Molecules, congested structures

Practically all dendrimers [ 1 ] known today have cores with a few, typically three to six functional groups to which the corresponding number of dendrons (dendritic wedges) are attached. The fact that these dendrons are connected to one another by a small, almost dot-like molecule results in considerable steric congestion in the space around the core. This congestion is a unique structural feature and has one led to view dendrimers, specifically those of high genera-... 

Design, synthesis and structure determination of molecules, the skeletons of which are distorted by steric congestion and/or by charge perturbation, therefore, has become an additional research project of the Frankfurt group since 1989 [2,3]. Of the altogether about 100 single crystal structures determined in the meantime, about a dozen are of silicon organic compounds [5] (cf. examples in Fig. 1). 

The complexes that can be starting points for addition of the next molecule of H2 are B3 (which we call Cl), B13(C4) and B21(C7). Because of atomic congestion, only Zr-P frozen structures were optimized for the expected TS s and products of these reactions. 

In the crystal structure of the in-isomer of the methylcyclophane shown in Fig. 14.3.7(e), there are two independent molecules with measured C-Me bond distances of 147.5(6) and 149.5(6) pm. The inward-pointing methyl group is forced into close contact with the basal aromatic ring, and the steric congestion accounts for compression of the C-Me bond. 

Byrne and Ross 5> have considered diffuseness in electronic spectra and have listed some twelve causes of line broadening. In an earlier work 6> they considered in detail a trivial explanation of the broadening, namely spectral congestion, with particular reference to the related molecules benzene, naphthalene, anthracene and tetracene. They showed that for the first three molecules spectral fine structure should be observable under the appropriate experimental conditions, but that for tetracene under practical experimental conditions no resolvable fine structure... 

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