Hydrocarbon molecules acetylene

Hydrocarbon molecules are abundant constituents of planetary atmospheres and major compounds in combustible gas mixtures and in fusion edge plasmas [7-11]. Methane is the simplest of these hydrocarbon molecules. Acetylene, C2H2, is the simplest hydrocarbon molecule that contains 2 carbon atoms. Thus absolute total and partial photon [24-27] and electron [15,28-34] ionization cross-sections and nascent fragment ion energy distributions [19,20,28,36-40] have been studied extensively for these molecules. For the deuterated methane molecule electron impact ionization and dissociative ionization cross-sections were determined for the CD (x=l—4) molecule and radicals applying a fast neutral beam technique [41]. Electron impact total ionization cross-sections have been determined also theoretically applying the BEB (Binary-Encounter-Bethe) model [42], the DM (Deutsch-Mark) method [43] and the JK (Jain-Khare) method [44], Partial electron impact ionization cross-sections were calculated for methane [45,46] as well as total electron impact cross-sections for various CH radicals [47]. The dissocia-... 

Nowadays silenes are well-known intermediates. A number of studies have been carried out to obtain more complex molecules having Si=C double bonds. Thus, an attempt has been made to generate and stabilize in a matrix 1,1-dimethyl-l-silabuta-l,3-diene [125], which can be formed as a primary product of pyrolysis of diallyldimethylsilane [126] (Korolev et al., 1985). However, when thermolysis was carried out at 750-800°C the absorptions of only two stable molecules, propene and 1,1-dimethylsilacyclobut-2-ene [127], were observed in the matrix IR spectra of the reaction products. At temperatures above 800°C both silane [126] and silacyclobutene [127] gave low-molecular hydrocarbons, methane, acetylene, ethylene and methylacetylene. A comparison of relative intensities of the IR... 

A technique that allows rapid evaluation of molecular stability using small (20-30 mg) samples has been demonstrated and applied to three different families of strained molecules. All of the molecules studied are stable at room temperature, though most must be stored in nonmetallic containers to avoid catalytic decomposition. Of the four molecules shown in Fig. 4.1, the least thermally stable was quadricyclane, for which decomposition lifetimes drop below 10 ms at about 500 K. The other three molecules had similar stabilities, with lifetimes dropping below 10 ms above 700 K. For all systems studied, decomposition by loss of small hydrocarbon fragments (acetylene or ethene) was an important decomposition mechanism in the gas phase. For all but AEBCB, isomerization was also a significant decomposition mechanism. At high pressures, one would expect more isomerization because the very rapid collision rate should allow collisional stabilization of the isomerization products. 

The length of a hydrocarbon molecule also has a depression effect on water solubility. Figure 6.8 shows the series of paraffins, 1-olefins, cycloparaffins, aromatics, and acetylenes. The longer hydrocarbon molecules with more —CH2— groups have much lower water solubility, so that each additional —CH2— group drops the water solubility by a factor of almost five. 

The chemisorption of acetylene, ethylene, benzene, and cyclohexane were also studied on the Ir(lll) and stepped Ir[6(111) x (100)] crystal surfaces (30). Chemisorption characteristics of the Ir(lll) and Pt(lll) surface are markedly different. Also, the chemisorption characteristics of the low Miller index Ir(l 11) surface and the stepped Ir[6(l 11) x (100)] surface are markedly different for each of the molecules studied. The hydrocarbon molecules form only poorly ordered surface structures on either the Ir(l 11) or stepped iridium surfaces. Acetylene and ethylene (C2H2 and C2H4) form surface structures that are somewhat better ordered on the stepped iridium than on the low Miller index Ir(l 11) metal surface. The lack of ordering on iridium surfaces as compared to the excellent ordering characteristics of these molecules on... 

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... 

Recently, carbon nanotubes, an important class of one-dimensional nanostructures, have been fabricated within the pores of anodic alumina via CVD (Davydov et al, 1999 Li et al, 1999 Iwasaki et al., 1999 Suh et al, 1999). A small amount of metal (e.g., Co) is first electrochemically deposited on the bottom of the pores as a catalyst for the carbon nanotube growth, and the template is heated to 700 to 800°C in a flowing gas mixture of N2 and acetylene or ethylene. The hydrocarbon molecules are then pyrolyzed to... 

Garrone et al. (168) have shown that the sensitization of MgO to electron donation by preadsorption extends to propene, butene, and acetylene. Ultraviolet reflectance measurements show bands characteristic of the carbanions, and the protons are assumed to react with 02c to form OH c. The addition of oxygen then leads to an electron transfer from the carbanion to form 02, whereas the radical then formed can oxidize or dimerize. They suggest that in this way OJ can be formed without the need for electron transfer from the solid to form a preexisting radical. However, it is clear that the oxide surface is involved and without the presence of 0 c the reaction will not proceed. The function of the Owould be to abstract a proton from the adsorbed molecule alternatively an electron could be donated to the hydrocarbon molecule and then the 0 c would abstract a hydrogen to form OH c. 

