Shielding phenomenon

The 13C NMR spectmm of the cation 581 shows substantial shielding of both the C(7) cationic and vinylic carbon chemical shifts (813C 34.0 and 125.9, respectively).1011 A similar shielding phenomenon is observed for ion 582. Interestingly, ion 582... 

The calculations of external exposure for works on piers, vessels, platforms, etc. were performed with due regard for the natmal shielding phenomenon using the following IAEA s shielding factors 0.2 for gamma-exposme and 0 for beta-particles [7]. 

The diffusion boundary layer of any given drop in chain interacts with the diffusion wake of the previous drop (located upstream). The concentration field in it is substantially nonuniform and is depleted because of the absorption of the solute at the surfaces of all preceding drops. By virtue of such interaction, the inner mass exchange will be appreciably retarded (the shielding phenomenon) compared with the case of isolated drops. 

Clearly, the smaller is the gold or rubin number, the more effective is the biopolymer at shielding the dispersion from coagulation by electrolytes. To describe this shielding phenomenon, Zsigmondy coined the term protective colloid ( Schutzkolloide ), which functions by protective action. From the results recorded in Table 2.1, it can be inferr that sodium casemate is considerably more effective in its protective action than is, say, dextrin or potato starch. Note that both of these latter polymers are nonionic in character. 

Figure 1.57 Schematic representation ofEMI shielding phenomenon and intrinsic/ extrinsic parameters that govern theoretical SEj, SE, and SE .

Crack Deflection Contribution to Toughening. Crack deflection is a phenomenon that leads both to toughening and to the formation of bridges that shield the crack tip from the appHed stress. Little is known of the bridge formation process, but its effect, that is, crack-tip shielding, is considered in the following section. 

Exact solutions to the electronic Schrodinger equation are not possible for many-electron atoms, but atomic HF calculations have been done both numerically and within the LCAO model. In approximate work, and for molecular applications, it is desirable to use basis functions that are simple in form. A polyelectron atom is quite different from a one-electron atom because of the phenomenon of shielding", for a particular electron, the other electrons partially screen the effect of the positively charged nucleus. Both Zener (1930) and Slater (1930) used very simple hydrogen-like orbitals of the form... 

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

All calorimeters are composed of an inner vessel (the calorimeter vessel, A in Fig. 1), in which the thermal phenomenon under study is produced, and of a surrounding medium (shields, thermostat, etc., B in Fig. 1). Depending upon the intensity of the heat exchange between the inner vessel and its surroundings, three main types of calorimeters may be distinguished theoretically as indicated in Fig. 1. 

All CCD chips have a leakage current, also called dark current, which is of thermal origin as in photomultipliers. As a result of this phenomenon, a charge slowly builds up in each pixel even when the detector is shielded from light. 

The blocking of the pores of the filter medium by particles is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media, and partly because the lines of movement of the particles are not well defined. At the start of filtration, the manner in which the cake forms will lie between two extremes — the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. Heertjes(11) considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked, so that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited. 

A similar phenomenon was postulated by Thomas and Pellon (10) to account for data obtained in the kinetics of acrylonitrile polymerization. They felt that it was possible to obtain unimolecular chain termination by a process of burial. This was conceived as a mechanism by which the growing chain became shielded from further growth by coiling or by embedding itself in the solid phase. At room temperatures we feel the... 

---