Orbital mixing degenerate

All alkenes contain a carbon-to-carbon double bond. We can use ethene as a typical example to explain the bonding in alkenes. On each carbon atom of the double bond, the 2s orbital mixes with two of the 2p orbitals to form three degenerate sp hybrid orbitals. The remaining 2p orbital is left unhybridised. 

Fig. 5.12 Simplified molecular orbital energy levels for diatomic molecules of elements in the second period, assuming no mixing of s and p orbitals. The three 2p orbitals are degenerate, that is, they all have the same energy and might also be...

From the electric dipole term in Equations (2) and (3), the MCD selection rule is AM , = 1, requiring that either the ground or excited state contains orbital angular momentum. Therefore, A- and C-term MCD transitions observed between spin degenerate orbital singlets occur by spin-orbit mixing with other states. 

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