Fragment concepts building blocks

It is of vital interest for a wider applicability of CTCB to examine how these two mechanisms can be accommodated in OCT. In Section 3, we shall argue that the mutual decoupling status of several subsets of basis functions, manifesting itself by the absence of any external communications (bond orders) in the whole system, calls for the separate unit normalization of its input probabilities since such fragments constitute the mutually nonbonded (closed) building blocks of the molecular electronic structure. It will be demonstrated, using simple hydrides as an illustrative example, that the fulfillment of this requirement dramatically improves the agreement with the accepted chemical intuition and the alternative bond multiplicity concepts formulated in the MO theory. 

There are other cases, where the idea of transferable submolecular building blocks is exploited. The concept of the chromophore in spectroscopy reflects the empirical observation that the appearance of certain characteristic absorption bands always indicate the presence of well-defined fragments, like carbonyl groups or aromatic rings, in the molecule [15]. Spectroscopic measurements in larger molecules or in extended systems, e.g. molecular crystals [16] or biopolymers can be conveniently interpreted by the concept of chromophores, while the effect of the chemical environment involves a relatively small perturbation. The optical absorption [15] and circular dichroism [17] spectra of even small and medium sized molecules can also be systematized with respect to the characteristic properties of the chromophores. 

Another novel extension of asymmetric organocatalysis was reported by Hintermann and Schmitz towards the successful development of an organo-catalytic enantioselective double-bond isomerisation, which has been previously associated with the field of metal catalysis. Therefore, an asymmetric synthesis of the 2,5-diphenylphosphol-2-ene fragment was achieved via the enantioselective cinchonine-catalysed double-bond isomerisation of a wc50-2,5-diphenyl-phosphol-3-ene amide into a 2,5-diphenylphosphol-2-ene amide with an enantioselectivity of up to 83% ee (Scheme 10.10). This new asymmetric concept opened the way to a catalytic enantioselective synthesis of 2,5-diarylphospho-lane building blocks for many applications in transition metal catalysis. 

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