Molecular periphery

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

Fig. 3.2 Molecular periphery on conclusion of a) divergent and b) convergent dendrimer growth (schematic). The step shown on the right generates the final periphery in each case...

The following sections are devoted primarily to the introduction of two different types of functional units. A distinction is made between dendrimers with bi-functionalised molecular periphery (Fig. 3.7 Types A, B, C, D) and those in which one function is located in the core and the other in the branching units or in the periphery (Types E, F). Multifunctional dendrimers of type G with different functional units in the core, scaffold, and periphery have so far played only a minor role and will therefore only be treated briefly here, particularly since compounds of this type will be considered in greater depth in Chapter 6. 

Bifunctionalised dendrimer with perfectly alternating molecular periphery. 

Alternatively, dendrimers with chirality in the molecular periphery can also be constructed by the convergent approach starting from chiral monomeric units. 

Fig. 4.75 POPAM dendrimer with chiral Boc-protected amino acids in the molecular periphery (according to Meijer et ai.)...

Poly(phenyl ester) dendrimers with six terminal platinum chelate complex units were used as highly selective sensors for sulphur dioxide, considered partly responsibly for the formation of smog and acid rain. In the presence of hydrogen peroxide or sunlight and ozone, S02 is oxidised to S03. The latter reacts spontaneously with water to form sulphuric acid. That is the reason why detection of minimal concentrations of sulphur dioxide with appropriate sensors is necessary. The arylplatinum(II) complexes attached to the molecular periphery of the dendrimers absorb sulphur dioxide with formation of a pentacoordi-... 

A different approach to mention here because it has some similarity to QM/MM is called RISM-SCF [5], It is based on a QM description of the solute, and makes use of some expressions of the integral equation of liquids (a physical approach that for reasons of space we cannot present here) to obtain in a simpler way the information encoded in the solvent distribution function used by MM and QM/MM methods. Both RISM-SCF and QM/MM use this information to define an effective Hamiltonian for the solute and both proceed step by step in improving the description of the solute electronic distribution and solvent distribution function, which in both methods are two coupled quantities. There is in this book a contribution by Sato dedicated to RISM-SCF to which the reader is referred. Sato also includes a mention of the 3D-RISM approach [6] which introduces important features in the physics of the model. In fact the simulation-based methods we have thus far mentioned use a spherically averaged radial distribution function, p(r) instead of a full position dependent function p(r) expression. For molecules of irregular shape and with groups of different polarity on the molecular periphery the examination of the averaged p(r) may lead to erroneous conclusions which have to be corrected in some way [7], The 3D version we have mentioned partly eliminates these artifacts. 

Figure 1.12 The arylsulfonylhydroxamate scaffold (top) served as template structure for compounds active on various members of the matrix metallo-protease family, as well as of phosphodiesterase 4. By variation of the molecular periphery (R-groups, X top), discriminating compounds (bottom left, bottom middle), as well as dual inhibitors (bottom right) were obtained [61, 62].

Starting from a selected /th atom on the molecule periphery, the vertex degrees 6 of the periphery atoms define an ordered code for the molecule two different codes are obtained whether the clockwise or the anticlockwise direction is chosen. These codes, referring to the / th atom, are then transformed by making partial sums, adding elements of the series successively the two codes obtained encode information on the asymmetry of the molecular periphery. 

In this way, different types of profiOies can be derived, such as shape profiles, which are local profiles taking into account only atoms on the molecular periphery ... 

Several shape descriptors are defined within more general approaches to - molecular descriptors. This is the case of - Kier shape descriptors, -> shape profiles, -> shadow indices, -> WHIM shape descriptors, - Sterimol shape parameters L/Bj and B1/B5, molecular - periphery codes, and -> centric indices. Other approaches to the study of molecular surface and shape are Mezey 3D shape analysis and Hopfinger - molecular shape analysis. -> Triangular descriptors have also been used to characterize molecular shape to search for similarities among molecules. 

The third route has been called the broken fenestrane route since it involves two broken fenestranes , viz. di-/ so-triindanes 5 and 120 (Scheme 52), instead of the respective intact fenestranes 9 and 244, as crucial intermediates. In this case, the final step consists of a two-fold Friedel-Crafts reaction of dibro-mocentropentaindane 265 incorporating a single solvent molecule. In total, the broken fenestrane route comprises seven steps from 1,3-indanedione and furnishes centrohexaindane 11 in 40% overall yield [102,105]. Pursuing these three routes, and maybe a further one [104] to 11, should make available a variety of centrohexaindanes with different substitent patterns at the molecular periphery. 

Here = 2 Rk./n is the average row sum of the matrix If only atoms at the molecular periphery are included in the constrnction of the matrices, one obtains the sequence... 

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