The Site Effect

From the viewpoint of the crystal symmetry, the crystal field effect may be divided into two parts  

we do not know the physical reason why there may be more than one asymmetric unit in the unit cell, nor do we have definite answer to the question When is the symmetry of the free molecule reduced by the condensation into a solid and when is this symmetry preserved Typical examples are the substituted 2,4,6-trichloro-l-X-benzenes. The symmetry of the gaseous molecule is preserved in the solid in so far as the Cl atoms in positions 2 and 6 are crystallographically equivalent for X = N02 and X = Br. Sites 2 and 6 of the molecule, however, are inequivalent in the solid for X = I, Cl, OH, and all other compounds for which the NQR spectrum is known. In the vast majority of molecules, the symmetry is reduced by condensation. We shall take the frequency splitting of NQR resonances due to this breakdown of symmetry as a qualitative measure of the crystal field effect. 

We have already considered the multiplicity of asymmetric units and shall now discuss the positioning of chemically equivalent atoms at different point positions. Table III.4 shows the splitting of the 3SC1 NQR frequencies at 77 °K in benzene derivatives 126 . Only substances with one asymmetric unit in the unit cell are considered. One recognizes the spread of the frequencies for chemically equivalent chlorine atoms. The crystal field effect Av is within the limit of Av % 500 kHz. 

In Fig. III. 1 a distribution function of Av (35C1) is drawn for a number of aromatic compounds. A mean crystal field effect of about 300 kHz shows up in this graph (more than one asymmetric unit, different point positions within one asymmetric unit, and different modifications are considered). 

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For any given catalytic reaction the active surface area is normally only a small fraction of the area of the active component (active phase). The term active sites is often applied to the sites effective for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. There is evidence that the centres required for some catalytic reactions are composed of a collection of several metal atoms (ensemble). This appears to be the case for such reactions as, for example, hydrogenolysis, hydrogenation of CO, and certain deuterium-exchange processes with hydrocarbons. 

In the solid state, V3 and V4 may split into two or three bands because of the site effect. In some cases, the MX4 ions are distorted to a flattened tetrahedron (D2 ) or a structure of lower symmetry (C ) [892,912]. According to an X-ray analysis [913] the unit cell of [(CH3)2CHNH3]2 [CUCI4] contains two square-planar and four distorted tetrahedral [CuCU4] ions. 

Acceleration FAS are often analyzed to decompose the observed ground motion into the spectral functions describing the source of the event, the propagation from the source to the site, and the site effect (Castro et al. 1990 Bindi et al. 2009). Acceleration FAS are also used to estimate the empirical site response, through the Standard Spectral Ratio (SSR Tucker and King 1984), based on the ratio between the FAS between the horizontal (or vertical) components of the selected site and a reference site or the Horizontal to Vertical Spectral Ratio (HVSR Lermo and Chavez-Garcia 1993) based on the ratio of the FAS between the horizontal and vertical component of a single station, that provides reliable estimates of the fundamental frequency when the site effects are mainly controlled by resonance phenomena. 

Broadening of spots can result from themial diffuse scattering and island fomiation, among other causes. The themial effects arise from the disorder in atomic positions as they vibrate around their equilibrium sites the sites themselves may be perfectly crystalline. 

Suppose now that the sites are not independent, but that addition of a second (and subsequent) ligand next to a previously bound one (characterized by an equilibrium constant K ) is easier than the addition of the first ligand. In the case of a linear receptor B, the problem is fonnally equivalent to the one-dimensional Ising model of ferromagnetism, and neglecting end effects, one has [M] ... 

LEER suffer from an artificial separation of a molecule into skeleton, reaction site, and substituent. The physicochemical effects mentioned in section 3.4.1 and the methods presented in section 7.1 for their calculation consider a molecule as a... 

The contribution of an atomj to the polarizability effect is attenuated by the number of bonds, H , between this atom and the site of protonation, i. 

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

Inductive effects depend on the electronegativity of the substituent and the num ber of bonds between it and the affected site As the number of bonds increases the inductive effect decreases... 

When a benzene ring bears two or more substituents both its reactivity and the site of further substitution can usually be predicted from the cumulative effects of its substituents In the simplest cases all the available sites are equivalent and substitution at any one of them gives the same product... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

Pharmaceutical powder aerosols have more stringent requirements placed upon the formulation regarding moisture, particle size, and the valve. For metered-dose inhalers, the dispensed product must be deflvered as a spray having a relatively small (3—6 -lm) particle size so that the particles can be deposited at the proper site in the respiratory system. On the other hand, topical powders must be formulated to minimize the number of particles in the 3—6-p.m range because of the adverse effects on the body if these materials are accidently inhaled. 

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