Cyclobutadiene molecular orbital energy

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Find the Hiickel energy levels and molecular orbitals for butadiene, cyclobutadiene, and pentatrienyl. 

Fig. 4.17 The n molecular orbitals and n energy levels for a cyclic four-p-orbital system in the simple Hiickel method. The MOs are composed of the basis functions (four p AOs) and the eigenvectors, while the energies of the MOs follow from the eigenvalues (Eq. 4.69). This particular diagram is for the square cyclobutadiene molecule. The paired arrows represent a pair of electrons of opposite spin, in the fully-occupied lowest MO, i/q, and the single arrows represents unpaired electrons of the same spin, one in each of the two nonbonding MOs, ij/2 and 1//3 the highest n MO, 1I/4, is empty in the neutral molecule...

Antiaromaticity [1] is the phenomenon of destabilization of certain molecules by interelectronic interactions, that is, it is the opposite of aromaticity [2], The SHM indicates that when the n-system of butadiene is closed the energy rises, i.e. that cyclobutadiene is antiaromatic with reference to butadiene. In a related approach, the perturbation molecular orbital (PMO) method of Dewar predicts that union of a C3 and a Ci unit to form cyclobutadiene is less favorable than union to form butadiene [3]. 

Although we can draw benzene-like resonance structures (Figure 16-3) for cyclobutadiene, experimental evidence shows that cyclobutadiene is unstable. Its instability is explained by the molecular orbitals, shown in Figure 16-6. Four sp2 hybrid carbon atoms form the cyclobutadiene ring, and their four p orbitals overlap to form four molecular orbitals. The lowest-energy MO is 771, the all-bonding MO with no nodes. 

The pi molecular orbitals of cyclobutadiene. There are four MOs the lowest-energy bonding orbital, the highest-energy antibonding orbital, and two degenerate nonbonding orbitals. 

An electronic energy diagram of cyclobutadiene shows that two electrons are unpaired in separate nonbonding molecular orbitals. 

Q Explain how to construct the molecular orbitals of a conjugated cyclic system similar to benzene and cyclobutadiene. Use the polygon rule to draw the energy diagram, and fill in the electrons to show whether a given compound or ion is aromatic or antiaromatic. 

Fig. 1.39 The three lowest-energy n molecular orbitals of rectangular cyclobutadiene...

Cyclobutadiene According to the molecular orbital picture, square planar cyclobn-tadiene shonld be a diradical (have two unpaired electrons). The fonr TT electrons are distribnted so that two are in the lowest energy orbital and, in accordance with Hund s rule, each of the two equal-energy nonbonding orbitals is half-filled. (Remember, Hnnd s rnle tells ns that when two orbitals have the same energy, each one is half-hlled before either of them reaches its full complement of two electrons.)... 

The distribution of electrons in the tt molecular orbitals of (a) benzene, (b) the cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. The reiative energies of the tt molecular orbitals in a cyclic compound correspond to the relative levels of the vertices. Molecular orbitals below the midpoint of the cyclic structure are bonding, those above the midpoint are antibonding, and those at the midpoint are nonbonding. 

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