Ethylene geometry

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

Figure Bl.22.10. Carbon K-edge near-edge x-ray absorption (NEXAFS) speetra as a fiinotion of photon ineidenee angle from a submonolayer of vinyl moieties adsorbed on Ni(lOO) (prepared by dosing 0.2 1 of ethylene on that surfaee at 180 K). Several eleetronie transitions are identified in these speetra, to both the pi (284 and 286 eV) and the sigma (>292 eV) imoeeupied levels of the moleeule. The relative variations in the intensities of those peaks with ineidenee angle ean be easily eonverted into adsorption geometry data the vinyl plane was found in this ease to be at a tilt angle of about 65° from the surfaee [71], Similar geometrieal detenninations using NEXAFS have been earried out for a number of simple adsorbate systems over the past few deeades.

Zaera F, Fischer D A, Carr R G and Gland J L 1988 Determination of chemisorption geometries for complex molecules by using near-edge X-ray absorption fine structure ethylene on Ni(IOO) J. Chem. Rhys. 89 5335-41... 

Figure 4-5 The Geometry of Ethylene with all Atoms at their Equilibrium Positions in the MM3 Eorce Eield.

The name of the TINKER input file in Eile 4-4 is ethylene.xyz, where the. xyz indicates that the geometry is given in Cartesian coordinates. (There are other... 

Files 4-4a and b. The Initial (top) and Final (bottom) Geometries of Ethylene Calculated by TINKER Using the MM3 Eoree Eield. 

We have just seen how to construct a TINKER input hie for ethylene. We shall now construct several new models and study their geometries. 

Procedure, a) Using the procedure shown in constructing the ethylene input file, construct an approximate input file for HjO. The atom type for oxygen is 6. The approximate input geometry can be taken as in File 4-.5, where all of the c-eourdinates are set at 0. Go to the tinker directory in the MS-DOS operating system and create an input (ile for your II2O calculation. Be sure the extension of the input file is. xyz. Rename or edit the (ile as neeessaiy. 

Placing the 5-Iine control block above the geometry specification block of Exercise 5-4 gives the complete minimal input file for benzene, which we can call miubeuz.inniS (or anything else you like with the extension. mm3). Aside from the geometry block, there are two important differences between miubenz.mm3 and the file miuimal.mm3 for ethylene in File 4-la. One is the switch in column 61 of the first line, the other is the set of switches (hat constitutes the entire second line. The first switch tells the system... 

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... 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

Liquid Hazards. Pure liquid ethylene oxide will deflagrate given sufficient initiating energy either at or below the surface, and a propagating flame may be produced (266,267). This requites certain minimum temperatures and pressures sensitive to the mode of initiation and system geometry. Under fire exposure conditions, an ethylene oxide pipeline may undergo internal decomposition either by direct initiation of the Hquid, or by formation and subsequent decomposition of a vapor pocket (190). 

As mentioned in Section 10.3.2, there has been recent interest in the use of the Dow constrained geometry catalyst system to produce linear low-density polyethylenes with enhanced properties based, particularly, on ethylene and oct-l-ene. 

Ethylene is a highly symmetric molecule. Here is the input file for an optimization ol its geometry ... 

In ethylene, QH4, there is a double bond between the two carbon atoms. The molecule has the geometry to be expected if each carbon atom had only three pairs of electrons around it... 

Along the bond axis itself, the electron density is zero. The electron pair of a pi (tt) bond occupies a pi bonding orbital. There is one tt bond in the C2H4 molecule, two in QH The geometries of the bonding orbitals in ethylene and acetylene are shown in Figure 7.13. 

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. 

These calculations have been conducted on the basis of RHF optimized geometries, considering the 6-31G basis set for the n-alkane compounds (11), and the 6-31G basis set for the polyacene series (12). In both cases, the basis set contention has been checked by comparison with more thorough investigations on small compounds, such as ADC[3] calculations (11a) on n-butane based on the 6-31IG, 6-31G and 6-31G basis, or the MRSDCI ionization spectrum of ethylene as obtained by Murray and Davidson (33) using a 196-CGTO basis set. 

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