Ethylene ground-state configuration

The ground-state configuration of the two-ethylene system is a b u (see Table 7-1). This state is totally symmetric, Ag. The excitation of an electron from the HOMO to the LUMO will give an electron configuration agbiub2u. The direct product is ... 

Omitting the occupied CH orbitals, because they retain their symmetry labels across the diagram, the ground-state configuration of ethylene is adequately characterized as 7r(62u) ]. Now, suppose that each of the methylenes is... 

Again, as was the case with ethene (ethylene), the most widely used (because it seems to work best) approximation avoids rehybridization of the electrons between the carbons into some new kind of bonding in favor of the formation of a and n bonds, with their corresponding a and rt antibonding orbitals (unoccupied in the ground-state configuration). 

For some systems a single determinant (SCFcalculation) is insufficient to describe the electronic wave function. For example, square cyclobutadiene and twisted ethylene require at least two configurations to describe their ground states. To allow several configurations to be used, a multi-electron configuration interaction technique has been implemented in HyperChem. 

Fig. 6 The n energy levels of ethylene. The asterisk, identifies the antibonding orbital, while the two arrows represent the two electrons with antiparallel spins corresponding to the configuration of the ground state (see text).

These results can be summarized as follows The triplet carbene ( 5i) adds nonstereospecifically because its complex and a ground state ethylene correlate with the triplet state of an excited trimethylene configuration, which has no barriers to rotation around terminal bonds. 

The same basic diagram provides a detailed description of the twisting of a -bond (Salem, 1973 Salem and Rowland, 1971). This is illustrated in Fig. 10. The -bond in ethylene may exist in a number of predetermined states, each of which may be approximated by the appropriate MO configuration. The possible states are S0, the ground state, ( 2) Tj, the first... 

Figure 7.10 An orbital correlation diagram for ethylene dimerization. Left two widely separated ethylene molecules. Center two ethylene molecules close enough for significant interactions to occur. Right cyclobutane electron configurations correspond to the ground state for each stage.

The ground-state complex between benzene and maleic anhydride was found to have the exo configuration. Bryce-Smith and Hems [44] have measured nuclear magnetic resonance chemical shifts of the ethylenic protons of maleic an-... 

Figure 29.5. Ethylene. Configuration of tt electrons in ground state and excited state.

The state function of ifo. The LE function of ethylene contributes mainly to S2(about 21 ). The result accords with the fact that the experimental data on the photochemical reaction of PVCi are explained on the bases of Woodward-Hoffmann s rule. CT configurations from the occupied MO(OMO) of the ground state of benzene to the unoccupied MO(UMO) of ethylene and the OMO of ethylene to the UMO of benzene increase 16.6 % and 4.6 respectively, compared with So.(Pig.6) These contributions decrease the bond order of the central double bond in S2. In S- the contributions of the LE of ethylene and CT are small... 

If the triplet states of 128 and 129 involve mainly single configurations with a half-occupied HOMO and a half-occupied LUMO, the pronounced regioselec-tivity may be rationalized on the basis of an interaction of the half-occupied LUMO of the aromatic moiety with the vacant LUMO of the ground-state ethylene moiety. Substitution removes the degeneracy of the benzene LUMO, and the product formed (meta or para) depends on the magnitude of the LCAO coefficients of the lower of these MOs. From Example 3.10 the MO is... 

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