The molecular orbitals associated with the Ï-bonding MOs insquare cyclobutadiene (C4H4) are, like benzene, form by overlap ofa single p-orbital on each carbon atom that is not involved in thesigma-bonding network of the molecule. These p-orbitals pointperpendicularly with respect to the plane defined by the carbonatoms. There is no central atom, so the only interactions thatmatter are between the p-orbitals among themselves. These MOs haveexactly the same phase relationship as the SALCs you just made inquestion 1. i) Use your results from question 1 to draw the fourÏ-orbitals of C4H4. ii) Use the idea that the energy of a MOincreases with number of new nodes made during bonding (i.e. nodesnot already present in initial AO functions) to sketch a molecularorbital energy level diagram for the Ï-molecular orbitals of C4H4.Remember that, by definition, degenerate orbitals are identicalexcept for trivial rotation in space, and have identical energy.iii) Fill the MOs with the electrons from the atomic p-orbitalsthat you used to form the Ï-MOs of C4H4. Use your diagram todetermine the overall bond order by considering each doublyoccupied bonding MO to contribute 1, each nonbonding orbital 0 andeach antibonding orbital -1. The result would be divided by 4 C-Cbonds to get the average contribution to each C-C bond fromÏ-bonding, which then should be added to a Ï-bonding contributionof 1.0. iv) Repeat step (iii) for the dication C4H4 2+ to determinethe net bond order for this species. Why do you think you got thisvalue? v) Predict which of C4H4 and C4H4 2+ could be observed byESR spectroscopy. vi) Which of C4H4 2+ and C4H4 is more likely tobe susceptible to electrophilic attack.
The molecular orbitals associated with the Ï-bonding MOs insquare cyclobutadiene (C4H4) are, like benzene, form by overlap ofa single p-orbital on each carbon atom that is not involved in thesigma-bonding network of the molecule. These p-orbitals pointperpendicularly with respect to the plane defined by the carbonatoms. There is no central atom, so the only interactions thatmatter are between the p-orbitals among themselves. These MOs haveexactly the same phase relationship as the SALCs you just made inquestion 1. i) Use your results from question 1 to draw the fourÏ-orbitals of C4H4. ii) Use the idea that the energy of a MOincreases with number of new nodes made during bonding (i.e. nodesnot already present in initial AO functions) to sketch a molecularorbital energy level diagram for the Ï-molecular orbitals of C4H4.Remember that, by definition, degenerate orbitals are identicalexcept for trivial rotation in space, and have identical energy.iii) Fill the MOs with the electrons from the atomic p-orbitalsthat you used to form the Ï-MOs of C4H4. Use your diagram todetermine the overall bond order by considering each doublyoccupied bonding MO to contribute 1, each nonbonding orbital 0 andeach antibonding orbital -1. The result would be divided by 4 C-Cbonds to get the average contribution to each C-C bond fromÏ-bonding, which then should be added to a Ï-bonding contributionof 1.0. iv) Repeat step (iii) for the dication C4H4 2+ to determinethe net bond order for this species. Why do you think you got thisvalue? v) Predict which of C4H4 and C4H4 2+ could be observed byESR spectroscopy. vi) Which of C4H4 2+ and C4H4 is more likely tobe susceptible to electrophilic attack.
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