The chemisorption of hydrocarbon molecules on surfaces presents another class of important and interesting systems for study. We shall discuss the case of acetylene chemisorption on the Ni (111), Rh (111) and Pt (111) surfaces, as they incorporate many features relevant to all hydrocarbon chemisorption systems. 

Here, the phenyl radical once again attacks the unsamrated bond. However, the steric effect and larger cone of acceptance (the methyl group screens the p carbon atom and makes it less accessible to addition) direct the addition process of the radical center of the phenyl radical to the a carbon atoms of methylacetylene and propylene (the carbon atom holding the acetylenic hydrogen atom). Consequently, crossed beam reactions with complex hydrocarbon molecules can be conducted and valuable information on the reaction pathways can be derived if (partially) deuterated reactions are utilized. 

FIGURE 11.11 Products formed in the reactions of ground state carbon atoms with unsaturated hydrocarbon molecules under single collision conditions via hydrogen atom elimination. Note that for the reaction of carbon atoms with acetylene, the tricarbon plus molecular hydrogen elimination channel was observed, too. 

Scheme 5 is a summary of the experimental frequencies of the v(CH), v(C = Q, and v(OH) modes for the 1 1 7i-bonded acetylene and methylacetylene complexes. Notice that the shift of v(C = C) and v(OH) bands is always negative with respect to the free hydrocarbon molecules (v(C = C) = 1974 and 2142cm for acetylene and methylacetylene, respectively) or Bronsted acid group, respectively. The shift of the v(OH) is a normal consequence of hydrogen-bonding and has been abundantly documented for many bases adsorbed molecularly in zeolites with proton affinities in the range of 420—840kJmol (20,21,156-161). Furthermore, the v(C = C)... 

Figure 2, Volume concentration of heavy hydrocarbon molecules and soot at different heights above the burner in an acetylene/oxygen flame. Pressure = 20 mm Hg fuel equivalence ratio = 3.0 cold gas velocity = 50 cm sec.

Huang, L.C.L. Chang, A.H.H. Asvany, O. Balucani, N. Lin, S.H. Lee, Y.T. Kaiser, R.I. Osamura, Y. Crossed beam reaction of cyano radicals with hydrocarbon molecules IV Chemical dynamics of cyanoacetylene formation from reaction of C,N(X E+) with acetylene. J. Chem. Phys. 2000, 113, 8656-8666. 

Kaiser, R.L Ochsenfeld, C. Head-Gordon, M. Lee, Y.T. Suits, A.G. Crossed-beam reaction of carbon atoms with hydrocarbon molecules. Ill Chemical dynamics of propynylidyne (/-C3H Y Hj ) and cyclopropynylidyne (c-C3H Y 52) formation from reaction of with acetylene, C2H2(X... 

In just the same way two or more mechanisms of coke formation were observed in the decomposition of the naphthenes, of the paraffin hydrocarbons, olefins, acetylene, carboxylic acids, etc. In all cases the mechanisms of coke and tar formation could be represented by a cyclic sequence of a number of elementary stages, which involve the addition of every new molecule of the coke-forming material. Depending on whether the initial organic substance when the temperature is rising can give one, two, or more kinds of molecules of coke-forming material capable of... 

Soot forms in a flame as the result of a chain of events starting with the oxidation and/or pyrolysis of the fuel into small molecules. Acetylene, C2H2, and polycyclic aromatic hydrocarbons (PAHs) are considered the main molecular intermediates for soot formation and growth (McKinnon and Howard 1990). The growth of soot particles involves first the formation of soot nuclei and then their rapid growth due to surface reactions (Harris and Weiner 1983a,b). 

In another reaction of [Mo3S4] +, its aqua complex [Mo3S4(H20)9] + reacts with acetylene at room temperature in I m HCl to yield complex 61, according to Scheme 20. Here a bond is formed between a carbon atom within a hydrocarbon molecule and a sulfur atom of a metal-sulfide cluster. The adduct can be viewed as representing the transition state or intermediate presumed to be formed when an organosulfur molecule reacts with a sulfide vacancy of a heterogeneous catalyst (cf. Section 2.6.4) or a cluster. However, whereas the reaction depicted in Scheme 20 is the first step of a sulfurization process (C-S bond formation), the direction of the conversion would have to be reversed to make it the second step of a desulfurization process (C-S bond cleavage). 

Surface irregularities play a crucial role in the decomposition of hydrocarbons. While acetylene (C2H2) remains intact on the smooth, densely packed (111) surface of nickel and shows no tendency to form CH fragments below 400 K, the molecule dehydrogenates completely on a stepped nickel surface at 150 K, while the C=C bond breaks already at 180 K. 

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