On the aromatic character of some ten π-electron systems

The electronic structure of 1,4,5-triaza-cyclohepta-2,6-diene has been studied. The treatment accounts for the failure of detecting, by experiment, a cyclic conjugation in this and related seven membered ring systems.

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Zusammenfassung Die Elektronenstruktur des 1,4,5-triaza-cycloheptadien-2,6 wurde theoretisch untersucht. Das Ergebnis erklärt das Mißlingen von Versuchen, in diesem und verwandten Siebenringsystemen zyklische Konjugation aufzufinden.

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Résumé La structure électronique du 1,4,5-triaza-cycloheptadiène-2,6 a été étudiée. Le résultat explique pourquoi une conjugaison cyclique n'a pas été trouvée expérimentalement ni dans ce composé ni dans d'autres systèmes cycliques semblables à sept membres.

     

A rapid procedure for obtaining hydrodynamic voltammetry plots for electrochemical detectors in high-pressure liquid chromatography systems

The construction of relatively simple and inexpensive apparatus for the determination of the operating conditions for electrochemical detectors in high-pressure liquid chromatography systems is described. A procedure has been developed for the characterization of electrochemically active compounds.     

On the Dirac–Kepler problem: The Johnson–Lippmann operator,supersymmetry, and normal-mode representations

The relativistic Kepler problem is discussed, with emphasis on the exact supersymmetry of the problem. It is shown that the supersymmetry is generated by the Johnson–Lippmann operator. Two related operators are found to generate new supersymmetries in an extended function space. Each of these supersymmetries may be disguised as radial supersymmetries. The radial supersymmetries are discussed and it is shown that each of them defines a normal-mode representation of the hydrogen-atom radial functions. Thus, one obtains two different, but equivalent, analytical expressions for these functions. The expressions are well known, but are rederived here in the light of the new understanding. Finally, the nonrelativistic image of the relativistic supersymmetry is constructed and its generators shown to be identical with those recently presented in the literature. © 1995 John Wiley & Sons, Inc.     

Improved synthesis of oligonucleotides with an allylic backbone. Oligonucleotides containing acyclic, achiral nucleoside analogues: N-1 or N-9-[3-hydroxy-2-(hydroxymethyl)prop-1-enyl]nucleobases

An improved phosphoramidite method is described to prepare oligonucleotides modified with the acyclic, achiral monomers 1. Examination of dimers, prepared on solid support or in solution, showed that phosphortriester dimers containing the allylic unit 1 were unstable towards bases, whereas phosphordiester dimers were stable. Phosphordiester dimers were obtained by replacing cyanoethyl phosphoramidites 2 with phosphoramidites 3, which gave phosphordiesters directly upon oxidation. The phosphordiester dimers were found to be stable towards capping and oxidation, but were somewhat labile towards acids. By reducing the contact time to acids during detritylation it was possible to prepare oligonucleotides containing 4 or 8 modified A, G or T units. The modified oligonucleotides hybridized to complementary DNA and RNA, although with reduced affinity (DeltaT(m) per modification -1 to -5 degrees C).     

Predicting catalysis: understanding ammonia synthesis from first-principles calculations

Here, we give a full account of a large collaborative effort toward an atomic-scale understanding of modern industrial ammonia production over ruthenium catalysts. We show that overall rates of ammonia production can be determined by applying various levels of theory (including transition state theory with or without tunneling corrections, and quantum dynamics) to a range of relevant elementary reaction steps, such as N(2) dissociation, H(2) dissociation, and hydrogenation of the intermediate reactants. A complete kinetic model based on the most relevant elementary steps can be established for any given point along an industrial reactor, and the kinetic results can be integrated over the catalyst bed to determine the industrial reactor yield. We find that, given the present uncertainties, the rate of ammonia production is well-determined directly from our atomic-scale calculations. Furthermore, our studies provide new insight into several related fields, for instance, gas-phase and electrochemical ammonia synthesis. The success of predicting the outcome of a catalytic reaction from first-principles calculations supports our point of view that, in the future, theory will be a fully integrated tool in the search for the next generation of catalysts.     

180 degree unidirectional bond rotation in a biaryl lactone artificial molecular motor prototype

A bifunctional biaryl lactone has been synthesized that should be capable of iterative unidirectional aryl-aryl bond rotation via: (1) a diastereoselective lactone ring opening, (S)-1 to (P,S)-2 or (M,S)-2; (2) a chemoselective lactonization, (P,S)-2 or (M,S)-2 to (S)-3; and (3) a chemoselective hydrolysis, (S)-3 to (S)-1. Preliminary results of a racemic sample have indicated unidirectional 180 degrees rotation with very high directional selectivity per individual artificial molecular motor molecule through the first two steps of this sequence. [reaction: see text]