Propeller-type structures

According to these simulations, the dendrimers based on core 10 adopt a false-propeller structure, while the dendrimers based on the hexaphenyl-benzene core 11 adopt a true-propeller type structure. The difference lies in... 

Carbanions of the type [HjCR ], [HCR R ] and [CR R R ] (R = NO and R R = CN, NO, NO2) can be considered to be resonance-stabilized, nonlinear pseudohalides. All experimentally known resonance-stabilized methanides are reported to be planar or nearly planar (Table 1). While the parent ion, the methanide anion HsC, adopts a pyramidal structure [Afipianar-pyramidai = 9.8 kJmol rf(CH) = 1.099 A, <(HCH) = 109.7° cf. rf(CH) = 1.093 A, <(HCH) = 109.6°] due to the lack of delocalization (no resonance for the p-AO-type lone pair possible) , substitution of one H atom by NO results in a planar anion since the empty jr -orbitals of the NO group are perfectly suitable to delocalize the carbon lone pair. Further substitution of the second H atom again results in planar anions, and the same holds for the third substitution in case of R = CN. In case of R = NO and NO2, the third substitution leads either to a propeller-type structure with only a small distortion from planarity or one NO2 group is twisted by 90°, nevertheless leaving the central carbon in an almost trigonal planar environment . ... 

A special example of chiral liquid crystals is the tilted smectic phases of bent-core molecules, which can be chiral even though the molecules do not contain any stereo-center carbons. The reason is that a bent-core tilted layer structure with polar order has a three-dimensional structure defined by the polar vector and the tilt direction, which can be illustrated either by our left or right hands of Figure 1.12. We note that the latest investigations indicate other sources of chirality of bent-core molecules, which are termed conformational chirality, due to a propeller type configuration of the two molecular legs. ... 

Propellants may be of a number of different types CFCs, hydrofluoroalkanes (HFAs), or alkanes. The composition impacts upon performance. A numerical system is employed to identify fluorinated propellants. The rules governing this numbering system allow the molecular structure to be derived from the numerical descriptor. The rules may be listed as follows ... 

Other structural concepts evolved, one of them being triptycenes (55). Here, additional wings are introduced into the molecules by Diels-Alder type reactions. The glass-forming properties are improved by the propeller-like rigid structure [89]. We end our overview of the different classes of molecular glasses with these materials and note that the number of published structures is growing every year. More details on the different materials can be found in one of the recent chemical reviews [102-104]. 

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