Solid-phase synthesis of 3-aminopyrrole-2,5-dicarboxylate analogues

An efficient strategy has been developed for the solid-phase parallel synthesis of 3-aminopyrrole-2,5-dicarboxylate analogues. A library of twenty-nine 2,3,5-trisubstituted pyrroles has been synthesized on Wang resin by a 5-6 step process. The attachment of (2S,4R)-4-hydroxy-N-(PhF)proline cesium salt (PhF = 9-(9-phenylfluorenyl)) to Wang bromide resin, followed by alcohol oxidation, produced the resin-bound 4-oxo-N-(PhF)prolinate as the pyrrole precursor. Resin-bound 3-aminopyrroles were synthesized by treatment of the oxo-N-(PhF)prolinate resin with different secondary amines and diversified at the 2-position by acylation with trichloroacetyl chloride and haloform reactions with primary amines. 3-Aminopyrrole-2,5-dicarboxylates were isolated in 81-99% purity and 51-99% yields after cleavage from the resin using TFA or sodium methoxide.     

Generalised G 2–Manifolds

We define new Riemannian structures on 7–manifolds by a differential form of mixed degree which is the critical point of a (possibly constrained) variational problem over a fixed cohomology class. The unconstrained critical points generalise the notion of a manifold of holonomy G ₂, while the constrained ones give rise to a new geometry without a classical counterpart. We characterise these structures by means of spinors and show the integrability conditions to be equivalent to the supersymmetry equations on spinors in type II supergravity theory with bosonic background fields. In particular, this geometry can be described by two linear metric connections with skew–symmetric torsion. Finally, we construct explicit examples by introducing the device of T–duality.On leave at: Centre de Mathématiques Ecole Polytechnique 91128 Palaiseau, France. E-mail: fwitt@math.polytechnique.fr     

Linear preselective policies for stochastic project scheduling

In the context of stochastic resource-constrained project scheduling we introduce a novel class of scheduling policies, the linear preselective policies. They combine the benefits of preselective policies and priority policies; two classes that are well known from both deterministic and stochastic scheduling. We study several properties of this new class of policies which indicate its usefulness for computational purposes. Based on a new representation of preselective policies as and/or precedence constraints we derive efficient algorithms for computing earliest job start times and state a necessary and sufficient dominance criterion for preselective policies.  A computational experiment based on 480 instances empirically validates the theoretical findings.     

Coating thickness measurements on gas-borne nanoparticles by combined mobility and aerodynamic spectrometry

An on-line method is described and validated to measure the thickness of coatings on gas-borne nanoparticles. The method is essentially a tandem technique which measures the aerodynamic diameter of a particle twice—before and after coating—by a single-stage low-pressure impactor (SS-LPI) for the same mobility equivalent diameter preselected via differential mobility analyzer (DMA). A shell thickness is then derived from the change in effective particle density determined by the SS-LPI. The method requires a difference in mass density between carrier particle and coating material. Its theoretical sensitivity is shown to range between about 0.1 and 1 nm, depending on the density ratio. One advantage of this approach is that both DMA and SS-LPI are situated in series but downstream of the coating step, so as not to interfere with the coating process. The method was validated against transmission electron microscopy (TEM) measurements, using spherical silica–titania particles coated with conformal shells of molybdenum and bismuth oxide by chemical vapor deposition (CVD). For such spherical particles, the agreement with TEM was excellent. The technique was able to provide layer thicknesses for sub-nanometer layers barely or not resolved by TEM. The paper also discusses the impact of ‘non-ideal’ phenomena such as the formation of doublet particles by coagulation, the effect of multiply charged particles, or the onset of homogeneous decomposition of the coating precursor. With supporting experimental data, it is shown that such phenomena can be interpreted reliably from certain features of the impactor penetration curve. The on-line method can thus be used for fast screening of process parameters and reliable process monitoring for gas-phase synthesis of composite nanopowders.     

Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an Fe^IINHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot ³MLCT state, from the initially excited ¹MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the ³MC state, in competition with vibrational relaxation and cooling to the relaxed ³MLCT state. The relaxed ³MLCT state then decays much more slowly (7.6 ps) to the ³MC state. The ³MC state is rapidly (2.2 ps) deactivated to the ground state. The ⁵MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the ³MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.     

