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    why is it that when you add together 2 ethenes to form conjugated bonds the pi orbitals are split in to 2 energy levels? wouldnt there already be two energy levels present with one double bond - the normal bonding level and the antibonding level for the simplified electronic energy level for a conjugated diene? and then when you put two together you would get 4?

    and why is the delta E for benzene higher in UV than in infrared?
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    But it is split into 4 energy levels, like your diagram shows! The diagram is a bit confusing, traditionally you would consider a diene as a row of 4 p-orbitals rather than 2 ethenes but the result is the same - you have 2 bonding and 2 antibonding molecular orbitals, the bonding ones being occupied.

    As for the delta E, Infrared spectra are detecting transitions in vibrational energy as the major absorption event whereas UV absorption comes from electronic transitions. These electronic transitions can be coupled to vibrational and rotational ones, but the gaps between electronic levels, like in butadiene, are smaller than the gaps between vibrational levels. I hope this is what the question's getting at, you haven't really specified 'delta E'..
 
 
 
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