Acetylene model

Figure 4.11 Important configurations for a bend acetylene model...

Table 4.4 Natural orbital occupation numbers for the distorted acetylene model in Figure 4.11. Only the occupation numbers for the six central orbitals are shown...

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

The excited states of the enediyne moiety were approximated by a weakly interacting ethylene acetylene model where the various states are expressed as linear combinations of the configurations for one ethylene and two acetylenes. The authors examined the first six excited states of 80, three triplets and three singlets. AH methods examined predicted that the spectroscopically forbidden triplet states (DBj, DBj, and DAj, respectively) He between the singlet ground state (S ) and the first singlet excited state (I Bj). 

Practice working with your Learning By Modeling software Construct molecular models of ethane ethylene and acetylene and compare them with respect to their geometry bond angles and C—H and C—C bond distances... 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

Because each carbon m acetylene is bonded to two other atoms the orbital hybridization model requires each carbon to have two equivalent orbitals available for CT bonds as outlined m Figure 2 19 According to this model the carbon 2s orbital and one of Its 2p orbitals combine to generate two sp hybrid orbitals each of which has 50% s character and 50% p character These two sp orbitals share a common axis but their major lobes are oriented at an angle of 180° to each other Two of the original 2p orbitals remain unhybridized... 

FIGURE 2 18 Acetylene is a linear molecule as indicated in (a) the structural formula and (b) a space filling model... 

An sp hybridization model for the carbon-carbon triple bond was developed in Section 2 21 and is reviewed for acetylene in Figure 9 2 Figure 9 3 compares the electrostatic potential maps of ethylene and acetylene and shows how the second tr bond m acetylene causes a band of high electron density to encircle the molecule... 

Tsai et al. have also used RAIR to investigate reactions occurring between rubber compounds and plasma polymerized acetylene primers deposited onto steel substrates [12J. Because of the complexities involved in using actual rubber formulations, RAIR was used to examine primed steel substrates after reaction with a model rubber compound consisting of squalene (100 parts per hundred or phr), zinc oxide (10 phr), carbon black (10 phr), sulfur (5 phr), stearic acid (2 phr). 

When a plasma polymerized acetylene film on a steel substrate was reacted with the squalene-containing model rubber compound at 155°C for 15 min, a new band assigned to zinc stearate appeared near 1539 cm in the RAIR spectra... 

Fig. 13. RAIR spectra of model rubber compound reacted with plasma polymerized acetylene films on steel substrates for (A) 0, (B) 15, (C) 0 and (D) 45 min. Adapted by permission of Gordon and Breach Science Publishers from Ref. [12].

Many applications of XPS to problems in adhesion science have been reported in the literature. One interesting example is provided by the work of Tsai et al. on the use of XPS to investigate reactions between model rubber compound and plasma polymerized acetylene films that was discussed above [22,23], Consideration of that system permits some interesting comparisons to be made regarding the type of information that can be obtained from RAIR and XPS. 

The Auger depth profile obtained from a plasma polymerized acetylene film that was reacted with the same model rubber compound referred to earlier for 65 min is shown in Fig. 39 [45]. The sulfur profile is especially interesting, demonstrating a peak very near the surface, another peak just below the surface, and a third peak near the interface between the primer film and the substrate. Interestingly, the peak at the surface seems to be related to a peak in the zinc concentration while the peak just below the surface seems to be related to a peak in the cobalt concentration. These observations probably indicate the formation of zinc and cobalt complexes that are responsible for the insertion of polysulfidic pendant groups into the model rubber compound and the plasma polymer. Since zinc is located on the surface while cobalt is somewhat below the surface, it is likely that the cobalt complexes were formed first and zinc complexes were mostly formed in the later stages of the reaction, after the cobalt had been consumed. 

Positive SIMS spectra obtained from plasma polymerized acetylene films on polished steel substrates after reaction with the model rubber compound for times between zero and 65 min are shown in Fig. 44. The positive spectrum obtained after zero reaction time was characteristic of an as-deposited film of plasma polymerized acetylene. However, as reaction time increased, new peaks appeared in the positive SIMS spectrum, including m/z = 59, 64, and 182. The peaks at 59 and 64 were attributed to Co+ and Zn, respectively, while the peak at 182 was assigned to NH,J(C6Hn)2, a fragment from the DCBS accelerator. The peak at 59 was much stronger than that at 64 for a reaction time of 15 min. However,... 

Negative SIMS spectra obtained from plasma polymerized acetylene films on polished steel substrates as a function of reaction time with the model rubber compound are shown in Fig. 45. The most important changes observed in the... 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

Using a mechanical model and a set of force constants, Popov and Lubuzh (66ZPS498) have calculated vibration frequencies for polyacetylenic groups. But these calculations are rather complex and the data on the IR spectra of acetylenic... 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

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