Fragment properties

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

The simplest representation of a molecule, with respect to computing physicochemical properties, is to assume the property to be the sum of the property values of the individual constituent atoms, or groups of atoms. Extensive data bases (1,2) of atomic and group (fragment) property values have been compiled to facilitate implementation of this model. The most notable physicochemical properties employed in QSARs using an additive property model are ... 

Steric descriptors account for both the size and shape of molecules and substituents, and are thus contemporarily related to -> size descriptors and - shape descriptors. However, in several QSAR models, size descriptors are encountered as estimates of steric molecular/fragment properties. 

Diverse sets of building blocks do not lead necessarily to an optimized diversity of the resulting library. The properties of the assembled final molecules are not a simple sum of the fragment properties. Especially physico-chemical constants (e.g., lipophi-licity, basicity, etc.) depend very much on interactions between structural fragments. It has been shown that assessing the diversity of the enumerated library leads to significantly more diverse libraries than from assessing merely the diversity of the reactants [129, 130]. 

The iipids are immediateiy converted to TMS or BDMS ether derivatives (see Chapter 4 for detaiis), the iatter now often being preferred because of their greater chemicai stabiiity and good MS fragmentation properties, in some of the eariywork [529], acetate... 

One further type of derivative of sphingoid bases for GC-MS purposes is worthy of note, i.e. cyclic boronates [277]. Usually, it is necessary to protect the amine group, but a b/s-boronate derivative is formed from trihydroxy bases. Model compounds, prepared from standards, had excellent GC properties and also exhibited distinctive fragmentation properties on mass spectrometry. For example, they gave molecular ions in reasonable abundance. Unfortunately, they do not appear to have been applied to the analysis of natural samples. 

M. T. Barakat and P. M. Dean, J. Comput.-Aided Mol. Design, 9, 341 (1995). The Atom Assignment Problem in Automated De Novo Design. 1. Transferabilitiy of Molecular Fragment Properties. 

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