Acetylene molecular shape

Prasad (1988) further demonstrated that 7 increases remarkably with the increase of the molecular length n. The relation of 7 and n of the para-acetylene (C H +2) is shown in Figure 6.35 in which 7 increases exponentially with n. Therefore, the long molecular shape and the linear conjugated system are still the essential requirements for a high x 3" -Apparently, liquid crystals are a good candidate. 

As an example refer to Fig. 1.34, the vibrations of acetylene. In vibrations 1 and 2 the molecular shape is different at the vibrational extremes illustrated so the polarizability ellipsoid will change in size or shape. In vibration 3 the shape is the same at the vibrational extremes but the polarizability ellipsoid will be changed in orientation because the effect of the rotation of the C=C bond is not compensated for by the counterrotation effect of the two CH bonds which are not symmetrically equivalent to the C=C bond. Vibrations 1, 2, and 3 are Raman active. Vibrations 4 and 5 are Raman inactive. The polarizability ellipsoid in 4 and 5 cannot change in size or shape because the vibrational extremes have the same shape. The ellipsoid cannot rotate because in vibration 5 the effect of the rotation of one CH bond is nullified by the counter-rotation of the other equivalent CH bond. 

Abstract The past two decades have profoundly changed the view that we have of elemental carbon. The discovery of the fullerenes, spherically-shaped carbon molecules, has permanently altered the dogma that carbon can only exist in its two stable natural allotropes, graphite and diamond. The preparation of molecular and polymeric acetylenic carbon allotropes, as well as carbon-rich nanometer-sized structures, has opened up new avenues in fundamental and technological research at the interface of chemistry and the materials sciences. This article outlines some fascinating perspectives for the organic synthesis of carbon allotropes and their chemistry. Cyclo[n]carbons are the first rationally designed molecular carbon allotropes, and... 

The thermal decomposition of 7, 8 and 9 into fullerenic substructures is a milestone in fullerene formation and represents the first example of a macroscopic preparation of closed-shell carbon particles from acetylenic precursors. However, molecular allotropes of carbon, such as Cgo or higher fullerenes were not found among the decomposition products. It is interesting to note in this context that 10 [24], a structural isomer of 7 with a saddle-shaped solid state conformation, also shows thermal transformations, but in this case they occur at temperatures ca. 50 °C lower than those of 7 and are accompanied by a release of 50 kj mol-1 more energy. Although an insoluble carbonaceous material is formed during this process, further details of its nature are currently not known. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

Normally, a 10-valence electron system such as acetylene, HCCH, possesses a linear structure, which can be rationalized by the shape of its bonding a-MOs, namely the C,C bonding (2a,) MO and the two C,H bonding MOs (2a and 3bonding overlap of 2pa(C) orbitals, which means that bending of acetylene leads to a decrease of C,H bonding and, hence, to an increase of the molecular energy. 

The second approach to a more tractable polyacetylene is to sidestep the compromises described above by developing multistep syntheses which proceed via a soluble precursor polymer [31, 48-50]. Although the final product is still insoluble, the solubility of the precursor allows both the determination of properties, such as molecular weight, as well as more facile manipulation of the polymer into a desired macroscopic shape. In addition, these routes also provide polyacetylene films that differ in microscopic morphology from those synthesized directly from acetylene. However, these advantages come with added complexity not only do these routes require a reaction, frequently carried out in the solid state, to go to completion, but there is also usually the need to remove an eliminated by-product from the solid product. 

Molecular packing in the low-temperature orthorhombic phase of perdeutero-acetylene shows a layer structure with a T-shaped interaction between pairs of molecules within a layer. The D—C distances form an isosceles triangle with edges of 2.738, 2.737 A and a D to C=C midpoint distance of 2.672 A. The D—C contacts are some 0.2 A shorter than those expected from van der Waals radii considerations. The geometrical arrangement clearly implies a donor-acceptor interaction between D and the triple-bond TT-electrons, a situation described as long ago as 1972 to account for close C=C—I interactions in crystals of diiodo-acetylene. More recently five O—H—C C and five NH—C=C TT-interactions with mean H—C distances of 2.69 (for O—H) and 2.61 (for N—H) have been located in the CSD. 

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