Degenerate Carbonium Ions

Carbonium ions are called “degenerate” when they rearrange through finite energy barriers to give products whose gross structures are identical with those of the starting ions. In such degenerate rearrangements (“automerizations”), individual atoms and groups become mixed but the overall structure is conserved. Automerizations can be observed by noting the dispersal of isotopic labels, the loss of optical activity, or the temperature dependence of NMR spectra. In certain cases, a combination of well-known rearrangement processes results in complete mixing of all atoms, C or H. The 2-norbornyl and 9-barbaralyl cations are examples of fully degenerate systems; conditions can be found where both give but single-line NMR spectra.     

V → V energy transfer in HCI with tunable optical parametric oscillator excitation

Energy transfer rates for the V → V processes:

have been measured following excitation of specific levels with a tunable optical parametric oscillator.     

Photodetachment threshold of CN by laser optogalvanic spectroscopy

Laser optogalvanic spectroscopy is used for the first time to obtain the photodetachment threshold for a molecular negative ion, CN⁻. The electron affinity for CN is determined to be 3.821 ± 0.004 eV.     

Growth characteristics of chloride‐based SiC epitaxial growth

In this study some aspects of the chloride‐based CVD growth process have been investigated by using both the approach to add HCl to the standard precursors and by using the single molecule precursor methyltrichlorosilane (MTS). The efficiency of the process for different precursors, the growth rate stability and the effect that the C/Si and Cl/Si ratios have on the growth are studied. It is found that MTS is the most efficient precursor and that the growth becomes carbon limited at C/Si < 1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoinitiated Energy Transfer in Porous-Cage-Stabilised Silver Nanoparticles

We report a new composite material consisting of silver nanoparticles decorated with three-dimensional molecular organic cages based on light-absorbing porphyrins. The porphyrin cages serve to both stabilize the particles and allow diffusion and trapping of small molecules close to the metallic surface. Combining these two photoactive components results in a Fano-resonant interaction between the porphyrin Soret band and the nanoparticle-localised surface-plasmon resonance. Time-resolved spectroscopy revealed the silver nanoparticles transfer up to 37 % of their excited-state energy to the stabilising layer of porphyrin cages. These unusual photophysics cause a 2-fold current increase in photoelectrochemical water-splitting measurements. The composite structure provides a compelling proof of concept for advanced photosensitiser systems with intrinsic porosity for photocatalytic and sensing applications.     

Effects of electron-donating ability of binding sites on coordination number: the interactions of a cyclic Schiff base with copper ions

The stepwise addition of Cu²⁺ ions to the nonplanar cyclic Schiff base 5,9,14,18-tetramethyl-1,4,10,13-tetraazacyclooctadeca-5,8,14,17-tetraene-7,16-dione (H₄daaden, C₁₈H₂₈N₄O₂), yields a one-end-open dinuclear copper chelate. The pyridine adduct of the dinuclear copper chelate, namely, [μ-6,11-dimethyl-7,10-diazahexadeca-5,11-diene-2,4,13,15-tetraolato(4−)](pyridine)dicopper(II), [Cu₂(C₁₆H₂₀N₂O₄)(C₅H₅N)], was characterized by single-crystal X-ray crystallography. The two Cu^II atoms of the copper chelate display different coordination modes, i.e. inner-N₂O₂ and outer-O₂O₂. The Cu atom which is bonded in the outer-O₂O₂ mode is axially bonded to a pyridine molecule, which suggests that the electron-donating ability of the O₂O₂ site to the Cu atom is poor. As a result, the O₂O₂-bonded Cu atom has a coordination number of five, showing square-bipyramidal geometry around the Cu atom. The N₂O₂-coordinated site provides sufficient electron density to the other Cu atom to be stabilized with a coordination number of four, showing square-planar geometry around the Cu atom. The electron-donating ability of the ligand coordination sites plays a key role in determining the coordination number of the Cu atoms of the dicopper chelate.     

Expanding Catch and Release DNA Decoy (CRDD) Technology with Pyrimidine Mimics

Catch and release DNA decoys (CRDDs) utilize photochemically responsive nucleoside analogues that generate abasic sites upon exposure to light. Herein, we describe the synthesis and evaluation of four candidate CRDD monomers containing nucleobases that mimic endogenous pyrimidines: 2-nitroimidazole ( 2-NI ), 2-nitrobenzene ( 2-NB ), 2-nitropyrrole ( 2-NP ) and 3-nitropyrrole ( 3-NP ). Our studies reveal that 2-NI and 2-NP can function as CRDDs, whereas 3-NP and 2-NB undergo decomposition and transformation to a higher-ordered structure upon photolysis, respectively. When incorporated into DNA, 2-NP undergoes rapid photochemical cleavage of the anomeric bond (1.8 min half-life) to yield an abasic site. Finally, we find that all four pyrimidine mimics show significantly greater stability when base-paired against the previously reported 7-nitroindole CRDD monomer. Our work marks the expansion of CRDD technology to both purine and pyrimidine scaffolds.     

Mononuclear metavanadate catalyses gas phase oxidation of methanol to formaldehyde employing dioxygen as the terminal oxidant

Multistage mass spectrometry experiments reveal a sequence of gas phase reactions for the oxidation of methanol to formaldehyde with a mononuclear oxo vanadate anion as the catalyst and dioxygen as the terminal oxidant.     

Ordered and Kinetically Discrete Sequential Protein Release from Biodegradable Thin Films

Multidrug regimens can sometimes treat recalcitrant diseases when single‐drug therapies fail. Recapitulating complex multidrug administration from controlled release films for localized delivery remains challenging because their release kinetics are frequently intertwined, and an initial burst release of each drug is usually uncontrollable. Kinetic control over protein release is demonstrated by cross‐linking layer‐by‐layer films during the assembly process. We used biodegradable and naturally derived components and relied on copper‐free click chemistry for bioorthogonal covalent cross‐links throughout the film that entrap but do not modify the embedded protein. We found that this strategy restricted the interdiffusion of protein while maintaining its activity. By depositing a barrier layer and a second protein‐containing layer atop this construct, we generated well‐defined sequential protein release with minimal overlap that follows their spatial distribution within the film.