Ethylene double bond interactions

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

It was suggested that dehydration proceeds almost completely at the GA monomer unit that contains Schiff base imine, but proceeds only little at other GA monomer units. Thus, the formed imino bond is conjugated with the ethylenic double bond, which interaction leading to its stability. 

DuPre DB, Yappert MC (2002) Cooperative hydrogen- and rrH-bonded interactions involving water and the ethylenic double bond. J Phys Chem A 106 567-574... 

FIGURE 1. Schematic illustration of the HOMO and LUMO of a supermolecule consisting of four strongly interacting ethylenic double bonds. See text for details. 

FIGURE 2. (Left) Illustration of the / -orbitals of phospholipid carbonyls interacting with the cholesterol double bond. All orbitals lie in the same plane (plane of the page). (Right) The basic, three ethylenic double bond unit in a helical array in SMQS, stabilized by an extra electron. The upper- and lowermost double bonds have suprafacial whereas the double bond in the middle has antarafacial contacts. 

The ethylenic double bond in cyclobutene-l,2-diones was unreactive in attempted cycloadditions with mesitonitrile oxide. Addition took place preferentially at the carbonyl groups to spiro-dioxazoles. The periselectivity has been disccused in terms of frontier MO interactions. 

The universally accepted model of the second-order dicarbon interaction, colloquially known as an ethylenic double bond, has developed from a misreading of a seminal paper which discussed the quantum theory of double bonds [7]. In this paper, it is shown that by linear combination of the eigenfunctions... 

FIGURE 6 4 Electro static potential maps of HCI and ethylene When the two react the interaction is between the electron rich site (red) of ethylene and electron poor region (blue) of HCI The electron rich region of ethylene is associ ated with the tt electrons of the double bond and H IS the electron poor atom of HCI... 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

Dative bonding can also occur in pi-type interactions, particularly involving the BH2 group. A good example is H2B=NH2, isoelectronic to ethylene, which is well described by a double-bonded NBO Lewis structure with dative 7Tb n bond ... 

Low torsional barriers in combination with strong steric interactions between donor and acceptor groups in push-pull ethylenes have in several cases been demonstrated to cause permanently twisted double bonds, in which a planar arrangement of substituents at the double bond may represent an energy maximum. 

The bonding in ethylene is based initially on one C-C CT bond together with four C-H a bonds, much as we have seen in ethane. We are then left with a p orbital for each carbon, each carrying one electron, and these interact by side-to-side overlap to produce a IX bond (Figure 2.15). This makes the ethylene molecule planar, with bond angles of 120°, and the TX bond has its electron density above and below this plane. The combination of the C-C ct bond and the C-C Jt bond is what we refer to as a double bond note that we cannot have Jt bond formation... 

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