Ethene orbital structure

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The four valence-bond structures or configurations, 4a-d, are combined mathematically to give four hybrid states, and of these, the lowest-energy one corresponds approximately to the normal state of the molecule. The calculation shows that the structures 4a and 4b, which have one electron in each p orbital, are the major contributors to the hybrid of ethene. The valence-bond structures, 4c and 4d, are ionic structures, which correspond to the conventional formulas, 4e and 4f ... 

One more example of the CASSCF procedure will be outlined calculating the barrier to rotation around the CC double bond in ethene. Step 2, orbital localization, showed nicely localized orbitals when NBO localization was used, but the orbitals were harder to identify with Boys localization. For a CAS(2,2)/6-31G optimization the active orbitals chosen were the n and 7t MOs, and for a CAS(4,4)/6-31G optimization the n, n, cr and cr MOs. The input structures were the normal planar ethene and perpendicular (90° twisted) ethene. Optimization and frequency calculations gave a minimum for the planar and a transition state for the perpendicular structures. The energies (without ZPE, for comparison with those calculated with the GVB method by Wang and Poirier [71]) were ... 

The great advantage of this method is that it can be used to build up structures of much larger molecules quickly and without having to imagine that the molecule is made up from isolated atoms. So it is easy to work out the structure of ethene (ethylene) the simplest alkene. Ethene is a planar molecule with bond angles dose to 120°. Our approach will be to hybridize all the orbitals needed for the C-H framework and see what is left over. In this case we need three bonds from each carbon atom (one to make a C-C bond and two to make C-H bonds). 

The overall energy of the two bonding butadiene molecular orbitals is lower than that of the two molecular orbitals for ethene. This means that butadiene is more thermodynamically stable than we might expect if its structure were just two isolated double bonds... 

These three views of the ethylene molecule emphasize different aspects of the disposition of shared electron pairs in the various bonding orbitals of ethene (ethylene), (a) The backbone structure consisting of sigma (a) bonds formed from the three sp2-hybridized orbitals on each carbon, (b) The % (pi) bonding system formed by overlap of the unhybridized pz orbital on each carbon. The pi orbital has two regions of electron density extending above and below the plane of the molecule, (c) A cutaway view of the combined sigma and pi system. 

New parameters of cyclopropene (155) have been calculated from existing MW data. A near-equilibrium structure has also been derived from scaled moments of iner-tia (Table 16). The lengths of the C—C single bond and the methylene C—H bond and H—C—H angle are similar to those in 1 (Table 1). The C=C bond is, however, considerably shorter than in ethene 1.337 (2) A, and (=)C—H is between C—H in ethene (Section II. A) and in acetylene, 1.0586 and 1.0547 A. Bond-length relations indicate that the methylene carbon in 155 uses approximately the same hybrid orbitals as 1, sp" and sp (Section II.A), to form bonds within the ring and to substituents, while the —CH= carbon in 155 is characterized by sp and sp hybrids, respectively ... 

Pi cation radicals, that is cation radicals in which the SOMO is a pi type orbital, are obviously key intermediates in the ET chemistry of alkenes and alkynes. The ethene cation radical, which has the novel twisted structure shown in Scheme 5, provides an excellent example of a significant structural adjustment which can accompany the loss of an electron [8]. 

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