Ethylene Hiickel level

A molecular orbital diagram for ethylene at the Hiickel level. An isolated p orbital has an energy of a, and the interaction energy for an adjacent carbon p orbital is 3. 

The Hiickel energy levels and p basis orbital coefficients for formaldehyde and ethylene are shown in Figure 4.9. If we examine the orbital interactions for case 1, we can see that in the orientation such that the lone pair on oxygen is oriented in a way that a bond can begin to form between the oxygen lone pair and the carbon ir orbital, there are two possible orbital interactions. 

Figure 2.25 Energy levels (left) and Hiickel MOs (right) for ethylene ground state (N = 2)...

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