Covalent structures

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 ... 

Silicon tetrachloride is a colourless liquid, b.p. 216.2 K, and again the molecule has a covalent structure. Silicon tetrachloride is hydrolysed by water ... 

Several nonmetallic elements and metalloids have a network covalent structure. The most important of these is carbon, which has two different crystalline forms of the network covalent type. Both graphite and diamond have high melting points, above 3500°C. However, the bonding patterns in the two solids are quite different... 

More than 90% of the rocks and minerals found in the earth s crust are silicates, which are essentially ionic Typically the anion has a network covalent structure in which Si044-tetrahedra are bonded to one another in one, two, or three dimensions. The structure shown at the left of Figure 9.15 (p. 243), where the anion is a one-dimensional infinite chain, is typical of fibrous minerals such as diopside, CaSi03 - MgSi03. Asbestos has a related structure in which two chains are linked together to form a double strand. 

In Section 18-6.3 the composition of proteins was given. They are large, amide-linked polymers of amino acids. However, the long chain formula (Figure 18-14, p. 348) does not represent all that is known about the structure of proteins. It shows the covalent structure properly but does not indicate the relative positions of the atoms in space. 

The increased speed of structure determination necessary for the structural genomics projects makes an independent validation of the structures (by comparison to expected properties) particularly important. Structure validation helps to correct obvious errors (e.g. in the covalent structure) and leads to a more standardised representation of structural data, e.g. by agreeing on a common atom name nomenclature. The knowledge of the structure quality is a prerequisite for further use of the structure, e.g. in molecular modelling or drug design. 

Investigations of the constitution and the configuration at the azo groups in diazoanhydrides are difficult because they decompose very rapidly if dissolved in organic solvents, and their water solubility is not sufficient. The results obtained by Kauffmann et al. using HCN in water are in favor of the covalent structure 6.13 rather than of a diazonium-diazoate salt they obtained (in 93% yield) two equivalents of (Z)-4-chlorobenzenediazocyanide. In ethanol the products shown in Scheme 6-8 were obtained, i.e., the same products as obtained by Huisgen and Nakaten (1951, 1954) under similar conditions from (Zi)-diazoacetate, the intermediate in the intramolecular rearrangement of AT-nitrosoacetanilide. 

Questions such as, for example, whether sphalerite contains Zn++ and S= ions or has a covalent structure similar to that of diamond, and whether ionic or covalent bonds are present in complexes such as [FeF% —, [Fe(CN)e]=, etc., have been extensively discussed it has, indeed, until recently not been at all clear whether or not they could be definitely... 

Both the ionic and the covalent structure of sphalerite, for instance, are singlet structures, with no unpaired electrons, so that either extreme or any intermediate is possible, and in such a case evidence from various properties of the particular substance must be considered to decide which extreme is more closely approached. On the other hand, in a crystal such as (NH FeFg or (7V774)3Z e(GW)s the lowest ionic state of the [FeXft] complex does not combine with the lowest covalent state, so that the transition from one extreme to the other is discontinuous. The actual state of the complex in the crystal can be determined from the multiplicity. With an ionic state, Fe+++ and -For ( C Nthe F or... 

Sometimes the atomic arrangement of a crystal is such as not to permit the formulation of a covalent structure. This is the case for the sodium chloride arrangement, as the alkali halides do not contain enough electrons to form bonds between each atom and its six equivalent nearest neighbors. This criterion must be applied with caution, however, for in some cases electron pairs may jump around in the crystal, giving more bonds than there are electron pairs, each bond being of an intermediate type. It must also be mentioned that determinations of the atomic arrangement are sometimes not sufficiently accurate to provide evidence on this point an atom reported equidistant from six others may be somewhat closer to three, say, than to the other three. 

Equation 1-1 with Ax = 1 for N—F leads to 22.1% ionic character and bond moment 1.46 D, a little above the straight line in Figure 1-3. Let us assume that the contribution of the pure covalent structure 1 has the value of 47.2%, calculated from the value 77.9% for each bond (22.1% ionic character). Since the three structures of type 2 contribute 3 X 2.74% = 8.2%, the structures of type 3 contribute the remainder, 44.6%. This value leads to 14.9% for the amount of double-bond character of each of the bonds in the NF3 molecule, close to the value 15% for CHF3, CC1F3, and C1F3 calculated from the shortening of the bond length,59 which is by 0.05 A. 

Bismuth is intermediate in the transition from a metallic to a normal covalent structure each atom shows the effect of its normal tricovalence by having three nearest neighbors, at 3.10 A. and it has also three near neighbors at the larger distance 3.47 A. The respective bond... 

The resulting active end-groups have a covalent structure, rather than ionic, with tetrahedrally bound ligands persisting unaltered during the lifetime of the polymeric chains 46). This persistence of the various initially formed structures accounts for the multimodal distribution of the molecular sizes of polymers initiated by Grignard compounds46). 

The role of oxygen in these inorganic polymers in important. In Zacheriasen s theory, oxygen was described as bridging if part of a covalent structure (10.4). When there was an ionic bond, the oxygen was described as non-bridging (10.5). 

Model analogs of the green type chromophore HBI have been chemically synthe-tized in different forms carrying blocking groups in place of the protein polypeptide chain [21, 24, 68, 69]. However, the covalent structure of HBI does not uniquely define its optical properties, because the molecule undergoes several protonation and conformational equilibria that directly affect its electronic structure. 

The state of the superoxide ion has been summarized by Naceache et al. 22). It appears probable that an ionic model is most suitable for the adsorbed species since the hyperfine interaction with the adjacent cation is relatively small. Furthermore, the equivalent 170 hyperfine interaction suggests that the ion is adsorbed with its internuclear axis parallel to the plane of the surface and perpendicular to the axis of symmetry of the adsorption site. Hence, the covalent structures suggested by several investigators have not been verified by ESR data. 

Edelman, G.M., Gall, W.E., Waxdal, M.J., and Konigsberg, W.H. (1968) The covalent structure of a human gG-immunoglobulin. I. Isolation and characterization of the whole molecules, the polypeptide chains, and the tryptic fragments. Biochemistry 7, 1950-1958. 

The actual structure of HF can be represented as a composite of the covalent structure H-F, in which there is equal sharing of the bonding electron pair, and the ionic structure H+ F, where there is complete transfer of an electron from H to F. Therefore, the wave function for the HF molecule wave function can be written in terms of the wave functions for those structures as... 

From the foregoing analysis, it appears that we can consider the polar HF molecule as consisting of a hybrid made from a purely covalent structure contributing 57% and an ionic structure contributing... 

Compound LII, on the other hand, can be made readily. It can have either the planar tricovalent boron structure or the "triptych tetra-covalent structure. In the latter structure the nitrogen is attached to boron and should be considerably less basic and nucleophilic than usual. It does in fact react unusually slowly with methyl iodide and with acids. The neutralization reaction with acids in water is not only slow but of zero order with respect to the acid. It is believed to have a rate-determining transformation from the triptych to the more basic form as the first step. 

Define network covalent bonds list four substances that have a network covalent structure. 

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