Planar principle

L. Botz, Sz. Nyiredy and O. Shelter, The principles of long distance OPLC, a new multi-layer development technique , 7. Planar Chromatogr. 3 352-354 (1990). 

The results described thus far sketch the synthetic demands for being able to prepare processable, structurally defined PPPs, in which the 7r-conjugalion remains fully intact or is even increased as compared to that of the parent PPP 1 system. The key step in the realization of this principle is the preparation of a PPP in which the aromatic subunits can be obtained in a planar or only slightly twisted conformation in spite of the introduction of substituents. 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

In principle, an accurate calculation of these barriers should require full geometry optimization for the planar molecule. This is no easy task, however, and the result is very... 

Suggest the form that the orbital energy-level diagram would take for a square planar complex with the ligands in the xy plane, and discuss how the building-up principle applies. Hint The d -orbital has more electron density in the xy plane than the dzx- or d -orbitals but less than the dXJ,-orbital. 

For the still-unsolved, hexagonal structure type of GdSI, only the cell dimensions are presented. The structures were described in detail by Dagron and Thevet (.96). Their common building-principle is a layer structure formed by planar layers [LS] (L = metal), separated by a double layer of bromine or iodine. The plane layers [LS] eu e formed by the juxtaposition of metal tetrahedra enclosing sulfur, [L4S]. These layers exhibit two different synunetries. 

Although rings of four carbons and larger are not generally planar (see p. 177), they will be treated as such in this section, since the correct number of isomers can be determined when this is done " and the principles are easier to visualize (see p. 173). 

Figure 4.28 shows an example where STM recognizes the individual metal atoms in an alloy, thus revealing highly important structural information on the atomic level. The technique does not require a vacuum, and can in principle be applied under in situ conditions (even in liquids). Unfortunately, STM only works on well-defined, planar, and conducting surfaces such as metals and semiconductors, and not on oxide-supported catalysts. For the latter surfaces, atomic force microscopy offers better perspectives. 

In principle, TPD can also be applied to high-surface area catalysts in plug-flow reactors. Often, however, the curves are seriously broadened by mass-transport phenomena. Hence, the use of single crystals or particles on planar supports offers great advantages for these investigations. 

FIGURE 6.14 (a) Principle of SBCD, elution with five interstitial volumes on 4-cm distance (5x4 cm) is faster than single development on 20-cm distance (thick line), (b) Rp values of sample components plotted as a function of modifier concentration. Optimal concentration (Y) for SBCD (5x4 cm) is lower than for development on the full distance of 20 cm (X). (Modified from Soczewinski, E., Chromatographic Methods Planar Chromatography, Vol. 1, Ed., Kaiser R.E., Dr. Alfred Huetig Verlag, Heidelberg, Basel, New York, 1986, pp. 79-117.)... 

Principles and Characteristics High-performance thin-layer chromatography (HPTLC), also known as planar chromatography, is an analytical technique with separation power and reproducibility superior to conventional TLC, which was first used in 1938 [7] and modified in 1958 [8]. HPTLC is based on the use of precoated TLC plates with small particle sizes (3-5 xm) and precise instruments for each step of the chromatographic process. 

Principles and Characteristics Many of the planar chromatography methods rely on fluorescence detection to achieve the required identification limits exploitation of sensitive and selective derivatisation reactions is of considerable importance. Most TLC scanning densitometers can be operated in the fluorescence mode and are able to record in situ excitation spectra of TLC... 

Principles and Characteristics Multidimensionality in planar chromatography takes a broader context than is common for column separations. For TLC, various modes of multidimensional separations are used ... 

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