Effect of: covalency

Parallel molecular dynamics codes are distinguished by their methods of dividing the force evaluation workload among the processors (or nodes). The force evaluation is naturally divided into bonded terms, approximating the effects of covalent bonds and involving up to four nearby atoms, and pairwise nonbonded terms, which account for the electrostatic, dispersive, and electronic repulsion interactions between atoms that are not covalently bonded. The nonbonded forces involve interactions between all pairs of particles in the system and hence require time proportional to the square of the number of atoms. Even when neglected outside of a cutoff, nonbonded force evaluations represent the vast majority of work involved in a molecular dynamics simulation. 

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

To examine the effect of covalency on the QS, the values of the QS of some compounds have been calculated with the aid of the Extended Hiickel MO method (i 82, 61). In these calculations the values for the empirical parameters used were obtained from comparison with EPR experiments (see EPR studies). This method is suitable only for molecules with low symmetry, because effectsoof spin-orbit coupling and thermal mixing have been neglected. 

In the REC model, the ligand is modelled through an effective point charge situated in the axis described by the lanthanide-coordinated atom axis, at a distance R, which is smaller than the real metal-ligand distance (Figure 2.6). To account for the effect of covalent electron sharing, a radial displacement vector (Dr) is defined, in which the polar coordinate R is varied. At the same time, the charge value (q) is scanned in order to achieve the minimum deviation between calculated and experimental data, whereas 9 and cp remain constant. 

Abuchowski A, McCoy JR, Palczuk NC, van Es T, Davis FF (1977) Effect of covalent attachment of polyethylene glycol on immunogenicity and circulating life of bovine liver catalase. J Biol Chem 252 3582-3586... 

One can visualize the effects of covalence on magnetic properties using a simple molecular orbital scheme. In the usual notation the orbitals considered for the transition metal M and the ligand X are ... 

The effects of covalently attaching a sulfhydryl-specific reagent to an accessible cysteine are due to local steric or electrostatic effects in the binding site and not due to nonlocal effects. 

Nafion " with intercalated heterocycles. Conductivities of >10 S/cm are easily accessible by such systems however, the volatility of the heterocycles requires some immobilization. The effect of covalent immobilization on proton conductivity is shown in Figure 23 for imidazole-based... 

The rf-orbital peaks near the iron atom now occur in the direction of the pyrrole-nitrogen ligand atoms, and thus lie along the bonds, indicating the effect of covalence (cr-donation) superimposed on the cylindrical distribution of the d6... 

The effect of covalently bonding the shell surface to the matrix was studied (Sue et al., 1996a). It is possible to introduce glycidyl methacrylate (GMA) in the last step of the CSR synthesis. GMA... 

However, in sulphides and related minerals, the effects of covalent bonding predominate and orbital overlap must be taken into account. Thus, concepts of molecular orbital theory are described in chapter 11 and applied to aspects of the sulfide mineralogy of transition elements. Examples of computed energy diagrams for molecular clusters are also presented in chapter 11. There, it is noted that the fundamental 3d orbital energy splitting parameter of crystal field theory, A, receives a similar interpretation in the molecular orbital theory. 

The failure of the Goldschmidt Rules in other cases, such as accounting for the geochemical behaviour of zinc, was attributed to effects of covalent bonding (Fyfe, 1951, 1954). The rules are stated in terms of ionic radius and... 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

The simplest model for a solvent is one of a continuous dielectric which effectively reduces charge-charge interactions but does not influence energies due to covalence. No matter how naive this model may be it stresses a very simple point. As the dielectric constant increases and ionic energies diminish then the effect of covalence in bringing about association becomes dominant. Consider the equilibrium... 

Proteolytic digestion is commonly used to overcome the effects of covalent cross-links that are formed in tissues during formalin fixation. Controlled proteolysis can improve the penetration of reagents into the tissue structures and restore the immunodominant conformation... 

The results obtained in these studies demonstrate dramatically the effects of covalent hydrates on reactivity. For direct bromination of 10.19, a rate coefficient of k2 5 M 1 sec-1 is indicated, but by virtue of it reacting as its covalent hydrate, it has an apparent value of 2.9 x 103, an enhancement of >>580 times. The enhancement is even greater (>> 104) for the 2-isomer (10.12, R = H), which exhibits a greater degree of covalent hydration (—0.05%). 

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