The Addition of Nitriles to Tetramesityldisilene: A Comparison of the Reactivity between Surface and Molecular Disilenes

The addition of acetonitrile, propionitrile, and phenylacetonitrile to tetramesityldisilene (Mes₂Si?SiMes₂) was examined. In general, 1,2,3‐azadisiletines and the tautomeric enamines were formed, although a ketenimine was formed as the major product in the addition of phenylacetonitrile to the disilene. In the presence of LiCl, the mode of addition changed for both acetonitrile and propionitrile: insertion into the α‐CH bond of acetonitrile and/or formation of the formal HCN adduct was observed. Preliminary investigations of the reactivity of the nitrile adducts are also reported. A comparison between the reactivity of nitriles with Mes₂Si?SiMes₂ and the Si(100)‐2×1 surface was made both in terms of the types of adducts formed and their reactivity. Some insights into the surface chemistry are offered.     

Mechanism of the addition of nonenolizable aldehydes and ketones to (Di)metallenes (R(2)X=YR(2), X = Si,Ge Y = C,Si, Ge): a density functional and multiconfigurational perturbation theory study

The mechanism of the addition of nonenolizable aldehydes and ketones to group 14 (di)metallenes has been examined through a theoretical study of the addition of formaldehyde to Si=C, Ge=C, Si=Si, Si=Ge, and Ge=Ge bonds at the B3LYP/6-311++G(d,p) and CAS-MCQDPT2/6-31++G(d,p) levels of theory. The reaction pathways located can be grouped as either involving the formation of singlet diradical or zwitterionic intermediates or as concerted processes. Within each group of reaction pathways, several different mechanisms have been located, with not all mechanisms being available to all of the (di)metallenes. It was found that for reactions in which a Si-O bond results (i.e., addition to Si=C, Si=Si, and Si=Ge) both diradical and zwitterionic intermediates are possible; however, the formation of diradical intermediates was not found for reactions that result in the formation of a Ge-O bond (addition to Ge=C and Ge=Ge). The underlying cause of this pathway selectivity is examined, as well as the effect of solvent on the relative energies of the pathways. The results of the study shed light on the cause of experimentally obtained results regarding the mechanism of the reaction of (di)metallenes with nonenolizable ketones and aldehydes.     

Muon spin relaxation in solid3He

Muon spin relaxation in solid³He depends non-monotonously on the temperature. It is shown that this is entirely due to the magnetic dipole-dipole interactions. The observed line narrowing cannot be explained by the mechanism of positive charge hopping described by an Arrhenius-like law. The relaxation rate at low temperatures was found to increase under the influence of an external electric field.     

Improved measurements of the neutral current from hadron and lepton production at LEP

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine ⁺ e ^– collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ ⁰ peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ ⁰ parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.     

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

The reaction of benzyl isocyanide, tert‐butyl isocyanide, and 2,6‐dimethylphenyl isocyanide with tetramesityldigermene (Mes₂Ge=GeMes₂) was examined. Whereas the addition of benzyl isocyanide gave the C?NC activation product, Mes₂Ge(CH₂Ph)Ge(CN)Mes₂, tert‐butyl isocyanide, and 2,6‐dimethylphenyl isocyanide did not give stable adducts, rather the rate of conversion of the digermene to the corresponding cyclotrigermane was accelerated. A comparison between the reactivity of the isocyanides with Mes₂Ge=GeMes₂ and the Ge(100)‐2×1 surface was made and some insights into the surface chemistry are offered.     

