Unshielded structures

Maxted in his study on the poisoning of metallic catalysts (30) has suggested that this phenomenon occurs because the unshielded structure of the poisoning atom allows strong bonding (chemisorption) between it and the surface of the catalyst. In another publication (31)... 

An important question is what is the probable radiation dose during exposure to the space environment In outer space, unshielded structures may be exposed to as much as 3500 Mrads per day during a solar storm. Organic structures with minimum shielding are exposed to perhaps 10-15 rads per hour. Even at the upper level of 50 rads per hour (Van Allen belt), it would take 2,000,000 hours (229 years) to reach the 100-Mrad dosage level, and mot missions should be completed by that time. ... 

Figure 1 shows that the oxidizing potential of the organism is enhanced by the availability of unshielded sulfur in the molecular structure. The sulfur-sulfur bonds of the di- and polysulfides are easily disrupted but access to the carbon-sulfur-carbon bond of the monosulfide seems... 

Polyelectrolyte behavior is exhibited by solutions of SPS in DMF, Fig. 12. At low polymer concentrations, the viscosity Increases as a result of repulsion between the unshielded anions, which Increases the hydrodynamic volume of the polymer. The structure of SPS in DMF solutions as determined by static and dynamic light scattering is also discussed in the chapter by Hara and Wu. 

Pt(lll) disk potential is swept across the first voltammetric peak, the associated adsorption of bromide is demonstrated by the concomitant decrease of the ring current below its unshielded value. The same observation is made for the subsequent, more positive peak, which clearly relates this process to further bromide adsorption on the Pt(lll) disk. Following these two characteristic voltammetric peaks, the Pt(lll) disk current diminishes to a double-layerlike structure above 0.2 V. At the same time, the ring current remains below until the positive potential Kmit is reached. 

Consideration of the chemical properties of quinuclidine derivatives led to the conclusion that this type of compound was characterized both by the presence of unshielded free lone pair electrons attached to the bridgehead nitrogen atom and by the specific conformation of the molecule. In contrast to aliphatic tertiary amines and iV-substituted piperidines quinuclidine has a rigidly fixed structure, each of the two rings in the structure having a bath form. 

DPT calculations at the B3LYP/6-31G(d,p) level of theory confirmed the presence in the 2 cla pseudorotaxane structure of two NH - O hydrogen bonds (Fig. 30.5, left). Analogously, in the H NMR spectrum of the 3a cla pseudo[2] rotaxane structure two sets of shielded and unshielded benzyl signals can be always observed (Fig. 30.4a) corresponding to their exo- or ent/o-cavity disposition. 

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