Linear molecular shape, 260 table

In any molecule in which there are no nonbonding pairs around the central atom, the molecular shape is the same as the molecular geometry. Thus, to use the examples from Table 6.2, all three two-substituent molecules have both a linear geometry and a linear shape. Both BH3 and H2CO have a triangular planar shape, CH4 has a tetrahedral shape, PF5 a triangular bipyramidal shape, and SF6 a square bipyramidal shape. 

Inspection of Table 11.3 reveals that there are relatively small differences between the corresponding values of H and E0 for NaY and HY. This is to be expected since the adsorbent-adsorbate interactions are essentially non-specific (see Chapter 1). Decationization of zeolite Y thus has a minimal effect on the energetics of adsorption of the paraffins. The molecular shape of the adsorptive is also unimportant. In accordance with the results in Figures 1.S and 1.6, the molar mass (number of carbon atoms) is much more important than the molecular shape. As before, there is a linear relation between E0 and Nc. An exponential increase of kH with Nc is of course consistent with the form of Equation (4.3). 

The second major variable determining the hydrodynamic volume of a molecule is its shape. The radius of gyration of a spherical polymer is less than that of a linear polymer of the same molecular weight. It can be seen in Fig. 5 that linear sulfonated polystyrenes (SPSs) do not penetrate the pores as much as globular proteins of the same molecular weight. Table 1 lists the radii of gyration for molecules with varying asymmetry. As the shape of the 500,000-Da model is... 

Three of the five basic molecular shapes are linear, trigonal planar and tetrahedral. Table 4.9 shows the arrangement of the electron pairs (charge centres) that results in minimum repulsion and the basic shapes of the molecules. The two other basic shapes adopted by molecules, trigonal bipyramidal and octahedral, are discussed in Chapter 14. 

A molecule can have only one center of symmetry, which may or may not coincide with an atom. For a molecule to have a center of symmetry, all atoms, with the exception of the one which may coincide with the center, must exist in paired sets. In carbon dioxide, the C atom is the center of symmetry. Of the typical molecular shapes mentioned in Tables 19.4 and 19.5, only the linear, square-planar, and the octahedral forms, as exemplified by BeCl2, XeF4, and SFe, respectively, have centers of symmetry. 

The shape-selectivity of ZSM-5 is particularly remarkable. Active centres at the inner walls of the catalyst readily release protons to organic reactant molecules forming carbonium ions, which in turn, through loss of water and a succession of C—C forming steps, yield a mixture of hydrocarbons that is similar to gasoline. The feedstock can be methanol, ethanol, corn oil or jojoba oil. The shape-selectivity of this catalyst is particularly striking, as can be seen from the product distribution obtained for the dehydration of three different alcohols (Table 8.2). The product distribution can be understood in terms of the intermediate pore size of ZSM-5, which can accommodate linear alkanes and isoalkanes as well as monocyclic aromatic hydrocarbons smaller than 1, 3, 5-trimethyl benzene. In Table 8.3, we list some of the recent innovations in catalysis, to highlight the important place occupied by molecular-sieve catalysts. 

Each stationary phase is characterised by a calibration curve made with iso-molecular standards of known masses, M polystyrene in THF or polyoxyethylenes, pullulanes, polyethyleneglycols in water (Fig. 7.3 and Table 7.1). The curves representing log(M) as a function of the elution volume have a sigmoidal shape. However, by mixing stationary phases, the manufacturers can provide columns for which the calibration curves are almost linear for a wide range of masses. 

Figure 6.3 Ultrafiltration membranes are rated on the basis of nominal molecular weight cut-off, but the shape of the molecule to be retained has a major effect on retentivity. Linear molecules pass through a membrane, whereas globular molecules of the same molecular weight may be retained. The table shows typical results obtained with globular protein molecules and linear polydextran for the same polysulfone membrane [8]...

The branched architecture has great influence on the packing of molecular chains. In general, dendrimers have smaller hydrodynamic radius and the melt and solution viscosity of a hyperbranched polymer is expected to be lower than that of a parent linear polymer. Viscosity measurements performed with a cone viscometer confirmed the decrease of viscosity of star-shape polymers compared to the respective high molecular weight arms (polymers B-R-4 and C-R-4, Tables 1 and 2). This observation is consistent with the decrease of hydrodynamic volume observed for... 

However, when hydrocarbons with different shape are adsorbed on activated carbons the values of Vg are not related to a property depending on the molecular weight [14]. This occurs with the adsorption of n-hexane (linear), benzene and cyclohexane (cyclic), and 2,2 dimethyl butane (2,2 DMB, branched) on activated carbons obtained from olive stones (Table 1). In order to explain these results we must consider the relationship between the molecular dimension of the adsorbates and the shape and size of the pores. 

PE) correlates with molecular surface, but the slope of the linear correlation is quite different for various subsets of chemically similar molecules. Table 12.3 shows that planar aromatics with a regular shape have the highest density, packing coeffi-... 

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