Acetylene, structure

A useful self-terminating catalyst system (77), employs a Pd catalyst [prepared from Pd(OAc)2, NaH, and r-AmOH in THF]. The solvent required for the hydrogenation depends on the acetylene structure monosubslituted acetylenes require solvents such as hexane or octane, whereas disubstituted acetylenes need ethanol, ethanol-hydrocarbon, or ethanol-THF mixtures. In all cases it was necessary to use quinoline as a catalyst modifier. The authors consider this system one of the best for achieving both high yield and stereoselectivity. 

The authors noticed that polylithiated phenylallenes have a vibrational behaviour similar to that of polylithiated propynes and butynes (Table 8) . The monolithium compounds from terminal acetylenes were found to have acetylenic structures (2050 cm ), whereas the monolithium derivatives of non-terminal acetylenes could exist in either acetylenic (2000 cm ) or allenic (1870-1850 cm ) forms (Table 8) °. The polylithium... 

The product distribution from the two types of anions is also very different. PropargyUde anions derivatize to give acetylenic products exclusively (except when steric interactions favour the aUenic products), while allenic anions with unhindered derivatizing agents give both aUenic and acetylenic structures " " " . 

The Mg promoted reaction between perchloropropene (232) and permethyl-l,3-dichlorotrisilane (233) produces an acetylenic structure with two l,2,3-trisilacyclopent-4-ene rings (234) (Equation (19)) <93JA3111>. 

Acetylene forms spontaneously an ordered (2 X 2) surface structure on the Pt(l 11) surface at 300 K, at low exposure under ultrahigh vacuum conditions. The intensity profiles reveal that this structure is metastable, and upon heating to 350-400 K for one hour, it undergoes a transformation to a stable structure with the same (2 X 2) unit cell. Ethylene adsorbs on the Pt(l 11) surface and at 300 K, it forms an ordered (2 X 2) surface structure that is identical to the stable acetylene structure as shown by the intensity profiles. 

Data was taken in the electron energy range of 10-200 eV, but little sensitivity to the organic adsorbate is found above 100 eV. The observed diffraction pattern arises from three equivalent 120° — rotated domains of (2 X 2) unit cells. The optimum agree "ent between calculated and experimental intensity data for the metastable acetylene structure is achieved for an atop site coordination. The molecule is located at a z-distance of 2.5 A from the underlying surface platinum atom. However, the best agreement is obtained if the molecule is moved toward a triangular site, where there is a platinum atom in the second layer, by 0.25 A, as shown in Fig. 7.2. 

Polydiacetylenes are produced by the solid-state polymerization of single crystalline diacetylenes of the form RC=C—C=CR by 1,4-addition (see Fig. 32). The fully conjugated backbone of the polydiacetylenes is a model quasi one-dimensional electronic system being capable of representation by two extremes of bonding, sequence (a) an acetylene structure -fRC—C=C—CR)- and sequence (b) a butatriene structure -fRC=C=C=CR)-A,. There is great current debate as to the best method of representation but, in line with the belief that the nature of the side groups affect the bond lengths (and hence the... 

Polydiacetylenes having mainly an acetylenic structure to the backbone are semiconductors along the chain but are poorer conductors normal to the chains. 

Very few A2B2 molecules have been studied. Strausz et a/.619 have examined F2C=C in its three forms (1)—(3). It is clear that the acetylene structure (2) is much more stable, as is also the case for the C2H2 species. 

The Conduction and Valence Band of cis-transoid and trans-cisoid Polyacetylenes and of Polydiacetylene (ideal Acetylene Structure) in eV s... 

The argument in favour of the latter was that the diradical should be lower in energy, because its formation requires disruption of only one carbon-carbon rc-bond instead of two in case of carbene formation. Both ESR work and optical spectroscopy have meanwhile confirmed the diradical mechanism for growth of oligomeric chains up to length of 5 repeat units. Upon further addition of monomers, the acetylenic structure becomes energetically more stable causing a cross-over to the carbene mechanism. For further discussion of this topic the reader is referred to the article by H. Sixl in this volume. 

Fully conjugated and fully chain-aligned polymer single crystals with planar polymer backbone are obtained, which may have the alternative acetylene (ynene) or butatriene structures of Eq. (1). From our experiment we know that the acetylene structure is dominant in the polymer molecules. Up to now the best investigated diacetylene crystals are the TS-6 monomer crystals and the corresponding polymer crystals (poly TS-6). The substituents R and the notation of further diacetylene crystals discussed below are listed in Table 1. 

At the ends of the polymer chains and at the ends of the short oligomer units (see for example the trimer molecule of Table 1) a bond defect structure is expected. For the acetylene structure of the polymer chain this is a carbene —C— with two free valence electrons and in the case of the butatriene structure this is a radical carbon atom —C= with one free valence electron. In both rases there is a reactive chain end, which allows reaction of the chain with the neighbouring monomer molecules. These reactive structures and a possible nonreactive structure are listed in Table 1 as examples of the trimer molecules. 

This structure is in accordance with the optical absorption spectra, where a sequence of butatriene chain structures with increasing n was postulated as responsible for the red shift of the absorption lines A to E (see Fig. 7). The longest diradical molecule (which has been detected in the optical system) therefore is the hexamer E with n = 6. For each value of n further mesomeric butatriene and acetylene structures are present up to maximally 10% of the real structure of the diradicals. 

The dominant structure of the oligomer chain attached to the carbene end is the acetylene structure = =RC—CsC—CR= 2. As discussed above this structure is also responsible for the spectral positions of the optical absorption lines b, c, d,... shown in Fig. 7. 

The individual chain ends of the DC intermediates exhibit the same mesomeric structures (4) of the corresponding asymmetric carbenes AC and a carbon backbone with acetylene structure. It has been shown in a quantitative theory of the DC quintet states that the fine structure parameters Dq of all DC intermediates are... 

Butatriene structure (theor.) Acetylene structure (theor.)... 

Two different reaction mechanisms have been postulated for the topochemical polymerization of diacetylenes involving diradical or carbene chain ends The first mechanism leads to a butatriene structure (I), the latter to the acetylene structure (II) of polydiacetylene chains. 

Within these limits the polydiacetylene chain seems to be best represented by the acetylene structure (II). This is especially true for those polymers (PTS, HDU-1, THD) which are obtained thermally under mild conditions. In the other cases devia-... 

Resonant raman spectroscopy has proved to be another valuable tool for the study of the structure of the polydiacetylene chain. Due to the resonance enhancement the spectra are compared to greatly simplified, infrared spectra and show as principle feature only the in-plane modes of the polymer chain. The correlation of the CsC and C = C stretching modes and their temperature dependence have been interpreted as resonances between the mesomeric structures (I) and (II) i32) Hoy(rever, a model using simple anharmonic force constants for the acetylene structure (II) is in good agreement with the experiment, e.g, the temperature and pressure dependence of the vibration frequency and the mechanical properties... 

The product of the reaction of MeAuPMe2Ph with perfluorobut-2-yne shows an unsymmetrical multiplet at S -52-4, indicating that the two CFj environments are different. It was originally (183) thought to have a bridging acetylene structure, but subsequent work has shown the structure to be [332] (184) in which the two gold atoms are in different oxidation states. 

FIGURE 6.8 Photoelectron spectrum of 20 produced by laser vaporization of graphite in helium gas (spectrum reproduced from Reference 37). Inset the totally symmetric tangential stretching mode of the acetylenic structure. 

The membrane properties of the microporous polypropylene/poly-acetylene structure were measured, and a comparison with the original properties is given in Table II. 

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