Hydrides critical concentrations

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy ... 

The transformation of 6-bromo-l-hexene (38) into methylcyclopentane by the action of tributyltin hydride (Scheme 7) typifies the richness of the C—C bond forming chemistry in question. A knowledge of the critical rate constants (kc, ku and Br in Scheme 7) allow, through control of substrate concentration, necessary selectivity criteria to be met. Specifically the 5-hexenyl radical (39) must undergo intramolecular addition to form the cyclopentylmethyl radical (40), 40 must abstract a hydrogen atom from tributyltin hydride and the tributylstannyl radical must abstract the halogen in 38 to form 39. These processes must proceed faster than any competing side reaction. 

Palladium is a special case in the Group 10 metals in contrast to Ni and Pt it forms bulk hydrides under mild conditions. The schematic Pd-H phase diagram is shown in Figure 10.7. At low hydrogen concentrations the atoms occupy a small fraction of the interstitial octahedral positions in the palladium fee lattice (the diluted PdH or a-phase), up to about 0.015 H per Pd atom at room temperature. With increasing hydrogen gas pressure (and below the critical temperature T, which is 295 °C for... 

Lithium which had been purified by filtration followed by gettering with titanium and yttrium at 753 K had a lower resistivity than metal purified by other methods. For the liquid, dp/dOc was positive but decreased with increasing temperature, whereas for the solid, the value increased with increasing temperature. The resistance of dissolved oxide and hydride impurities in eutectic alloys of sodium and potassium appears to be a complex function of the concentration of each impurity, which can be attributed to chemical interaction in the metal to form hydroxide. Dissolved hydride causes a considerable increase in the resistance of the alloy but hydroxide has a much smaller effect. Dissolved lithium hydride affects the resistance of the alloy more than does sodium or potassium hydride but, again, hydroxide, as lithium hydroxide, has a smaller effect. Information on the solubility of lithium salts in liquid lithium has been critically reviewed. Recommended solubilities are provided for solutions of oxide and nitride as... 

Intramolecular hydride transfer in equation 4 proceeds with an enzymelike EM of 6.5 X 106 M. In other words, the intramolecular reaction is 6.5 X 106 times faster than the intermolecular counterpart at 1 M concentration (11). Davis et al. (II) argued that relief of strain cannot explain the fast rate because (a) the equilibrium constant in equation 4 is close to unity and (b) force-field calculations show that hydroxy ketone is only 1.7 kcal/mol more strained than the corresponding diketone, which lacks nonbonded H/C=0 interactions. The extremely fast nucleophilic attack on the carbonyl is, however, expected from our spatiotemporal hypothesis. Because the mobile hydrogen is held rigidly only 2.35 A away from the carbonyl carbon, well under the suspected critical distance of 2.8 A (6), the conditions for an enzyme-like acceleration are met. 

The determination of Ge by means of atomic absorption spectrometry (AAS) during the late 1960s was replaced by introduction of the hydride technique with sodium tetrahydroborate as a means of reduction. The detection limits indicated are 2xl0 g and 0.01 pg mL Abbasi et al. (2001) showed that there is a possibility of losing Ge in the presence of chloride when acid digestion procedures are carried out in open vessels. This is more pronounced if the Ge concentration is very low. In fact, 100 pg g can be considered as a critical value above which both open and closed vessels digestion methods can be used without any significant loss. 

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