Level structure of 100Tc

Gamma and electron spectra following thermal neutron capture on ⁹⁹Tc have been studied with a bent-crystal spectrometer, Ge(Li) and Si(Li) detectors and a magnetic spectrometer. Prompt and delayed γ-γ coincidences with Ge(Li) detectors have been performed. A level scheme is proposed for ¹⁰⁰Tc comprising 21 excited states up to 640 keV. The binding energy of the last neutron in ¹⁰⁰Tc was deduced. For most levels, spin and parity values were assigned. Two isometric transitions of respective half-lives 10.2 and 4.6 μs have been identified using the ¹⁰⁰Mo(d, 2n)¹⁰⁰Tc reaction with a pulsed beam of deuterons. From the comparison of the present (n, γ) study and the collaborative study of the ⁹⁹Tc(d, p) reaction, several members of the multiplets $\text{π}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{ν}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$, $\text{π}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{ν}\text{g}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$, $\text{π}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{ν}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{π}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{ν}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ have been identified.     

Crystalline thiophene. II: a comprehensive study of stable and metastable phases by means of heat capacity,thermally stimulated currents and raman spectroscopy measurements

This calorimetric study of crystalline thiophene between 77 and 250 K reveals the existence of four phase transitions at 174.50, 170.49, 136.8 and 111.26 K, respectively. The transition II-III (170.49 K) is easily avoided during cooling, leading to metastable phases, which, in turn, exhibit their own phase transitions.These findings explain previously confused observations. The existence of stable and metastable phases was confirmed by thermally stimulated current measurements. Intermolecular Raman spectra were observed between 10 and 300 K and assigned to the observed phases. Some predictions are made concerning the low-temperature behaviour of thiophene. Tentative rules are proposed to number metastable phases relative to stable ones.     

Practical aspects of shimming a high resolution magic angle spinning probe

High resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample to average out to zero magnetic susceptibility differences in the sample and to obtain resonance linewidths approaching those of liquid state NMR. Shimming such samples therefore becomes an important issue. By analyzing the different sources of magnetic field perturbations present in a sample under MAS conditions, we propose a simple protocol to obtain optimum shim settings in HRMAS. In the case of aqueous samples, we show that the lock level cannot be used as a reliable indicator of the quality of the shims at high spinning speeds. This effect is explained by the presence of temperature gradients imparted by the sample rotation.     

Ambient noise cross correlation in free space: theoretical approach

It has been experimentally demonstrated that the Green's function between two points could be recovered using the cross-correlation function of the ambient noise measured at these two points. This paper investigates the theory behind this result in the simple case of a homogeneous medium with attenuation.     

Stepwise and concerted electron-transfer/bond breaking reactions. solvent control of the existence of unstable pi ion radicals and of the activation barriers of their heterolytic cleavage

Available data from various sources seem to indicate an important role of solvation in the cleavage rates of intermediate pi ion radicals, in the passage from concerted to stepwise electron-transfer/bond breaking reaction pathways and even in the very existence of pi ion radicals. After preliminary computations treating the solvent as dielectric continuum, these expectations are examined with the help of a simple model system involving the anion radical of ONCH(2)Cl and two molecules of water, which allows the application of advanced computational techniques and a treatment of these solvent effects that emphasizes the role of solvent molecules that sit close to the charge centers of the molecule. A pi ion radical minimum indeed appears upon introduction of the two water molecules, and cleavage is accompanied by their displacement toward the leaving anion, thus offering a qualitative mimicry of the experimental observations.     

Photoinduction of surface-relief gratings during all-optical poling of polymer films

Using the all-optical poling method, we observed the formation of a surface-relief grating (SRG) in an amorphous polymer film containing azo dye molecules in side chain positions. The experiment consists of a seeding-type process. We used a recently described experimental setup that permits a periodic nonlinear pattern to be produced by the index dispersion of glass. The particular configuration permits photoinduced translation diffusion of the azo chromophores to be observed as the origin of the SRG formation. Analyses of the gratings recorded by use of s (TE) and p (TM) polarization of the writing beams are conducted by atomic-force microscopy. The effect is attributed to mass transport from regions of high isomerization activity to regions of lower activity.     

Activation barriers in the homolytic cleavage of radicals and ion radicals

As revealed by several experimental examples, radicals and ion radicals may, in contrast with closed-shell molecules, undergo exothermic homolytic cleavages (.A..B --> A: +.B) with substantial activation barriers. A two-state semiclassical model is proposed for explaining the existence of the barrier and estimating its magnitude. It is based on the intersection of the potential energy surfaces characterizing the dissociation of a bonding state, .A..B -->.A. +.B, on one hand, and the approach to bonding distance of a repulsive state, A: +.B --> A therefore B, on the other. After inclusion of the bond cleavage and formation as Morse curves in the normal-mode analysis, a simple activation driving force relationship is obtained, the two main ingredients of the intrinsic barrier being the triplet excitation energy of the A moiety and the pi*--> sigma* excitation energy in .A-B. The model is then tested by quantum chemical calculations, first on a simplified system to evaluate the calculation techniques and then on a real system. A comparison of the model predictions with experiment is finally performed using the rate data recently gathered for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover a respectable range of driving forces, showing satisfactory agreement between theoretical predictions and experimental data.     

Chemical kinetics at high pressure

Abstract

Numerous studies have demonstrated the importance to include pressure as a kinetic parameter in the elucidation of inorganic reaction mechanisms. These studies have specially led to a better understanding and a systematic classification of solvent exchange and ligand substitution reactions of octahedral complexes of transition metal elements. The mechanistic picture for substution reactions on square planar complexes is well established and involves a concurrent bimolecular attack by solvent and the nucleophile on the substrate with a considerable discrimination among different entering groups. The search for factor promoting the conversion of the normal associative mode of activation into a dissociative process has then attracted much attention. Two attempts to induce dissociation, as studied by high-pressure NMR, are presented: one is to prevent the formation by means of sterically hindered ligands, the other one is to promote bond weakening at the leaving group.