Molecular orbitals 1.3- butadiene

Butadiene The ti molecular orbitals of 1 3 butadiene are shown m Figure 10 10 The four sp hybridized carbons contribute four 2p atomic orbitals and their overlap... 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

Refer to the molecular orbital diagrams of allyl cation (Figure 10 13) and those presented earlier in this chapter for ethylene and 1 3 butadiene (Figures 10 9 and 10 10) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Floffmann rules... 

Active Figure 14.2 Four - molecular orbitals in 1,3-butadiene. Note that the number of nodes between nuclei increases as the energy level of the orbital increases. 

Figure 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from i / , the highest occupied molecular orbital (HOMO), to 1//3, the lowest unoccupied molecular orbital (LUMO).

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

Figure 23.3 The - bonding molecular orbitals of a conjugated erone (propenal) and a conjugated diene (1,3-butadiene) are similar in shape and are spread over the entire %< system.

Problem 30.1 Look at Figure 30.1, and tell which molecular orbital is the HOMO and which is the LUMO for both ground and excited states of ethylene and 1,3-butadiene. 

Coulson, C. A., and Jacobs, J., Proc. Roy. Soc. London) A206, 287, Electronic levels in simple conjugated systems. II. Butadiene." Best possible molecular orbital is obtained. The idea is the same as that developed by Roothaan (1951). The work has been done quite independently with the similar work by Parr and Mulliken (1950). 

Mixed labeling involving both x and a orbitals occurs in certain molecules the 5BU molecular orbitals of ra is-2-butene (III.78) and transoid 1,3-butadiene (III.65) are labeled ch3> ( cc) and 7rcn2> ( cc) because one lobe of the x orbital overlaps well with the adjacent CC bond-orbital to form a delocalized combination. In cisoid acrolein, orbitals 9A and 10A are labeled TCH2y nodal surfaces of the two localized orbitals coincide and allow for a delocalized combination (III.G8). 

Butadiene has two n bonds. The interaction between the two n bonds is one of the simplest models to derive molecular orbitals from bond orbitals. A n bond in butadiene is similar to that in ethylene. The n bond is represented by the bonding and antibonding orbitals. The interactions occur between the n bonds in butadiene. The bond interactions are represented by the bond orbital interactions. 

The bonding orbitals and of ethylenes are combined in phase to be the lowest TZ molecular orbitals (tTj) of butadiene (Scheme 18). The out-of-phase combined orbital (jtj) is the highest occupied molecular orbital (HOMO). The in-phase combination of... 

Scheme 18 The Jt molecular orbitals of butadiene from the bond orbitals...

The energies, the phases and the amplitudes of the tz molecular orbitals of butadiene are shown in Scheme 19. The, n, and orbitals corresponds to half, one,... 

All the atoms of butadiene lie in a plane defined by the s p hybrid orbitals. Each carbon atom has one remaining p orbital that points perpendicular to the plane, in perfect position for side-by-side overlap. Figure 10-42 shows that all four p orbitals interact to form four delocalized molecular orbitals two are bonding MOs and two are antibonding. The four remaining valence electrons fill the orbitals, leaving the two p orbitals empty. 

Serrano-Andres, L., Merchan, M., Nebot-Gil, I., Lindh, R., Roos, B. O., 1993, Towards an Accurate Molecular Orbital Theory for Excited States Ethene, Butadiene, and Hexatriene , J. Chem. Phys., 98, 3151. 

In a concerted reaction, orbital and state symmetry is conserved throughout the course of the reaction. Thus a symmetric orbital in butadiene must transform into a symmetric orbital in cyclobutene and an antisymmetric orbital must transform into an antisymmetric orbital. In drawing the correlation diagram, molecular orbitals of one symmetry on one side of the diagram are connected to orbitals of the same symmetry on the other side, while observing the noncrossing rule. 

Figure 13.4 The p orbitals of 1,3-butadiene, stylized as spheres. (See Figure 13.5 for the shapes of calculated molecular orbitals for 1,3-butadiene.)...

---