Mass Spectrometric Observation of Doubly Charged Alkaline‐Earth Argon Ions

Doubly charged diatomic ions MAr²⁺ where M=Mg, Ca, Sr or Ba have been observed by mass spectrometry with an inductively coupled plasma ion source. Abundance ratios are quite high, 0.1 % for MgAr²⁺, 0.4 % for CaAr²⁺, 0.2 % for SrAr²⁺ and 0.1 % for BaAr²⁺ relative to the corresponding doubly charged atomic ions M²⁺. It is assumed that these molecular ions are formed through reactions of the doubly charged metal ions with neutral argon atoms within the ion source. Bond dissociation energies (D₀) were calculated and agree well with previously published values. The abundance ratios MAr⁺/M⁺ and MAr²⁺/M²⁺ generally follow the predicted bond dissociation energies with the exception of MgAr²⁺. Mg²⁺ should form the strongest bond with Ar [D₀ (MgAr²⁺)=124 to 130 kJ mol^?1] but its relative abundance is similar to that of the weakest bound BaAr²⁺ (D₀=34 to 42 kJ mol^?1). The relative abundances of the various MAr²⁺ ions are higher than those expected from an argon plasma at T=6000 K, indicating that collisions during ion extraction reduce the abundance of the MAr²⁺ ions relative to the composition in the source. The corresponding singly charged MAr⁺ ions are also observed but occur at about three orders of magnitude lower intensity than MAr²⁺.     

Neutron imaging at PSI: a promising tool in materials science and technology

Neutron imaging as a method for non-destructive studies takes advantage of the alternative contrast mechanism of neutrons compared to X-rays: most of the light elements feature high contrast, whereas heavy elements are relatively transparent to neutrons. In the previous decade, the neutron imaging technique has made substantial progress in well-tailored and well-equipped beam lines, in improvements of the image recording quality and efficiency by new and modern imaging techniques, and in modern image processing tools.     

Beyond the x-K relations

Explicit formulae are given for all off-diagonal 1–1 and 2–2 resonance terms in the vibrational hamiltonian following the first contact transformation. Such formulae are needed to test the quantitative accuracy of the generalized x-K relation recently published by Della Valle. These formulae are used in a test calculation for linear HCN and DCN and are found only modestly to improve the predicted energy levels for the former, but dramatically improve those of the latter.     

Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands

Enantiomerically pure imidazolium triflates can be readily prepared from bioxazolines and oxazolineimines; deprotonation of imidazolium triflate 2 gives a chiral N-heterocyclic carbene that can act as a ligand in a catalytically active palladium complex.     

Photochemistry of 4'-benzophenone-substituted nucleoside derivatives as models for ribonucleotide reductases: competing generation of 3'-radicals and photoenols

Ribonucleotide reductases (RNRs) catalyze the 2'-reduction of ribonucleotides, thus providing 2'-deoxyribonucleotides, the monomers for DNA-biosynthesis. The current mechanistic hypothesis for the catalysis effected by this class of enzymes involves a sequence of radical reactions. A 3'-hydrogen abstraction, effected by a radical at the enzyme's active site, is believed to initiate the catalytic cycle. As models for this substrate-enzyme interaction, the photochemically induced intramolecular hydrogen abstraction in a series of 4'-benzophenone-substituted nucleoside analogues was studied. Model compounds with hydroxy-, methoxy-, mesyloxy-groups or a cyclic carbonate in 2'- and 3'-positions were investigated. Depending on the substitution pattern, two different types of photoproducts were observed: Those which result from photoenol formation (gamma-H-abstraction) and those which result from abstraction of the 3'-H-atom (delta-H-abstraction). Photoenol formation was further supported by H/D-exchange experiments. Thus, the 3'-H-abstraction postulated as the initial step in RNR action was successfully modeled by photolysis of 4'-benzophenone-substituted nucleoside analogues. The regioselectivity of the photochemical H-abstraction and thus of the product distribution as a function of the 2'- and 3'-substituents was rationalized on the basis of a conformational analysis of the four model systems, utilizing molecular mechanics simulations.     

New electron centers in neutron-irradiated natural quartz

EPR measurements on neutron-irradiated natural quartz crystals from Brazil revealed the presence of two new electron centers, each with hyperfine structure due to one proton. In one of these centers this hyperfine splitting is typical for H^? which most likely substitutes for an oxygen whereas in the other a much smaller splitting due to an OH group was observed. The properties of these centers are compared with those of the E^′_n centers with closely related structures.     

Initial State Interaction in the (π+,3p) Reaction on N, Ar and Xe

The pion absorption reaction (π^{+,3 p} ) on Ar was studied at pion energies of 70, 118, 162 and 239 MeV, and on N and Xe at 239 MeV. The 3p cross section with a 50 MeV cut on the missing energy is presented. The existence of an initial state interaction (ISI) component to the reaction was investigated. The portion of the 3p cross section assigned to the ISI process is less than one half and at lower energies significantly so. Received: 11 December 1997