alcohols and diamines as building blocks. Couplings were performed via S(N)2 alkylation of methanesulfonates with amines. The final introduction of N-4-nitrobenzyloxycarbonyldiamines allowed easy purification of the cleaved compounds.     

Reaction of 2,5‐dimethylpyrroles with quinones. Synthesis of new pyrrolylquinones

The reaction of 1,4‐naphthoquinone ( 1 ) with 2,5‐dimethylpyrroles ( 7a‐7d ) gives only 3‐(1,4‐naphtho‐quinonyl)‐2,5‐dimethylpyrroles. Extending the reaction to other quinones: 5‐hydroxy‐1,4‐naphthoquinone ( 2 ), 1,2‐naphthoquinone ( 3 ), quinoline‐5,8‐dione ( 4 ) and quinoxaline‐5,8‐dione ( 5 ), of which nothing was known, allows the synthesis of new pyrrolylquinones.     

Gas-phase reactions of Sc+, Y+, and Lu+ with alcohols. Effects of the class and chain length of alcohols on the nature of primary products

The primary gas-phase reactions between methanol, ethanol, propan-1-ol, propan-2-ol, and 2-methyipropan-2-ol and the isovalent rare earth metal ions Sc⁺, Y⁺, and Lu⁺ generated by laser desorption-ionization of metal targets have been investigated by using a Fourier transform ion cyclotron resonance mass spectrometer. The three metal ions react exothermically with all the alcohols. The overall reactivity is controlled by the high oxophilicity of these metals, and the primary metallated ions obtained are principally oxygenated species. However, the number and the nature of these primary products depends on the electronic configuration of the metal ions as well as on the class and the principal chain length of alcohols. The order of reactivity is Y⁺ > Sc⁺ > Lu⁺. The Y⁺ and Sc⁺ ions principally react via C—O and O—H insertions, whereas Lu+ reacts by direct abstraction or via various five-center electrocyclic mechanisms as a function of the class and the alcohol chain length.     

The electronic structure of a diarsaallene –As=C=As– and a phosphaarsaallene –P=C=As–: UV photoelectron spectroscopy and theoretical studies

The electronic properties of a diarsaallene ArAs=C=AsAr and a phosphaarsaallene ArP=C=AsAr (Ar: 2,4,6-tri-tert-butylphenyl) have been investigated by using UV photoelectron spectroscopy and by density functional calculations on model compounds [(CH₃)₂C₆H₃Pn=C=AsC₆H₃(CH₃)₂, Pn: As, P]. Moreover, a comparison of the geometrical and electronic structures of the parent heteroallenes with those of the arsaethene H₂C=AsH and phosphaethene H₂C=PH has also been undertaken in order to determine the magnitude of the interaction between the π bond and the pnictogen lone pair n_Pn.     

Synthesis of N alpha-(1-phenyl-2-mercaptoethyl) amino acids, new building blocks for ligation and cyclization at non-cysteine sites: scope and limitations in peptide synthesis

A new and convenient method for the synthesis and incorporation of N(alpha)-(1-phenyl-2-mercaptoethyl)-derivatized amino acids applicable to chemical ligation at non-cysteine sites is presented. N(alpha)-Auxiliary derivatives of glycine and alanine were easily prepared using reductive amination approaches. Several strategies for the incorporation of these derivatives into peptide chains were investigated: coupling without protection, with acid-labile protection, with base-labile protection, and via a novel protection strategy using the thiazolidine derivative. All amino acid derivatives were successfully coupled to various peptide resins, and with the exception of those incorporating Boc-protected derivatives, all resins yielded the desired peptide fragments. However, the coupling of the two alanine derivative diastereomers generated some epimerization. Finally, N-terminal auxiliary glycine and alanine peptides were cyclized, and the corresponding native circular peptides were obtained upon successful removal of the auxiliary.