Dynamical screening by Σ-electrons in π-electron transitions: Treatment in the equations-of-motion method

Dynamical screening in π-electron systems is studied by the equations of motion method. By using a partitioning technique on the equations of motion we can obtain simple expressions for the effect of dynamical screening directly on the transition energies and transition moments in π-electron systems. These results are used to study the effect of such screening in the N → V transition in ethylene. This procedure can be used to extend the equations-of-motion method to larger π-electron systems.     

Why not use point charge models for the determination of bond angles?

Bond angles of several free radicals have been estimated using the point charge model together with ESR data and compared with those derived by Coulson's equation and the INDO method. The model is found to work well in predicting bond angles for most radicals just like the INDO method. Some relationships between the model and the INDO method are discussed.     

New methods and reagents in organic synthesis. 49. A highly efficient stereoselective synthesis of L-daunosamine through direct C-acylation using diphenyl phosphorazidate (DPPA)

L-Daunosamine, the carbohydrate component of a group of important anticancer anthracycline antibiotics, has been efficiently prepared from L-lactic acid in a completely stereoselective manner through direct C-acylation using diphenyl phosphorazidate (DPPA).     

Proposed structure of the cyclic peptide dolastatin 3, a powerful cell growth inhibitor,should be revised!

The cyclic peptide proposed for the structure of the powerful cell growth inhibitory dolastatin 3, its diastereomers, and their isomers on the reverse order have been synthesized by the use of diphenyl phosphorazidate(DPPA) and diethyl phosphorocyanidate(DEPC). Their physicochemical and biological properties have shown that the proposed structure is untenable.     

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.     

Chiral carbene approach to gold-catalyzed asymmetric cyclization of 1,6-enynes

Chiral C₂-symmetric N-heterocyclic carbenes (NHCs) were tested for their stereocontrolling abilities in gold(I)-catalyzed asymmetric cyclization of 1,6-enynes giving the corresponding cyclopentane derivatives with moderate enantioselectivity of up to 59%.     

Pheromone synthesis. Part 244: Synthesis of the racemate and enantiomers of (11Z,19Z)-CH503 (3-acetoxy-11,19-octacosadien-1-ol), a new sex pheromone of male Drosophila melanogaster to show its (S)-isomer and racemate as bioactive

The enantiomers of (11Z,19Z)-3-acetoxy-11,19-octacosadien-1-ol were synthesized from the enantiomers of 3,4-epoxy-1-butanol PMB ether. Its racemate was also synthesized. Its (S)-isomer and racemate were shown to possess the same pheromone activity as CH503, a long-lived inhibitor of male courtship in Drosophila melanogaster, although the racemate was less active.     

Spatio-temporal chaos for the Gray–Scott model

A new geometrical criterion for the transition to spatio-temporal chaos (STC) arising in the Gray–Scott model is presented. This is based on the inter-relationship of global bifurcating branches of ordered patterns with respect to supply and removal rates contained in the model. This viewpoint not only gives us a new criterion for the onset of STC but also clarifies how the orbit itinerates among several ordered patterns in infinite-dimensional space. Moreover, the geometrical characterization gives us a universal viewpoint about the onset and termination of STC. There are at least two different mechanisms that cause re-injection dynamics and drive the STC: one is a generalized heteroclinic cycle consisting of self-replication and self-destruction processes, and the other involves annihilation of colliding waves instead of self-destruction.     

Analytical solutions describing the phase separation driven by a free energy functional containing a long-range interaction term

We are primarily concerned with the variational problem with long-range interaction. This functional represents the Gibbs free energy of the microphase separation of diblock copolymer melts. The critical points of this variational problem can be regarded as the thermodynamic equilibrium state of the phase separation phenomenon. Experimentally it is well-known in the diblock copolymer problem that the final equilibrium state prefers periodic structures such as lamellar, column, spherical, double-diamond geometries and so on. We are interested in the characterization of the periodic structure of the global minimizer of the functional (corresponding to the strong segregation limit). In this paper we completely determine the principal part of the asymptotic expansion of the period with respect to epsilon (interfacial thickness), namely, we estimate the higher order error term of the period with respect to epsilon in a mathematically rigorous way in one space dimension. Moreover, we decide clearly the dependency of the constant of proportion upon the ratio of the length of two homopolymers and upon the quench depth. In the last section, we study the time evolution of the system. We first study the linear stability of spatially homogeneous steady state and derive the most unstable wavelength, if it is unstable. This is related to spinodal decomposition. Then, we numerically investigate the time evolution equation (the gradient flow of the free energy), and see that the free energy has many local minimizers and the system have some kind of sensitivity about initial data. (c) 1999 American Institute of Physics.