Barriers to internal rotation in 1,3,5-trineopentylbenzenes—II: Unsymmetrically halogen-substituted compounds

The temperature dependence of the three different methylene AB spectra due to the 1-, 3- and 5-neopentyl groups in 2-chloro-4-iodo, 2-bromo-4-iodo and 2-bromo-4-chloro-1,3,5-trineopentyl-benzene was studied by proton magenetic resonance measurements at 60 MHz. The identification of the lines of the AB quartets at low temperature was carried out on an HA-100 spectrometer by means of the INDOR technique. The activation parameters for all three barriers in each compound were found to be approximately the same, which perhaps reflects the interdependence of the rotations, with the smaller of the two halogens determining the size of the barrier. An explanation in terms of magnetic nonequivalence induced in the 1- and 5-methylenes by the 3-neopentyl group is considered to be most plausible.     

A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols

An experimental and modeling study of irradiated toluene–NO_x–air, toluene–benzaldehyde–NO_x–air, and cresol–NO_x–air mixtures at part-per-million concentrations has been carried out. These mixtures were irradiated at 303 ± 1 K in a 5800-liter Teflon-lined, evacuable environmental chamber, with temperature, humidity, light intensity, spectral distribution, and the concentrations of O₃, NO, NO₂, toluene, PAN, formaldehyde, benzaldehyde, o-cresol, m-nitrotoluene, and methyl nitrate beingmonitored as a function of time. For the toluene and toluene–benzaldehyde–NO_x–air runs a variety of initial reactant concentrations were investigated. Cresol–NO_x–air runs were observed to be much less reactive in terms of O₃ formation and NO to NO₂ conversion rates than toluene–NO_x–air runs, with the relative reactivity of the cresol isomers being in the order meta » ortho > para. The addition of benzaldehyde to toluene–NO_x–air mixtures decreased the reactivity, in agreement with previous studies. Alternative mechanistic pathways for the NO_x photooxidations of aromaticsystems in general are discussed, and the effects of varying these mechanistic alternatives on the model predictions for the toluene and o-cresol–NO_x–air systems are examined. Fits of the calculations to most of the experimental concentration–time profiles could be obtained to within the experimental uncertainty for two of the mechanistic options considered. In both cases it is assumed that (1) O₂ adds to the OH–toluene adduct ～75% of the time forming, after a further addition of O₂, a C₇ bicyclic peroxy radical, and (2) this C₇ bicyclic peroxy radical reacts with NO ～75% of the time to ultimately form α-dicarbonyls and conjugated γ-dicarbonyls (e.g., methylglyoxal + 2-butene-1,4-dial) and ～25% of the time to form organic nitrates. The major uncertainties in the mechanisms concern (1) the structure of the bicyclicperoxy intermediate, and (2) the γ-dicarbonyl photooxidation mechanism. Good fits to the o-cresol concentration–time profiles in the toluene–NO_x runs are obtained if it is assumed that o7-cresol reacts rapidly with NO₃ radicals. However, it is observed that the model underpredicts nitrotoluene yields by a factor of ～10, but this is in any case a minor product. It is concluded that further experimental work will be required toadequately validate the assumptions incorporated in the aromatic photooxidation mechanisms presented here.     

Oligoacenes: theoretical prediction of open-shell singlet diradical ground states

A series of oligoacenes from benzene to decacene were studied computationally with DFT and CASSCF methods. In contrast to the common view that acenes are closed-shell systems or may have a triplet ground state, these results offer the first theoretical predictions for the singlet ground state and diradical character for oligoacenes. The nature of the ground states of these molecules arises from the disjoint nature of the NBMOs that are singly occupied in the diradical.     

Determination of the α parameter for the decay Ξ− → Λπ−

The α parameter for Ξ^? → Λπ^? decay has been determined to be α_Ξ^?=?0.462 ± 0.015, from a sample of 1.5 × 10⁵ events recorded in the CERN SPS hyperon beam. This value is 2.5 standard deviations higher than the current world average.     

Fine structure in the energy region of the isoscalar giant quadrupole resonance: characteristic scales from a wavelet analysis

Fine structure in the energy region of the isoscalar giant quadrupole resonance in nuclei is observed in high-resolution proton scattering experiments at iThemba LABS over a wide mass range. A novel method based on wavelet transforms is introduced for the extraction of scales characterizing the fine structure. A comparison with microscopic model calculations including two-particle two-hole (2p2h) degrees of freedom identifies the coupling to surface vibrations as the main source of the observed scales. A generic pattern is also found for the stochastic coupling to the background of the more complex states.     

Self-assembly via adsorbate-driven dislocation reactions

Deposition of S onto a monolayer of Ag/Ru(0001) transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [Nature (London) 397, 238 (1999)]]. Time-resolved scanning tunneling microscopy reveals that this nanometer-scale restructuring occurs by a cooperative mechanism involving the sequential formation of triangular regions with fcc and hcp stacking. Using a 2D Frenkel-Kontorova model, we can simulate the creation of these triangular building blocks via basic dislocation motions and reactions.     

Microcionamides A and B, bioactive peptides from the philippine sponge Clathria (Thalysias) abietina

Microcionamides A (1) and B (2) have been isolated from the Philippine marine sponge Clathria (Thalysias) abietina. These new linear peptides are cyclized via a cystine moiety and have their C-terminus blocked by a 2-phenylethylenamine group. Their total structures, including absolute stereochemistry, were determined by a combination of spectral and chemical methods. Compound 1 was shown to slowly isomerize about the C-36/C-37 double bond when stored in DMSO. Microcionamides A (1) and B (2) exhibited significant cytotoxicity against the human breast tumor cells lines MCF-7 and SKBR-3 and displayed inhibitory activity against Mycobacterium tuberculosis H(37)Ra.     

Filtered optical feedback induced frequency dynamics in semiconductor lasers

We demonstrate experimentally and numerically that, by spectrally filtering the delayed optical feedback into a semiconductor laser, one can elicit novel dynamics in the frequency of the laser output light on a time scale that is set by the delay time of the feedback. In particular, we show that through a judicious choice of the filter bandwidth, and its frequency relative to that of the laser, one can produce controlled oscillations in the frequency of light from the laser.     

Sweet success: Ionic liquids derived from non-nutritive sweeteners

The anions of the sweeteners saccharin and acesulfame form ionic liquids when paired with a variety of organic cations.