Large molecular complexes, phase extension

The equations and plots presented in the foregoing sections largely pertain to the diffusion of a single component followed by reaction. There are several other situations of industrial importance on which considerable information is available. They include biomolecular reactions in which the diffusion-reaction problem must be extended to two molecular species, reactions in the liquid phase, reactions in zeolites, reactions in immobilized catalysts, and extension to complex reactions (see Aris, 1975 Doraiswamy, 2001). Several factors influence the effectiveness factor, such as pore shape and constriction, particle size distribution, micro-macro pore structure, flow regime (bulk or Knudsen), transverse diffusion, gross external surface area of catalyst (as distinct from the total pore area), and volume change upon reaction. Table 11.8 lists the major effects of all these situations and factors. 

A large number of rare-earth adducts have been synthesized and characterized (see Gmelin Handbook of Inorganic Chemistry, Part D4, 1986 for a general review or Cunha et al. (1992) for a review on europium trigonal complexes). However, the molecular structure of only a few compounds has been established by means of single-crystal X-ray diffraction. In table 16, we compare the coordination numbers in the solid state and in solution for the solvates for which extensive data are at hand, that is, adducts with DMSO. Solid solvates R(N03)3 nDMSO display a CN change from 10 to 9 between Eu and Tb. X-ray diffraction data for 1M solutions of Er(N03)3 in DMSO yield the same coordination number (CN=9) and similar distances as in the crystalline phase (Johansson et al. 1991). 

Older soft ionization methods that are related to MALDl—such as fast atom bombardment (FAB), hquid secondary ion mass spectrometry (LSIMS), or field desorption (FD)— in principle have potential for the generation of intact gas-phase ions from noncovalent complexes. In particular, the extensive clustering often found in FAB-MS, which is generally thought of as a nuisance, can be viewed as evidence for this. However, the limitation of these methods lies in their inabiUty to ionize very large molecules. It is generally very difficult or impossible to obtain useful mass spectra from compounds with molecular weights above a few thousand daltons. For this reason, FAB-MS, LSIMS, and FD-MS only play a very minor role in this field. 

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