Building blocks, permutations

Thus, as we did for the allyl radical case, here too the bonding characteristics of the two covalent states of benzene can be deduced from the respective wave functions. As discussed in Chapter 5, each Kekule structure can be generated by a product of the corresponding bond wave functions, each having a spin factor a(l)(3(2)- 3(l)a(2). Each Kekule structure possesses the two spin alternant QC determinants, which are related to each other by a cyclic permutation of the spins over the ring, and are shown in Fig. 7.4d. To illustrate clearly the building blocks of the two Kekule structures, we express them as a sum of all the permutations of the QC determinants, as follows ... 

Figure 1 The "split-mix synthesis" method to generate a one-bead one-compound combinatorial library (a) and a number of permutations for random peptide libraries (b). P, , and T are building blocks (in this case amino acids).

This simple sketch shows that the type and the permutability of building blocks A, B and C and the assembly strategy play an equally important role in determining the number and the nature of the compounds that can be obtained. It is striking to note that the p>ermutability of building blocks A, B and C (compare Figure 1.3.5 charts b) and c)) not only increases the number of final compounds from 27 to 162 but maybe even more importantly increases the number of core... 

DCC should be an efficient approach to the generation of sophisticated molecular and supramolecular systems-diversity and complexity are obtained not through complicated synthesis, but through different combinations and permutation of simple building blocks linked by reversible reactions. 

Alkyd resins first found commercial application over 50 years ago, but, even with the wide array of other polymers that have appeared in more recent years, they still rank as the most important coating resin. The fundamental building blocks of alkyds are oils or fatty acids, dibasic acids and polyhydric alcohols. Although they are few in number, the permutation and combination possibilities of only these three basic components are enormous and there is no resin-forming reaction which lends itself to greater useful variation and modification than the formation of alkyd resins. 

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