Steric/size exclusion model

ISEC is a size-exclusion chromatography technique, in which the stationary phase is the CFP to be to characterized [16-18] and the eluates are geometrically well-defined steric probes. From the determined retention volumes in a given solvent and on the basis of suitable morphological models, ISEC analysis provides the... 

Before investigating the qualitative concepts of the VSEPR model it is worth noting that the details of the interactions between the electron pairs have been ascribed to a size-Pauli exclusion principle result . But objects do not repel each other simply because of their sizes (i.e. interpenetrations) only if the constituents of the objects interact is any interaction possible10). If we are to use the idea of orbital size at all we must avoid the danger of contrasting a phenomenon (electron repulsion) with one of its manifestations (steric effects). The only quantitative tests which we can apply to the VSEPR model are ones based on the terms in the molecular Hamiltonian specifically, electron repulsion. 

Concerning the microscopic origin of the local off-centre instabilities, most common explanations are based on steric effects (differences of ionic radii and/or electrostatic effects) [94,100,110,112,178] but it is not satisfactory because there are many exceptions to this rule. For example, it can be observed in Table 1 that Li+ remains on-centre in NaCl, NaBr, KBr and RbCl lattices despite the smaller size of the impurity with respect to the substituted cation. Qualitative PIT models have been proposed by several authors [20,110-113] considering the admixture of the ground state with different excited states. It should also be noted that a model based exclusively on size properties would always provide the same kind of distortion, i.e. in the < 111 > direction and could not be able to explain that Li+ goes in < 111 > or <110> directions. We believe that this can only be explained taking into account the chemical bond like in PIT models. 

Gel-type, microporous, resins must swell to expose their catalytically active sites, whereas macroreticular resins have a permanent pore structure (inside these pores, catalytically active sites reside). Pores of the macroreticular resins can be described acceptably in terms of the conventional cylindrical pore model (pore diameter and volume). Pore structure, size, pore volume, and so on have been studied intensively in recent years. Examples of analytical techniques include X-ray microprobe analysis, ESR spectroscopy, NMR, and inverse steric exclusion chromatography (ISEC) the latter yields the best quantitative assessment of the nanomorphology of swollen resins. 

The above simple model of a steric exclusion mechanism was considered by several authors attempting to describe quantitatively the gel chromatographic separation process. Distribution coefficients were expressed on the basis of the model considerations of the dimensions of both the separated molecules and the pores of gel, as well as of the stochastic model approaches (for reviews see e.g.. Refs. 1, 3-6), and also of the thermodynamic reasoning on the changes of conformational entropy of macromolecules due to their transfer from the interstitial volume into the pores in the course of separation [7]. However, besides the steric exclusion from the pores, at least two other size-based mechanisms are operative in the ideal gel chromatography ... 

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