Influence of the crystal

Here it can be seen that the nucleation rate is a decreasing function of growth rate (and supersaturation). The physical explanation is believed to be the mechanical influence of the crystallizer on the growing suspension and/or the effect of Bujacian behavior. 

The influence of the crystal lattice on the optical spectra comes into play for the molecules considered here at two points. First, due to the polarization of the local environment the transition energy is shifted with respect to the transition energy, which is observed for the isolated molecules (solvent shift). In the case of 4T, the shift has a size of (1158 20) cm-1. 

A more complete study was then carried out in the same research group over Id-Beta zeolites.[43] In particular, the influence of the crystal size and of the hydrophobic versus hydrophilic properties of the catalysts on the initial reaction rate and product distribution was examined. 

Table 7.4 Influence of the crystal size on the initial reaction rate and on product distribution at 60 % glucose conversion...

At least with intermetallics, the effect of melting points and atomic radii on the sequence of occurrence of compound layers at the A-B interface seems to be more or less straightforward. On the contrary, the influence of the crystal structure of the compounds is rather obscure. Probably, those with less symmetrical and loosely packed structures may be expected to form first under highly non-equilibrium and stressed conditions usually encountered in reaction-diffusion experiments. 

Hush, N. S. Pryce, M. H. L. (1958) Influence of the crystal field potential on interatomic separation in salts of divalent iron-group ions. J. Chem. Phys., 28, 424-9. 

Hush, N. S. and M. II. L. Pryce Influence of the Crystal-Field Potential on Interionic Separation in Salts of Divalent Iron-Group Ions. J. Chem. Phys. 28, 244 (1958). 

The unfortunate situation is that this model interprets NQR results in terms of the intramolecular chemical bond only. The influences of the crystal field on the NQR frequencies are neglected. 

Figure 2.6-2 Variation of the frequencies by the incorporation of a tetraatomic molecule with two degenerate vibrational states ( ) in a crystal lattice, a spectrum of the free molecule, R = rotations, T = translations b static influence of the crystal lattice. The degenerate states split, the free rotations change into librations L c dynamic coupling of the vibrations of molecules within a primitive unit cell with z = 2 molecules. Each vibrational level of a molecule splits into z components and 3 z - 3 translational vibrations TS and 3 z librations L appear d dependence of the vibrational frequencies on the wave vector k of the coupled vibrations of all unit cells in the lattice. The three acoustic branches arise from the three free translations with = 0 (for k 0) of the unit cell all vibrations of the unit cells with / 0 (for k 0) give optical branches .

In the case of some ion-transfer reactions the chemical desolvation step controls the rate of the overall process and the currents observed are lower than those expected for the process limited solely by the mass transport rate. The formation of such less-hydrated species was attributed [210] in the case of the electroreduction of nick-el(II) in water to a slow exchange of water molecules from the first solvation sphere of Ni(II) under the influence of the crystal field stabilization. A similar mechanism was found for Ni(II) and Co(II) in methanol [211]. 

As mentioned above, it is difficult to separate the influence of the crystal size and of the external surface on the shape-selectivity of zeohtes rigorously. Increasing the crystal size of the zeolite means at the same time reducing the influence of the external sur ce. For the industrial production it is desirable to operate with small crystal sizes. 

The choice of a theoretical model usually depends on the goals of the study, but sometimes other considerations, such as computer resources available, can also play a role in this selection process. A study of zeolite framework acidity or catalytic activity requires an explicit consideration of the electronic structure of the system, and quantum mechanical (QM) models are best suited for such investigations. Since the high CPU demands greatly limit the size of the systems that can be simulated with quantum mechanical models, the zeolite framework is often represented in these simulations by a cluster that presumably resembles the active site. The cluster approximation has the obvious drawback that influences of the crystal lattice are neglected. 

Earlier investigations in our laboratory 56) have shown that several chain reaction polymerizations at temperatures below the melting or dissolution temperature of the final polymer lead to a thermodynamically more stable state than crystallization of the identical polymer from the polymer melt. Bawn and Ledwith mentioned in a review on stereoregular addition polymerization 57) that crystallization of the growing polymer chain might influence the polymerization step such that a more stereoregular polymer results. Ham 58—60) has finally pointed out a possible influence of the crystallization on the tacticity of the preceeding poljonerization which should only be possible when both processes are practically simultaneous. 

Muller BW. Polymorphism of magnesium stearate and the influence of the crystal structure on the lubricating behavior of excipients. Acta Pharm Suec 1981 18 7A—75. 

Table 7. Characteristics of RNi2 indicating the influence of the Crystal Field Interaction (86,104).

The shapes of dehydration nuclei and rates of interface advance at different crystal surfaces of MgS04.7H20 are different [72], Reaction rates at 323 K for several crystals of equal size, but varied habits, were compared [72] and it was shown that the rate of penetration of the reactant-product interface normal to the (111) face was considerably greater than that normal to the (110) face. This anisotropy was attributed [73] to the influence of the crystal lattice. The rate of dehydration measured [74] by microscopy from the advance of the interface agreed with that measured from mass-losses. 

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