Formation of the dimethylbenzimidazole ligand of coenzyme B(12) under physiological conditions by a facile oxidative cascade

Dimethylbezimidazole, the axial ligand of vitamin B(12), is synthesized from riboflavin by a two-electron oxidation, a retro-aldol condensation, and a second two-electron oxidation. This oxidative cascade readily takes place nonenzymatically under physiological conditions. [reaction: see text]     

Sub-Tc electron transfer at the Hg-HTSC/liquid-electrolyte interface

The cyclic voltammetry of ferrocene (CpFeCp) adsorbed as a monolayer of CpFeCpCO2(CH2)8SH, self-assembled onto the Hg-based high-temperature superconductor Hg0.8Re0.2Ba2Ca2Cu3O10 (Tc = 134 K), via an ultrathin (3.1 nm) Ag film, has been performed in liquid electrolyte (16:7:1 EtCl/THF/2-MeTHF; 0.2 M LiBF4) at a range of temperatures spanning the superconducting transition. Kinetic analysis based on the Marcus density-of-states theory affords standard heterogeneous rate constants, k degrees , for the ferrocene/ferricinium electron-transfer reaction. Casting these data in Arrhenius form yields a value of k degrees (273 K) = 357 s-1, which is 10-fold lower than that previously reported for the same reaction at a metal electrode in a similar electrolyte, while the reorganizational energy of 0.92 eV for the superconductor interface is very close to that for the related metal interface of 0.95 eV. There is, however, no effect of the onset of superconductivity on the electron-transfer rate for this system; the Arrhenius plot is linear through Tc. This is the first sub-Tc electrochemistry of any kind on a Hg-based superconductor and demonstrates the ease with which kinetic data can be obtained for these very high-Tc materials, opening the way for the routine study of a range of electron-transfer reactions as novel probes of the superconducting state.     

A pi-basic rhenium center that effects cyclohexene isomerization to a beta-agostic carbene ligand

The molecule (PNPR)Re(H)4 (PNPR = (R2PCH2SiMe2)2N, R = iPr or cyclohexyl) reacts at 20 degrees C with 2 mol of cyclohexene to form equimolar cyclohexane and (PNPR)Re(H)2[=C(CH2)5]. This product is characterized by 1H, 13C, and 31P NMR and by X-ray diffraction as having one CH2 hydrogen (from a carbon located beta to Re) donating to the metal ("agostic CH"). This interaction occurs in preference to PNPR amide nitrogen pi-donation. DFT calculations confirm this agostic interaction, and show that the (PNPR)Re(H)2 fragment indeed reverses the greater stability of free olefin vs free carbene.     

Definition of displacement probability and diffusion time in q-space magnetic resonance measurements that use finite-duration diffusion-encoding gradients

In q-space diffusion NMR, the probability P(r,td) of a molecule having a displacement r in a diffusion time td is obtained under the assumption that the diffusion-encoding gradient g has an infinitesimal duration. However, this assumption may not always hold, particularly in human MRI where the diffusion-encoding gradient duration delta is typically of the same order of magnitude as the time offset Delta between encoding gradients. In this case, finite-delta effects complicate the interpretation of displacement probabilities measured in q-space MRI, and the form by which the signal intensity relates to them. By considering the displacement-specific dephasing, , of a set of spins accumulating a constant displacement vector r in the total time Delta+delta during which diffusion is encoded, the probability recovered by a finite-delta q-space experiment can be interpreted. It is shown theoretically that a data analysis using a modified q-space index q=gammadeltaetag, with gamma the gyromagnetic ratio and eta=square root (Delta-delta/3)/(Delta+delta), recovers the correct displacement probability distribution if diffusion is multi-Gaussian free diffusion. With this analysis, we show that the displacement distribution P(r,texp) is measured at the experimental diffusion-encoding time texp=Delta+delta, and not at the reduced diffusion time tr=Delta-delta/3 as is generally assumed in the NMR and MRI literature. It is also shown that, by defining a probability P(y,Delta) that a time tdeltac then eta is not equal to square root (Delta-delta/3)/(Delta+delta) which implies that we can no longer obtain the correct displacement probability from the displacement distribution. In the case that /g/=18 mT/m and Delta-delta=5 ms, the parameter deltac in ms is given by "deltac=0.49a2+0.24" where a is the sphere's radius expressed in microm. Simulation of q-space restricted diffusion MRI experiments indicate that if eta=square root (Delta-delta/3)/(Delta+delta), the recovered displacement probability is always better than the Gaussian approximation, and the measured diffusion coefficient matches the diffusion coefficient at time texp=Delta+delta better than it matches the diffusion coefficient at time tr=Delta-delta/3. These results indicate that q-space MRI measurements of displacement probability distributions are theoretically possible in biological tissues using finite-duration diffusion-encoding gradients provided certain compartment size and diffusion encoding gradient duration constraints are met.     

Vibronic structure in the luminescence spectrum of potassium octachlorodimolybdate(ii)

The luminescence spectrum of K₄ [Mo₂Cl₈] has been measured in a pressed KCl disc at ≈ 10 K. The spectrum shows a progression in the Mo.     

Intramolecular photocycloadditions: synthesis of stericallycongested substituted [4.5.5.5] Fenestranes. Models for the total synthesis of laurenene and silphinene

The synthesis of the first example of a fenestrane substituted at opposing ring junctures utilizing an intramolecular photocycloaddition at elevated temperatures as the key step is reported.     

Stagnation temperature and pressure distortion effects upon the response of an aerofoil

A series of experiments is described in which an uncambered aerofoil, set at a constant incidence was traversed incrementally through a range of combined stagnation temperature/stagnation pressure distortions. Measurement of the static pressure distribution around the aerofoil yielded lift coefficient variations. It was concluded that the aerofoil behaviour was in agreement generally with an associated theoretical development although agreement was somewhat confused by the presence of wakes in the experiment and also by local incidence variations created by circumferential variations in static pressure. The data obtained were directly applicable to wing/wake interactions or to a compressor stator in a flow with distortion. It was recognized that in passing through such a distortion the rotor of a turbomachine would also sense a change of incidence, a feature not included in these experiments.