Cluster properties of peptides on (100) semiconductor surfaces

Peptide adhesion on semiconductor surfaces is quantitatively investigated by atomic force microscopy. The selected peptides are shown to cluster at the surface, with the larger, higher, and softer clusters appearing on the surfaces with lower peptide adhesion. Average cluster diameters vary from 40 nm on GaAs (100) to 300 nm on Si (100). Direct adhesion of the peptides to the surface competes with forming molecular aggregates that offer an overall reduced surface contact.     

Time-resolved study of solvent-induced recombination in photodissociated IBr-(CO2)n clusters

We report the time-resolved recombination of photodissociated IBr-(CO2)n (n = 5-10) clusters following excitation to the dissociative IBr-A' 2Pi12 state of the chromophore via a 180 fs, 795 nm laser pulse. Dissociation from the A' state of the bare anion results in I- and Br products. Upon solvation with CO2, the IBr- chromophore regains near-IR absorption only after recombination and vibrational relaxation on the ground electronic state. The recombination time was determined by using a delayed femtosecond probe laser, at the same wavelength as the pump, and detecting ionic photoproducts of the recombined IBr- cluster ions. In sharp contrast to previous studies involving solvated I2-, the observed recombination times for IBr-(CO2)n increase dramatically with increasing cluster size, from 12 ps for n = 5 to 900 ps for n = 8,10. The nanosecond recombination times are especially surprising in that the overall recombination probability for these cluster ions is unity. Over the range of 5-10 solvent molecules, calculations show that the solvent is very asymmetrically distributed, localized around the Br end of the IBr- chromophore. It is proposed that this asymmetric solvation delays the recombination of the dissociating IBr-, in part through a solvent-induced well in the A' state that (for n = 8,10) traps the evolving complex. Extensive electronic structure calculations and nonadiabatic molecular dynamics simulations provide a framework to understand this unexpected behavior.     

Coherence stability in three-level systems

We study the decoherence rate for estimating the time at which the coherence instability of a quantum pure state is onset. We analyze the coherence stability of pure states of a three-level quantum system under the effect of a bosonic reservoir and driven by two Raman classical fields. By assuming the boson systems to be in thermal states we find for a symmetric V-system a set of three states free from decoherence and, for a symmetric cascade-system, a two-dimensional subspace whose states are stable against the considered decoherence mechanism.     

Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li‐doped MgO that are much smaller (12±6 kJ mol^?1) than the barriers inferred from different experimental studies (80–160 kJ mol^?1). This raises further doubts that the Li⁺O^.? site is the active site as postulated by Lunsford. From temperature‐programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH₄ on both Li‐doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol–gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH₄ binds heterolytically on Mg²⁺O^2? sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O₂.     

Chiroptische effekte von lanthanoidorganylen mit optisch aktiven liganden III. Erschließung wohldefinierter chiraler organosamarium(III)-verbindungen durch umsetzung von tris(cyclopentadienyl)samarium mit (1S,2R,5R)-Isomenthol, (R)-(+)-Isobutyllactat und (R)-(+)-Methyl-p-tolylsulfoxid

While each of the three organosamarium(III) title complexes: [Cp₂Sm{μ-OC₁₀H₁₉}]₂ (5; Cp = C₅H₅, OC₁₀H₁₉ = isomenthoxide), [Cp₂Sm{μ-OCH(Me)COOⁱBU}]₂ (6) and [Cp₃SmOS(Me)-p-C₆H₄Me] (7) contains a chiral ligand atom (i.e. C or S) next to the metal-bonded oxygen atoms, only the dinuclear compounds 5 and, even better, 6 display (below ca. 600 nm) significant circular dichroism of discrete f---f-crystal field transitions. According to a successful single-crystal X-ray study of 5, the cyclohexyl ring of its (1S,2R,5R)-isomenthoxide ligand adopts a conformation with axial OSm- and ⁱPr-substituents, which is energetically less favourable at least for neat (1S,2R,5R)-isomenthol.

Zusammenfassung

Obwohl jeder der drei neuen Organosamarium(III)-Komplexe: [Cp₂Sm{μ-OC₁₀OH₁₉}]₂ (5; Cp = C₅H₅, OC₁₀H₁₉ = Isomentholat), [Cp₂Sm{μ-OCH(Me)COOⁱBU}]₂ (6) und [Cp₃SmOS(Me)-p-C₆H₄Me] mindestens ein chirales Ligandenatom (C oder S) unmittelbar am metallkoordinierten O-Atom enthält, zeigen nur die dimeren Systeme 5 und noch ausgeprägter 6 (unterhalb von ca. 600 nm) signifikanten Circulardichroismus von f---f-Kristallfeldübergängen des Sm³⁺-Ions. Auf Grund einer erfolgreichen Kristallstrukturanalyse von 5 liegt der Cyclohexylring des (1S,2R,5R)-Isomentholatliganden ausschließlich in der Konformation mit axialen OSm- und ⁱPr-Substituenten vor, die für freies (1S,2R,5R)-Isomenthol energetisch deutlich unvorteilhafter ist.     

Improved synthesis of several methoxynitronaphthalenes

Summary An improved procedure for the synthesis of several isomeric methoxynitronaphthalenes is described. The key intermediates in the synthesis are the respective nitronaphthylamines and nitronaphthols. The syntheses of 1-methoxy-3-nitronaphthalene (1), 1-methoxy-5-nitronaphthalene (2), and 2-methoxy-5-nitronaphthalene (3) are reported.Part of this work was carried out at the University of Texas at El Paso, El Paso, TX, during the tenure of one of the authors there (C.P.)     

A new way of probing reaction networks: analyzing multidimensional parameter space

Technically relevant partial oxidation reactions represent complex reaction networks. Establishing a kinetic model for a system of multiple consecutive and parallel reaction steps is a challenging goal. The synthesis of acrylic acid by oxidation of propane using MoVTeNb mixed oxide as catalyst is such a reaction network. In an on-going study, a 10- fold parallel reactor set-up is used to vary systematically reaction conditions in a broad range over a single, well-defined MoVTeNb oxide. Selectivity and product yield in a multidimensional parameter space can give insight into the reaction network. Apparent activation energies and reaction orders of propane are derived for several conditions. Optimum reaction conditions within the investigated parameter space are specified. The results presented within this contribution contain about 200 data points measured in steady states each corresponding to reaction conditions that differ in temperature, contact time, and propane feed concentration. The fact that this data was collected in less than two months shows clearly the advantage of parallel screening of reaction conditions for mechanistic studies.