Expedient parallel synthesis of 2-amino-4-heteroarylpyrimidines

[reaction: see text] An expedient synthesis of diverse 2-amino-4-heteroarylpyrimidines via a 2-chloropyrimidine intermediate is described. A series of potentially biologically active analogues have been synthesized in two parallel steps to afford focused arrays.     

Rate and Computational Studies for Pd-NHC-Catalyzed Amination with Primary Alkylamines and Secondary Anilines: Rationalizing Selectivity for Monoarylation versus Diarylation with NHC Ligands

The relative rates of arylation of primary alkylamines with different Pd-NHC catalysts have been measured, as have the relative rates of arylation of the secondary aniline product in an attempt to understand the key ligand design features necessary to have high selectivity for the monoarylated amine product. As the substituents on the N-aryl ring of the NHC increase in size, selectivity for monoarylation increases and this is further enhanced by chlorinating the back of the NHC ring. Computations have been performed on the catalytic cycle of this transformation in order to understand the selectivity obtained with the different catalysts.     

Isomerism and decoherence

In the present paper we address the problem of optical isomerism embodied in the socalled “Hund’s paradox”, which points to the difficulty to account for chirality by means of quantum mechanics. In particular, we explain the answer to the problem proposed by the theory of decoherence. The purpose of this article is to challenge this answer on the basis of a conceptual analysis of the phenomenon of decoherence, that reveals the limitations of the theory of decoherence to solve the difficulties posed by optical isomerism and, in general, by quantum measurement.     

Stark Hole-Burning Spectroscopy of Cresylviolet Perchlorate in Amorphous Hosts

Stark hole-burning spectroscopy is used to investigate the effective dipole moment change of cresylviolet perchlorate (CVP) in various glass and polymer hosts such as ethanol:methanol (EM), polyvinyl alcohol (PVA), poly (2-hydroxyethyl) methacrylate (PHEMA), polyvinylbutyral (PVB), and formamide. The strong correlation between effective dipole moment change of the guest molecule and the holeburning efficiencies of the host matrices illustrates the sensitivity of the dipole moment change as a direct measure of guest-host interactions. Hole-burning is found to be more efficient as the dipole induced reaction field increases. This relationship is discussed in terms of the unusual hole-burning mechanism suggested for this molecule. The effective dipole moment change of cresylviolet perchlorate ranges from 0.14 to 0.59 Debye.     

Temporal organization of ongoing brain activity

Ongoing brain activity results from the mutual interaction of hundred billions non-linear units and represents a significant part of the overall brain activity. Although its complex dynamics has been widely investigated, a large number of fundamental questions are still open, many of them concerning its temporal structure. Why does a certain population of neurons fires synchronously? Are these synchronized bursts following each other randomly or are they correlated according to some organizing principle? Far from addressing the fundamental problem of its functions, in the present article we focus on the problem of temporal correlations of ongoing cortical activity. We first overview the major features of its temporal structure and review recent experimental results, with particular emphasis on alternative approaches inspired in the theory of stochastic processes; then we introduce a neuronal network model inspired in self organized criticality and compare numerical results with experimental findings.     

Enhancement of electromagnetically induced transparency in room temperature alkali metal vapor

Electromagnetically induced transparency (EIT) has led to several quantum optics effects such as lasing without inversion or squeezed light generation. More recently quantum memories based on EIT have been experimentally implemented in different systems such as alkali metal atoms. In this system the excited state of the optical transition splits into several sublevels due to the hyperfine interaction. However, most of the theoretical models used to describe the experimental results are based on a Λ-system with only one excited state. In this article, we present a theoretical model for the Λ-type interaction of two light, fields and an atomic system with multiple excited state. In particular we show that if the control and probe fields are orthogonally circularly polarized the EIT effect in an alkali-metal vapor can almost disappears. We also identify the reasons of this reduction and propose a method to recover the transparency via velocity selective optical pumping.     

Intrinsic d-wave effects in YBa(2)Cu(3)O(7-delta) grain boundary Josephson junctions

We have measured the angular dependence of the Josephson critical current density (J(C)) in c-axis tilt biepitaxial grain boundary YBa(2)Cu(3)O(7-delta) junctions. We observe for the first time intrinsic d-wave pairing symmetry effects manifested as an oscillatory dependence of J(C) on angle. This intrinsic effect is evident even though spontaneous currents, possibly induced by faceting or barrier impurities, are observed in the grain boundaries.     

On the possibility to determine weak hyperfine structure interaction constants by magnetic depolarization

A new method to measure weak hyperfine structure interaction constants (aτ ? 1) by magnetic depolarization, involving interaction between nuclear orientation and electronic alignment, was used on the 3 ¹P₁ level of ³He. It was found that a/Γ = 0.075 ± 0.010.     

Hybrid character and rotational analysis of the 2700 Å band of m-fluorochlorobenzene

The rotational structure of the 2700 Å (π^∗←π) transition of m-chlorofluorobenzene is observed and analyzed. Assuming the molecule to be planar in the excited state, the changes in rotational constants are found to be

δA=−0.0035cm⁻¹

Simultaneous two-frequency oscillation in a dye laser system

We describe a continuously tunable dye laser operating two-wavelengths simultaneously over a tuning range of about 250 Å. With a beam splitter, two polarizers and two gratings the two output beams are spatially coherent and can be of any desirable polarizations with respect to each other.