Concise formal synthesis of (-)-peduncularine via ring-closing metathesis

[reaction: see text] A synthesis of the 6-aza[3.2.1]bicyclooctene (-)-2 has been completed by a short sequence of reactions that required only six operations from (S)-malic acid and featured a novel ring-closing metathesis to form the bridged bicyclic ring. Because 2 was previously converted into (-)-peduncularine (1), its preparation constitutes a formal enantioselective synthesis of 1.     

Lithographically-defined 3D porous networks as active substrates for surface enhanced Raman scattering

Interferometric lithographically fabricated porous carbon acts as active substrates for Surface Enhanced Raman Scattering (SERS) applications with enhancement factors ranging from 7 to 9 orders of magnitude.     

Desorption and photopolymerization behavior of mixed and multilayered styrene-pyrrole nanofilms

The molecular arrangement and polymerization of adsorbed styrene-pyrrole films were explored using two different dosing schemes: simultaneous dosing to form an intimately mixed film of the two monomers and sequential dosing to form films with distinct layers of styrene and pyrrole. It was found that the desorption of pyrrole was delayed until 175 K when either simultaneously dosed with styrene or in the presence of an overlying styrene layer, whereas it normally desorbs at 165 K when dosed alone. No such effects were observed with styrene in any dosing configuration. The polymerization rate of pyrrole was unaffected in the films when present as the top layer and was observed to decrease by 50% when beneath a styrene layer. Moreover, the rate of polymerization increased by two orders of magnitude in the case of the intimately mixed film. The rate of styrene polymerization was not found to change in any of the explored geometries. These effects are explained by considering the proximity of the components and the potential for screening UV radiation based on their molecular structure.     

Suppression of pump-induced frequency noise in fiber-laser frequency combs leading to sub-radian f ceo phase excursions

We discuss the origins and the suppression of the large frequency jitter on the carrier-envelope offset frequency (f_ceo) of fiber-laser frequency combs. While this frequency noise appears most prominently on f_ceo, its effects are felt across the frequency comb and it is a potential limiting factor in applications of fiber-laser frequency combs. Here we show that its origin lies in the white amplitude noise of the pump laser output. We dramatically reduce this noise by operating the pump laser in a lower-noise state, i.e. at higher pump current, and by more aggressively feeding back to the pump current with an optimal feedback network. We demonstrate instrument-limited f_ceo linewidths and an integrated f_ceo phase jitter of 1 rad. PACS 42.55.Wd; 42.62.Eh; 42.60.Lh; 06.30.Ft     

Molekulargewichtsbestimmungen

Ohne Zusammenfassung     

Representations and the Jacobian Conjecture

In this paper we apply the representation theory of the Lie algebra sl₂(C) to the problem of describing Hessian nilpotent polynomials, which are important in the theory of the Jacobian Conjecture. In the two variable case we describe them as the maximal and minimal weight vectors of the irreducible representations of sl₂(C). For the first time this gives a characterization of the Hessian nilpotent polynomials in terms of linear differential operators.     

Solid H2: Has the quadrupolar glass phase a well-defined transition?

We report measurements of the NMR lineshape, the second and fourth moments (M₂ and M₄), and of the longitudinal relaxation tine in H₂ single crystals. Although a sharp phase transition cannot be excluded, our results for ortho concentrations between 0.15 and 0.4 suggest rather a continuous freezing of rotational motion to a “glass state” as T decreases. In this regime; the NMR spectrum is found to be very nearly isotropic.