Hydrogen structures, tetrahedral

Figure 99.1 Water s charge distribution and hydrogen bonding tetrahedral structure in water.

H2O is a major detonation product. A simple exp-6 potential model, however, does not naturally represent the hydrogen-bonded tetrahedral structure of water. We find that an effective two-species model is effective in representing the equation of state of supercritical water over a wide range of conditions. In the two-species model, we represent water by two species in chemical equilibrium non-associated water (H2O) and associated water H20(a). The non-associated water has standard state parameters given by gaseous water. In associated water, however, standard state parameters are chosen closer to that of liquid water the standard enthalpy and entropy are both less than that of gaseous water. 

Diffusivity and diffusion coefficient are synonymous [25]. In dense membranes, diffusivity is associated with the rate of movement of dissociated hydrogen from site to site within a crystal lattice. In general, diffusivity of hydrogen is greater in metals with the body centered cubic structure (bcc) relative to metals with the face centered cubic stmcture (fee) [26]. According to Wipf, dissociated hydrogen occupies tetrahedral interstitial sites in bcc metals and hops between such sites. Tetrahedral interstitial sites are only 1.01-1.17 A apart in common bcc metals [26]. This relatively short distance permits quantum mechanical tunneling, which accord-... 

The peaks at higher r values gradually disappeared with an increase in temperature and pressure, indicating the breaking of the ice-like hydrogen-bonded tetrahedral structure of water. The peak analysis was performed for the first peak around 200 00 pm of the radial distribution function in the form of D(r)/4TT/ pQ, and the peak was de-convoluted into two peaks, I and n, as seen in Fig. 12. The structure parameters, r, n and half-width at the half-height of the peak, cr, which corresponds to the mean-square amplitude of bonds, are summarized in Table 2. 

An important and numerous class of intermetallic compounds AM2 are the cubic Laves phases (MgCu2 type, C15). Many representatives with A or M being one of the transition metals forming stable binary hydrides (see in.B.l) absorb considerable amounts of hydrogen up to compositions AM2H7. Hydrogen occupies tetrahedral interstices of the crystal structure, thereby expanding it. 

Rush et al. (1%6) studied the Y-H2 system. In YH2 they observed a single peak which they assigned to the vibration of hydrogen in tetrahedral sites in the face centered cubic lattice. The spectrum for YH3 was considerably more complex, exhibiting several maxima, due to hydrogen atoms in different sites in the hexagonal lattice, but the spectrum could be correlated reasonably well with the known crystal structure. Karimov et al. (1%7) found one optical peak in... 

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

In anhydrous phosphoric(V) acid, tetrahedral PO4 groups are connected by hydrogen bonds, a structure which can be represented... 

Methane is a tetrahedral molecule its four hydrogens occupy the corners of a tetra hedron with carbon at its center We often show three dimensionality m structural for mulas by using a solid wedge ) to depict a bond projecting from the paper toward you and a dashed wedge (i 111 ) for one receding away from you A simple line (—)... 

The axes of the sp orbitals point toward the corners of a tetrahedron Therefore sp hybridization of carbon is consistent with the tetrahedral structure of methane Each C—H bond is a ct bond m which a half filled Is orbital of hydrogen over laps with a half filled sp orbital of carbon along a line drawn between them... 

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups. 

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