Extremes of nuclear structure

With the advent of medium and large gamma detector arrays, it is now possible to look at nuclear structure at high rotational forces. The role of pairing correlations and their eventual breakdown, along with the shell effects have showed us the interesting physics for nuclei at high spins — superdeformation, shape co-existence, yrast traps, alignments and their dramatic effects on nuclear structure and so on. Nuclear structure studies have recently become even more exciting, due to efforts and possibilities to reach nuclei far off from the stability valley. Coupling of gamma ray arrays with ‘filters’, like neutron wall, charged particle detector array, gamma ray total energy and multiplicity castles, conversion electron spectrometers etc gives a great handle to study nuclei produced online with ‘low’ cross-sections. Recently we studied, nuclei in mass region 80 using an array of 8 germanium detectors in conjunction with the recoil mass analyser, HIRA at the Nuclear Science Centre and, most unexpectedly came across the phenomenon of identical bands, with two quasi-particle difference. The discovery of magnetic rotation is another highlight. Our study of light In nucleus, 107In brought us face to face with the ‘dipole’ bands. I plan to discuss some of these aspects. There is also an immensely important development — that of the ‘radioactive ion beams’. The availability of RIB, will probably very dramatically influence our ‘conventional’ concept of nuclear structure. The exotic shapes of these exotic nuclei and some of their expected properties will also be touched upon.     

Decomposition of peroxycarbamates and their initiation of vinyl polymerization

A review is given of the synthesis, physical properties, and decomposition kinetics of organic peroxycarbamates. The activity of these compounds in initiating free-radical vinyl polymerization is discussed. The decomposition of hexamethylene N,N′-bis-(α-cumyl peroxycarbamate) has been measured in different solvents between 65 and 95°C and found to be a first-order reaction governed by the specific rate constant k_d = 2.21 × 10¹⁶ exp {?34 100/RT}. The initiator exhibits normal free-radical polymerization kinetics and in polymerization of styrene at 80°C shows an initiating efficiency of 0.53. Cobalt naphthenate can act as both accelerator and retarder of styrene polymerization, depending upon its concentration and the temperature of the polymerization.     

Investigation of low-temperature quantum diffusion in α-iron byμ + SR experiments on a single-crystal sphereSR experiments on a single-crystal sphere

The longitudinalμ ⁺-spin relaxation rate has been measured on a high-purity spherical α-iron single crystal at temperaturesT down to 20 mK and in applied magnetic fieldsB _appl parallel to 〈111〉 up to 3 T. Only above 1 K can the data be satisfactorily described by one rate constantГ. At 1 T≤B _appl≤2 T and 50 mK≤T≤300 mK, oscillations (“wiggles”) were in addition superimposed on the longitudinal relaxation. A qualitative understanding of the measurements may be achieved in terms of the increasing influence of internal stresses onμ ⁺ diffusion as the temperature is lowered.     

Observation of diamagnetic domains in beryllium

We have observed diamagnetic domains (Condon domains) in a beryllium single crystal in magnetic fields H⩽3 T (H∥[0001]) at liquid-helium temperatures. The formation of the domain structure was determined according to magnetic-breakdown quantum oscillations of the resistance thermoelectric power as well as according to the splitting of the resonance peak of the free spin precession frequency of muons (μSR). The alternation of a uniform state (with one μSR peak) and a state with domain structure (with two peaks) is consistent as regards the periodicity with the de Haas-van Alphen effect, the period is ΔH≅78 Oe, and the range of existence of domains and the difference in their magnetizations are ΔB=4πΔM=B ₂−B ₁≅30 Oe. Fiz. Tverd. Tela (St. Petersburg) 40, 524–526 (March 1998)     

Influence of elastic strain onμ + SR in α-iron single crystalsSR in α-iron single crystals

The spin-precession frequencies and the transverse spin relaxation rates of positive mouns (μ ⁺) have been measured on two elastically strained α-Fe single crystal platelets as well as on an unstrained reference α-Fe crystal at temperatures down to 2.7 K in applied magnetic field 0≤B _appl≤3 T. The drastic effects of the strains may be qualitatively understood in terms of their influence on both the magnetic domain structure and theμ ⁺ energies at the various interstitial sites. This leads to the conclusion that at low temperaturesμ ⁺ in α-Fe occupy configurations related to octahedral interstitials with dipolar fieldB _dip=0.70 T.