Concentration dependence of the thermal boundary resistance between dilute<Superscript>3</Superscript>He−<Superscript>4</Superscript>He solutions and sintered silver powder

We have measured the³He concentration dependence of the thermal boundary resistanceR _B between³He?⁴He dilute mixtures and submicron sintered silver between 10 and 150 mK. For concentrations greater than one percent, the results for the boundary resistance per inverse volume are insensitive to the concentration and have a magnitude similar to that predicted by the acoustic mismatch theory. For concentrations less than one percent, we observe an increase ofR _B for decreasing concentrations.     

Enantioselective partial reduction of 2,5-disubstituted pyrroles via a chiral protonation approach

[reaction: see text] The partial reduction of 2,5-pyrrole diester 1 followed by enantioselective protonation in situ to furnish synthetically useful building blocks is described. An enantiomeric excess of up to 74% was achieved using (-)-ephedrine and related analogues as chiral proton sources. The pyrroline product obtained could be recrystallized to give enantiomerically pure material.     

Dynamic covalent chemistry

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.     

Rearrangement of pyrrolines derived from the Birch reduction of electron-deficient pyrroles: radical ring-expansion to substituted tetrahydropyridines

Access to the synthetically important tetrahydropyridine motif has been achieved by radical rearrangement of pyrrolines obtained from the Birch reduction of electron-deficient pyrroles.     

Surface-enhanced vibrational Raman spectroscopy of C60 and C70 on rough silver surfaces

The observation of the surface-enhanced vibrational Raman spectra of vapor-deposited C₆₀ and C₇₀ on rough silver films is reported. Both near-monolayer and multilayer films of pure C₆₀ and of C₆₀/C₇₀ mixtures are studied. The films are obtained by evaporating fullerene samples at temperatures of 683–875 K in ultra-high vacuum. Mixed fullerene samples were greatly enriched in C₇₀ by making use of the slightly different vapor pressures of the two major components at the low end of this temperature range. The spectra contain all the lines of the normal Raman spectra as well as several additional lines caused by a reduction in the stringency of the normal Raman selection rules. These results demonstrate the potential of this technique for detecting small quantities of fullerenes and obtaining their vibrational spectra.     

Critical velocity of continuous vortex nucleation in a slab of superfluid 3He-A

A flow-induced Fréedericksz transition is observed in a 0.26 mm thick disk-shaped slab of superfluid 3He-A using a rotating cryostat and a torsional oscillator, and it is used to detect vortices in zero magnetic field. The phenomena are studied as a function of magnetic field normal to the slab. In defect-free l texture the critical velocity for vortex nucleation is 0.5 mm/s, but in the presence of a domain wall it is reduced to approximately Planck's over 2pi /2ma(c), where a(c)(H) is the field-dependent radius of the vortex soft core. The vortices nucleate at a distance at least 0.3 mm from the outer edge of the disk.     

Semisuperfluidity of 3He in aerogel?

According to hydrodynamic, acoustic, and NMR studies the superfluid transition temperature of 3He in aerogel ( T(a)(c)) is significantly suppressed with respect to that of bulk 3He. We have found in the range of temperatures between T(c) and T(a)(c) a large and unexpected NMR satellite line attributable to the liquid inside the aerogel. We propose that this anomalous behavior of liquid 3He corresponds to a new type of superfluid ordering related to magnetic and possibly orbital coherence.     

Frequency effects on the conduction electron spin resonance linewidth in aluminium

The CESR linewidth of pure aluminium is shown to have a temperature dependent phonon contribution which is largely frequency independent. A second term in the linewidth is found to be temperature independent but linearly dependent upon frequency giving 0.11 mT per GHz. Finally it is shown that if partial breakdown of motional narrowing occurs it contributes a term proportional to the frequency and not, as expected, to the frequency squared.