Acetylene molecules dimers

Photoexcitation of an acetylene molecule results in either dimerization or dissociation of the molecule (C2 + H2 or C2H + H). In the mass spectrometer, the major positive ions are C2H2+ (75%) and C2H+ (15%). However, at STP no gaseous products are seen under radiolysis. There are only two major products, benzene and a polymer, cuprene with an empirical formula QJH. The detailed mechanisms are still debatable. However, the following remarks may be made ... 

Depending on the substitution pattern and reaction conditions, however, dimerization may take place that is, the intermediate vinyl cation alkenylates another acetylenic molecule (self-alkenylation)305,307 [Eq. (5.115)]. 

Recent experiments by Wolkow43 and by Mezhenny and coworkers44 yielded another adsorption configuration an end-bridge structure with an acetylene molecule binding to two adjacent dimers and oriented perpendicular to the dimer rows (Figure 4a) a tetra-coordinated structure with the acetylene binding to the four Si-atoms of two adjacent... 

The THT and SMe2 adducts have structures of the type (18-B-V). Their chemistry has been extensively studied and it is summarized in Fig. 18-B-7. The diverse, and in some cases unique, reactivity of these compounds includes substitution with preservation of the geometry or with conversion to (MX4)2(/t-X)2 species, oxidative-addition,53 cluster formation, splitting of C—N bonds,54 and above all coupling of the molecules with triply bonded carbon atom.55 They catalytically trimerize and polymerize terminal acetylenes, and dimerize nitriles and isonitriles with incorporation of the new ligand into the complex. Another remarkable reaction of M2C16L3 is the metathesis of M=M and N=N bonds into two M=N bonds upon reaction with azobenzene. 

Dinuclear cobalt acetylene complexes [Co2(RCCR)(CO)6] in which the alkyne molecule constitutes a bridge are very stable. In these compounds, as in other similar complexes, acetylenes are 4e ligands. In complexes of the type [Co4(RCCR)(CO)io], the alkyne molecule is bonded to all cobalt atoms. In dimeric cobalt complexes, acetylene molecules may undergo exchange reactions ... 

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. 

The symmetry analysis is consistent with the experimental results reported by Chapman et al. [19]. Photolysis of CBD in an argon matrix produces a pair of caged acetylene molecules that diffuse apart on warming to 35°K. If the matrix is thawed without being irradiated, CBD does not fall apart but dimerizes to syn-tricyclo[4.2.0.0] octa-3,7-diene, as described in Section 7.3.1. 

The zirconacyclopropene 1, which was prepared by treatment of Cp2ZrCl2 with magnesium metal in the presence of bis(trimethylsilyl)acetylene, reacted with one molecule of C02 under atmospheric pressure at room temperature to give the dimeric zirconacycle 2 in good yield (Scheme iy6>6a>6b Further insertion of C02 did not occur, although 2 has... 

If two identical molecules combine chemically a dimer is obtained. Acetylene for instance, is dimerised to vinylacetylene. If smaller molecules of a substance unite then a large molecule, a polymer, of high Molecular weight is obtained. The individual small molecule from which a polymer is formed is called a monomer. The chemical process for the formation, of a polymer is called polymerisation. This is exemplified in the following equation. 

The first examples of what can be categorized as [2 + 2] type cycloaddition product formed by reaction between an alkene and a silicon surface were reported in the late 1980s. Alkenes such as ethylene, as well as the related alkyne molecule acetylene, were reacted with the clean Si(100)-2 x 1 surface in vacuum [196-213]. The adsorption of these unsaturated C2 molecules (ethylene and acetylene) on Si(100)-2 x 1 is also discussed in Chapter 1. The alkenes were found to chemisorb at room temperature, forming stable species that bridge-bonded across the silicon dimers on the surface. The reaction proceeded by formation of two new a bonds between Si and C atoms, hence the bonding was referred to as di-sigma bonding. In addition, it was shown that while the bonds of the alkene and of the Si—Si dimer are... 

From the ethylene results, and similar results on acetylene [306], it is evident that interdimer reactions play an important role in the chemistry of organic molecules on Ge(100)-2 x 1. The simple picture of reaction across a single Ge-Ge dimer, while capturing a number of important reaction pathways, is incomplete. Even small C2 molecules such as ethylene and acetylene can bridge across dimers along a dimer row. Other molecules are found to bridge across the wider trench. Furthermore, these studies indicate that multiple reaction products can form even for simple systems. 

The simplest hydrocarbon molecule is acetylene HC=CH, which in vacuum possesses a triple carbon carbon bond. If this molecule attaches to a clean silicon surface, it has essentially two options it can either adsorb on the tip of a silicon dimer, where the C-C bond in this case is reduced to a double bond or it can attach to two adjacent dimers, if the C-C bond is reduced to a single bond. There was some controversy, a few years ago, about the preferred adsorption site. Different methods seemed to reach a different conclusion concerning the actual adsorption geometry under different thermal conditions (for an outline of the discussion, see [57]). There were essentially two diverging opinions (i) There are only two adsorption... 

The reactions of ethylene and acetylene with Si( 100)-(2 x 1) were initially described as being [2 + 2] cycloadditions, with the di-configuration predicted by this mechanism believed to provide the dominant reaction products. A variety of alternate reaction products could actually be formed as a result of the chemisorption, with, e.g., the organic molecule spanning silicon atoms in different dimer rows, adhering above a row oriented perpendicular to the silicon dimers, or adhering above a row and parallel to the dimers. As reviewed in Sec. 3, a variety of alternate structures have now indeed been found for chemisorbed acetylene. Hence, while the [2 + 2] cycloaddition mechanism appears apt for ethylene chemisorption, its applicability to similar processes in acetylene is questionable. 

---