Ionic liquid effects on Mizoroki-Heck reactions: more than just carbene complex formation

Reaction profiles for a Mizoroki-Heck reaction in either an ionic liquid or a molecular solvent with different palladium sources demonstrate that the rate enhancements observed in ionic liquids cannot be solely attributed to Pd-carbene complex formation.     

Relaxation and Recovery in Hydrogel Friction on Smooth Surfaces

Background

Hydrogels are crosslinked polymer networks that can absorb and retain a large fraction of liquid. Near a critical sliding velocity, hydrogels pressed against smooth surfaces exhibit time-dependent frictional behavior occurring over multiple timescales. The origin of these dynamics is unresolved

Objective

Here, we characterize this time-dependent regime and show that it is consistent with two distinct molecular processes: sliding-induced relaxation and quiescent recovery.

Methods

Our experiments use a custom pin-on-disk tribometer to examine poly(acrylic acid) hydrogels on smooth poly(methyl methacrylate) surfaces over a variety of sliding conditions, from minutes to hours.

Results

We show that at a fixed sliding velocity, the friction coefficient decays exponentially and reaches a steady-state value. The time constant associated with this decay varies exponentially with the sliding velocity, and is sensitive to any precedent frictional shearing of the interface. This process is reversible; upon cessation of sliding, the friction coefficient recovers to its original state. We also show that the initial direction of shear can be imprinted as an observable “memory”, and is visible after 24 hrs of repeated frictional shearing.

Conclusions

We attribute this behavior to nanoscale extension and relaxation dynamics of the near-surface polymer network, leading to a model of frictional relaxation and recovery with two parallel timescales.

     

Small antenna input impedances that contradict Chu-Wheeler Q criterion

Analysis of the small tuned folded dipole input impedance gives a limiting bandwidth proportional to frequency and antenna dimension. A simple antenna mode expansion also directly contradicts the Chu-Wheeler Q criterion. A line-integral potential method gives an additional logarithmic Q reduction factor. Untuned field sensors may have additional coupling losses.     

Fine structure splitting of high 2D states of 39K

The technique of Doppler free two photon absorption is used to measure the fine structure interval in the ²D states of potassium for principal quantum numbers from 8 to 19.     

Nuclear magnetic resonance diagnosis of an anaplastic astrocytoma

A patient presented with an 8-month history of a progressive left homonymous visual field deficit, left hemiparesis, and a left thalamocortical sensory deficit that was not detectable by repeated conventional neurodiagnostic evaluations. Proton nuclear magnetic resonance (NMR) imaging revealed a right parietal lesion characterized by a prolonged T₂ (spin-spin relaxation time). At surgery, the mass proved to be an anaplastic astrocytoma. NMR appears to be more sensitive than x-ray computerized tomography scanning in some patients with malignant gliomas and offers the clinician an additional probe with which to evaluate these patients.     

Diborane: High resolution infrared rovibration studies of 11B2H6 and 11B2D6

Ground state rotation and quartic distortion constants were obtained for ¹¹B₂D₆ from the analysis of high resolution (～0.05 cm^?1) Fourier transform infrared spectra. The bands studied comprised the ν₁₇, ν₁₈ type A, and ν₁₄, ν₉ + ν₁₅ type C bands of ¹¹B₂H₆ and the ν₁₆, ν₁₇, ν₁₈ type A, ν₈ type B, and ν₁₄ type C bands of ¹¹B₂D₆. In the case of ¹¹B₂H₆, the authors' ground state data were combined with those of Lafferty et al. obtained from a previous study (J. Mol. Spectrosc.33, 345–367 (1970)) at comparable resolution of the ν₁₆ type A and ν₈ type B fundamentals. Information on the ground state rotational energy manifold of ¹¹B₂H₆ was accumulated up to J = 23, K_a = 18, and of ¹¹B₂D₆ up to J = 32, K_a = 22. This permitted rather precise determination of the distortion constants Δ_J⁰, Δ_JK⁰, Δ_K⁰, although δ_J⁰ and δ_K⁰ proved to be too small (< 10^?7 cm^?1) and were constrained to values calculated from the force field. Sets of upper state parameters were determined for all vibrational levels studied. Although these appear to be essentially unperturbed globally, several localized perturbations were observed and identified.     

NMR relaxation in the localized regime in InSb

Further experiments in doped InSb are reported on the anomalous NMR relaxation peak, the B-peak, first reported by Bridges and Clark. Data on the peak now extends up to donor concentrations of 8 × 10²¹ m^?3 and permits a sharp reduction in the number of possible explanations of the peak which remain plausible.The peak occurs well above the critical field required for magnetic freeze-out, and appears to follow a dependence of magnetic field at the peak, B_B, against density, n_D, of B_B ∝ n^0.60±0.01_D at constant temperature. The possibility that the peak is due to the tuning of the ESR frequency through the typical electron hopping frequency ω_H in the crystal is explored; a consequence of this model is that ω_H must be dominated by nearest-neighbour hops, involving only a very small minority of the donor centres.     

Infrared television: performance of pyroelectric vidicon at 8 to 14 ?m

The performance of a television camera using the 2.54cm pyroelectric vidicon has been measured and compared with an analysis that attempts to identify the factors limiting the performance of the system.     

The role of partial saturation of the λ3 ladder in SF6 absorption under high power infrared laser excitation

A rate process model is developed to interpret the experimental results of Smith, Schmid, Tablas and Kompa on the time and intensity dependence of the infrared absorption of SF₆. In this model incoherent excitation of the λ₃ molecular mode by the processes of absorption and stimulated emission is followed by radiationless energy transfer from a maximum level to an inactive mode heatbath. This achieves control of the level population by an empirical transfer rate parameter. Good theoretical fits of the experiment absorption decays are obtained with values of approximately 0.1 ns.