p Molecular Orbitals of 1,3-Butadiene (H2C=CH-CH=CH2)

s-trans 1,3-Butadiene top view:      Side view: 



p4*This molecular orbital has no bonding interactions. This p* antibonding molecular orbital is of higher energy than p3* (which still has one bonding interaction). The net bond order for p4* is -3. In ground state of 1,3-butadiene, this orbital is unoccupied.  It has four nodes.

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p3*This is an antibonding (p*) orbital, because it has more orbital overlaps that are out of phase (nodes between C1 and C2, and between C3 and C4) than overlaps that are in phase (between C2 and C3). The net bond order is -1. In the ground state, this orbital is the lowest energy unoccupied orbital, so it is the LUMO for 1,3-butadiene.  It has three nodes.

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p2:   This orbital has a node between C2 and C3 (area of no overlap between blue are red regions), resulting in a net bond order of 1. p2 is higher in energy than p1 because it has less total bonding (lower bond order). This is the HOMO for 1,3-butadiene, and contains two p electrons.  It has two nodes.

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p1:   The lowest energy p orbital is called p1, and is fully bonding. This orbital contains two p electrons, and has a bond order of 3.  It has just one node.

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