Mapon and single passage NMRON spectroscopy on125SbFe and125SbNi single crystals

The signs, mode magnitudes and distributions of the electric quadrupole hyperfine interactions (EQI) in single crystal¹²⁵SbFe and¹²⁵SbNi have been examined along a principal hard axis for each system and are compared with earlier easy axis results. The surface preparation was essentially the same for both hosts but the results of the distribution width and anisotropy in the mode values of the measured EQI’s are remarkably different between the two hosts. For¹²⁵SbFe the mode value of the EQI is a factor of two larger along the hard axis <111> and the same sign, negative, as for the easy axis <100>. For¹²⁵SbNi the mode values of the EQI along the same two principal directions are comparable in magnitude but the efg distributions are much broader than in the¹²⁵SbFe case. Single passage results on¹²⁵SbNi provide a weak sweep asymmetry with indications of an apparent change in sign in EQI from negative along the <111> easy axis to positive along the hard <100> axis.     

AC susceptibility and temperature modulation studies of gadolinium near the Curie point

Measurements of the ac susceptibility and temperature modulation studies of the ac and dc magnetic properties of polycrystalline gadolinium in the vicinity of the Curie point T_c are reported. Field-independent exchange-enhanced paramagnetism was observed above T_c. Below T_c the initial field-independent susceptibility was observed for applied fields < 8 A/m rms and exhibited a frequency dependence characteristic of a magnetic after-effect. Within about 3 K of T_c this susceptibility is strongly influenced by domain nucleation and a temperature dependent magnetic relaxation time was observed.     

A multi-component coupling approach to benzo[b]furans and indoles

A single step access to multiply substituted benzo[b]furans and indoles has been developed.     

MAPON EFG Result for 54Mn in Cubic Cobalt

Modulated adiabatic passage on oriented nuclei (MAPON) spectroscopy has been used to measure the electric quadrupole interaction at dilute ⁵⁴Mn impurity probes in crystallographically cubic (fcc) cobalt. The measured value is P/h=+5.2(5) kHz and using Q=+0.33(3) b, leads to an electric field gradient of V _zz =+1.3(2)×10¹⁹ V m⁻². This result is consistent with established trends for Mn and Co probes in the 3d ferromagnetic hosts. This revised version was published online in September 2006 with corrections to the Cover Date.     

The production of frenkel defects in fcc metals

Experimental results from polycrystalline and single crystalline samples of aluminum are analyzed in terms of how electron irradiation causes atomic displacements along 〈100〉 and 〈110〉 crystallographic directions. This interpretation is made for atoms that recoil with energies sliihtly greater than the threshold energy. It is noted that definite correlations exist between some of the substages of stage I and the natural required behavior of recoiling atoms. By applying this criterion to polycrystalline samples, one establishes a means for identifying other phenomena; e.g. multiple atomic displacements, spontaneous recombination, etc.     

Stored energy recovery of irradiated copper

Abstract

The stored energy released in Stage I recovery of reactor neutron irradiated copper was measured by differential thermal analysis calorimetry for three fluences up to a maximum of 3.5 × 10¹⁸ n/cm² (E>0.1 MeV) after irradiation at temperatures of less than 10 K. The stored energy dependence upon fluence, and a tendency toward saturation, were observed. Theoretical reaction rate processes were compared directly to the experimental rates of stored energy release, and the parameters associated with the theory were compared with results from previous resistivity measurements. Good agreement was found in several parameters, but major differences with previous D + E substage results lead to the conclusion that the point defect model may not describe materials experiencing severe neutron damage. Computer studies of warmup rates were made for first and second order and for correlated recovery processes as a function of defect concentration and of external power input. First and second order processes show definite distortion in their recovery rate curves for high defect concentrations; the correlated recovery process shows a much less pronounced effect.

This investigation of stored energy used several new approaches. The use of one sample in both experimental and control roles was not unique, but the use of induced radioactivity within the sample as the heating source, and the use of computer generated theoretical stored energy release curves to analyze the data were unique.     

Tyrosine or Tryptophan? Modifying a Metalloradical Catalytic Site by Removal of the Cys–Tyr Cross-Link in the Galactose 6-Oxidase Homologue GlxA

The concerted redox action of a metal ion and an organic cofactor is a unique way to maximize the catalytic power of an enzyme. An example of such synergy is the fungal galactose 6-oxidase, which has inspired the creation of biomimetic copper oxidation catalysts. Galactose 6-oxidase and its bacterial homologue, GlxA, possess a metalloradical catalytic site that contains a free radical on a covalently linked Cys–Tyr and a copper atom. Such a catalytic site enables for the two-electron oxidation of alcohols to aldehydes. When the ability to form the Cys–Tyr in GlxA is disrupted, a radical can still be formed. Surprisingly, the radical species is not the Tyr residue but rather a copper second-coordination sphere Trp residue. This is demonstrated through the introduction of a new algorithm for Trp-radical EPR spectra simulation. Our findings suggest a new mechanism of free-radical transfer between aromatic residues and that the Cys–Tyr cross-link prevents radical migration away from the catalytic site.     

Termination Mechanism in the Anionic Polymerization of Methyl Methacrylate

Recent studies have revealed that at temperatures around 200°K in tetrahydrofuran solvent, poly(methy1 methacrylate) ion pairs are long-lived and very reactive. At higher temperatures however termination of the ion pairs occurs, as evidenced by the broadening of the molecular weight distribution of the resultant polymer and by the incomplete polymerization of the monomer. Three mechanisms have been proposed to describe these termination reactions; an inter molecular reaction with the monomer ester function, an intramolecular cyclization of the anion, or reaction with the polymer ester function. In the absence of monomer only the last two mechanisms can be operative. A series of experiments was undertaken in which the molecular weight distribution broadening with temperature increase was measured under typical polymerization conditions or in the absence of monomer. The effect of each of the three counterions Li, Na, and K was also monitored. The results obtained are discussed in terms of these three possible termination mechanisms. Termination rate constants calculated from the molecular weight distribution are also presented.