It antibonding molecular orbital

Compared to an H atom, electrons with the function y/, are less energetic, and those with the function y/2 are more energetic. When the two available electrons occupy the molecular orbital y/(, this is energetically favorable y/, is the wave function of a bonding molecular orbital. y/2 belongs to an antibonding molecular orbital its occupation by electrons requires the input of energy. 

The third step hypothesized [eq. (9)] is electron demotion from the singly occupied antibonding molecular orbital of XIII to the low energy, singly occupied oxygen p orbital which originally lost the electron in the n-ir excitation process. It is important and of interest to note that this demotion process is an electronic transition thus XIII and XIV are... 

For a 7t system to function as an electron acceptor, it must have unfilled orbitals available to accept electrons. In the case of olefins or dienes those are n antibonding molecular orbitals. Thus interaction of the HOMO of one n system with file LUMO of a second n system produces a donor-acceptor pair (HOMO donating to LUMO) enabling electrons to be transferred from one n system to another with resulting bond formation. 

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