Shape selectivity molecular descriptors

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

The optimized geometries of (4.55a)-(4.55d) are shown in Fig. 4.17 and selected geometrical parameters are summarized in Table 4.13. The molecular shapes and NBO descriptors (not presented) generally agree with the idealized Lewis-like sd/x picture for Os(CH2)2, HW(CH2)(CH), and W(CH2)3. However, theC—W—C angle (42°) in the ground state of W(CH)2 is much smaller than expected for idealized sd1 geometry, and the optimal NBO description corresponds to a metallacyclopropene,... 

While some concepts in chemistry that serve us well are non-numerical and will remain qualitative, others have been from the start numerical and quantitative, even if not uniquely defined. For example, molecular shape and molecular similarity are qualitative, descriptive concepts that may not be characterized uniquely by a single number, or even by a sequence of numbers. Shape at best will require numerous descriptors that may not even be easy to order, while similarity may depend to a great extent on descriptors selected for the representation of objects considered. Thus, molecules that may be similar in one aspect are not necessarily similar when some other structural or experimental feature is considered. 

As we have seen in Chapters 2 and 4, there are various possibilities to select physical functions or molecular models for the representation of molecular shapes, and in Chapter 5 we have reviewed a variety of topological methods which can be applied and lead to topological shape descriptors for their characterization. When quantifying similarity of molecular shapes by topological techniques, it is necessary to specify the following [108] ... 

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