Covalent bond components

The negative 0p values are ascribed to exchange interactions. The [Xeff value indicates trivalent Nd [6], also see the comparative data on pp. 56/7. An Nd valence of only 2.7 pointing to an appreciable covalent bond component is suggested by measurements of [3] on powdered material at 130 to 470 K. The characteristic data given are 0p = OK, fXeff = 3.35 [Xg, and Xa = 4820x10-6 cm /mol Nd. 

The covalent bonding component is of essential importance for the understanding of the bonding mechanism. The local partial /-like charges and the partial /-like DOSs allow only a rough estimate of this bonding component. Much more detailed information can be obtained from valence electron densities (VEDs) and the VEDs of special states. 

Many enzymes carry out their catalytic function relying solely on their protein structure. Many others require nonprotein components, called cofactors (Table 14.2). Cofactors may be metal ions or organic molecules referred to as coenzymes. Cofactors, because they are structurally less complex than proteins, tend to be stable to heat (incubation in a boiling water bath). Typically, proteins are denatured under such conditions. Many coenzymes are vitamins or contain vitamins as part of their structure. Usually coenzymes are actively involved in the catalytic reaction of the enzyme, often serving as intermediate carriers of functional groups in the conversion of substrates to products. In most cases, a coenzyme is firmly associated with its enzyme, perhaps even by covalent bonds, and it is difficult to... 

In most cases, the formation of complexes in molten salts leads to an increase in the molar volume relative to the additive volume. This phenomenon is usually explained by an increase in bond covalency. Nevertheless, the nature of the initial components should be taken into account when analyzing deviations in property values, as was shown by Markov, Prisyagny and Volkov [314]. In particular, this rule applies absolutely when the system consists of pure ionic components. The presence of initial components with a significant share of covalent bonds leads to an S-shaped isotherm [314]. 

While sharing of electrons, i.e., covalent bonding, is the major component of the cohesive force in intermetallics, rationalization of their structure formation based on such chemical bonding is not trivial, because of the failure of the common electron counting rules that chemists have developed over the years from the studies of covalent compounds. The origin of the problem is the well-delo-... 

The covalent bond is the strongest force that holds molecules together (Table 2-1). Noncovalent forces, while of lesser magnitude, make significant contributions to the structure, stability, and functional competence of macromolecules in living cells. These forces, which can be either attractive or repulsive, involve interactions both within the biomolecule and between it and the water that forms the principal component of the surrounding environment. 

The majority of trichloroethylene present on soil surfaces will volatilize to the atmosphere or leach into the subsurface. Once trichloroethylene leaches into the soil, it appears not to become chemically transformed or undergo covalent bonding with soil components. When trichloroethylene was absorbed onto kaolinite and bentonite, the nuclear magnetic resonance (NMR) spectra showed no evidence of chemical reactions (Jurkiewicz and Maciel 1995). Because trichloroethylene is a dense nonaqueous phase liquid, it can move through the imsaturated zone into the saturated zone where it can displace soil pore water (Wershaw et al. 1994). 

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