Single-Photon Ionization of Organic Molecules Beyond 10 kDa

The volatilization and soft ionization of complex neutral macromolecules at low energies has remained an outstanding challenge for several decades [1]. Most volatilization techniques in mass spectrometry produce ions already in the source and most of them lead to particle velocities in excess of several hundred meters per second. For many macromolecules, post-ionization is inefficient since electronic or optical excitations can be followed by competing non-ionizing internal conversion, electron recapture, or fragmentation processes. Here, we explore the laser-assisted volatilization of neutral perfluoroalkyl-functionalized tetraphenylporphyrins as well as their single-photon ionization using vacuum ultraviolet (VUV) light at 157 nm. A systematic investigation of the ionization curves allows us to determine the molecular velocity distribution and ionization cross sections. We demonstrate the detection of single photon ionized intact organic molecules in excess of 10 kDa from a slow molecular beam.      

Enhancing Hydrogen Evolution Activity of Au(111) in Alkaline Media through Molecular Engineering of a 2D Polymer

The electrochemical splitting of water holds promise for the storage of energy produced intermittently by renewable energy sources. The evolution of hydrogen currently relies on the use of platinum as a catalyst—which is scarce and expensive—and ongoing research is focused towards finding cheaper alternatives. In this context, 2D polymers grown as single layers on surfaces have emerged as porous materials with tunable chemical and electronic structures that can be used for improving the catalytic activity of metal surfaces. Here, we use designed organic molecules to fabricate covalent 2D architectures by an Ullmann‐type coupling reaction on Au(111). The polymer‐patterned gold electrode exhibits a hydrogen evolution reaction activity up to three times higher than that of bare gold. Through rational design of the polymer on the molecular level we engineered hydrogen evolution activity by an approach that can be easily extended to other electrocatalytic reactions.     

Insight into Protein–Polymer Conjugate Relaxation Dynamics: The Importance of Polymer Grafting

The bio and chemical physics of protein–polymer conjugates are related to parameters that characterize each component. With this work, it is intended to feature the dynamical properties of the protein–polymer conjugate myoglobin (Mb)–poly(ethyl ethylene phosphate), in the ps and ns time scales, in order to understand the respective roles of the protein and of the polymer size in the dynamics of the conjugate. Elastic and quasi‐elastic neutron scattering is performed on completely hydrogenated samples with variable number of polymer chains covalently attached to the protein. The role of the polymer length in the protein solvation and internal dynamics is investigated using two conjugates formed by polymers of different molecular weight. It is confirmed that the flexibility of the complex increases with the number of grafted polymer chains and that a sharp dynamical transition appears when either grafting density or polymer molecular weight are high. It is shown that protein size is crucial for the polymer structural organization and interaction on the protein surface and it is established that the glass properties of the polymer change upon conjugation. The results give a better insight of the equivalence of the polymer coating and the role of water on the surface of proteins.     

One-Step Ring Opening Metathesis Block-Like Copolymers and their Compositional Analysis by a Novel Retardation Technique

Using a one-step synthetic route for block copolymers avoids the repeated addition of monomers to the polymerization mixture, which can easily lead to contamination and, therefore, to the unwanted termination of chain growth. For this purpose, monomers ( M1 – M5 ) with different steric hindrances and different propagation rates are explored. Copolymerization of M1 (propagating rapidly) with M2 (propagating slowly), M1 with M3 (propagating extremely slowly) and M4 (propagating rapidly) with M5 (propagating slowly) yielded diblock-like copolymers using Grubbs’ first ( G1 ) or third generation catalyst ( G3 ). The monomer consumption was followed by ¹H NMR spectroscopy, which revealed vastly different reactivity ratios for M1 and M2 . In the case of M1 and M3 , we observed the highest difference in reactivity ratios (r₁=324 and r₂=0.003) ever reported for a copolymerization method. A triblock-like copolymer was also synthesized using G3 by first allowing the consumption of the mixture of M1 and M2 and then adding M1 again. In addition, in order to measure the fast reaction rates of the G3 catalyst with M1 , we report a novel retardation technique based on an unusual reversible G3 Fischer-carbene to G3 benzylidene/alkylidene transformation.