Pi* antibonding molecular orbital

Consequently, the bond order decreases to 2.5 from 3.0. In forming the 02 ion from O2, an electron is removed from the pi antibonding molecular orbital. Consequently, the bond order increases to 2.5 from 2.0. 

The combination of parallel 2p atomic orbitals by in-phase and out-of-phase addition of their wave functions to give a pi (tt) bonding molecular orbital and a pi antibonding molecular orbital tt ) is shown in Figure 1.21. A tt bonding molecular orbital has a nodal plane that cuts through both atomic nuclei, with electron density above and below the nodal plane concentrated between the nuclei. We picture all isolated tt bonds between carbons to have orbitals such as those of Figure 1.21. 

Pi bonding and antibonding molecular orbitals. The sideways overlap of two p orbitals leads to a 77 bonding MO and a 77 antibonding MO. A pi bond is not as strong as most sigma bonds. 

This is well explained by the MO theory, which predicts that the six 2p orbitals of the C atoms overlap and mix to form three pi-bonding and three pi-antibonding molecular... 

Because of their similar shape and orientation, each 2p orbital overlaps two others, one on each adjacent carbon atom. According to the rules listed on p. 398, the interaction of six 2p orbitals leads to the formation of six pi molecular orbitals, of which three are bonding and three antibonding. A benzene molecule in the ground state therefore has six electrons in the three pi bonding molecular orbitals, two electrons with paired spins in each orbital (Figure 10.28). 

FIGURE 10.28 (a) The six 2p orbitals on the carbon atoms in benzene, (bj The delocalized molecular orbital formed by the overlap of the 2p orbitals. The delocalized molecular orbital possesses pi symmetry and lies above and below the plane of the benzene ring. Actually, these 2p orbitals can combine in six different ways to yield three bonding molecular orbitals and three antibonding molecular orbitals. The one shown here is the most stable. 

Define the following terms bonding molecular orbital, antibonding molecular orbital, pi molecular orbital, sigma molecular orbital. 

Unlike the a bond formed as a result of end-on overlap, side-to-side overlap of two p atomic orbitals forms a pi (tt) bond (Figure 1.6). Side-to-side overlap of two in-phase p atomic orbitals forms a tt bonding molecular orbital, whereas side-to-side overlap of two out-of-phase p orbitals forms a rr antibonding molecular orbital. The TT bonding molecular orbital has one node—a nodal plane that passes through both nuclei. The TT antibonding molecular orbital has two nodal planes. Notice that a bonds are cylindrically symmetrical, but tt bonds are not. 

Since the electron probability lies above and below the line between the nuclei, both the orbitals are pi (it) molecular orbitals. They are designated as iTzp for the bonding MO and 772/ for the antibonding MO. A similar set of tt molecular orbitals is formed from overlap of the parallel atomic orbitals. 

Pi (it) molecular orbital (Section 1.13) A molecular orbital formed when parallel J) orbitals on adjacent atoms overlap. Pi molecular orbitals may be bonding (p lobes of the same phase sign overlap) or antibonding (p orbitals of opposite phase sign overlap). 

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