Molecular Orbital Description of 1,3-Butadiene

Two p orbitals combine to form two ir molecular orbitals. When these orbitals are occupied by two electrons, both electrons occupy the low-energy, bonding orbital, leading to a net lowering of energy and formation of a stable bond. The asterisk on t//./ indicates an antibonding orbital. 

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

In describing the 1,3-butadiene molecular orbitals, we say that the v electrons are spread out, or delocalized, over the entire n framework rather than localized between two specific nuclei. Electron delocalization always leads to lower energy and greater stability of the molecule. 

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The molecular orbital description of 1,3-butadiene shown in Figure 7.9 represents the electronic configuration of the molecule in its ground state. If the molecule absorbs light of an appropriate wavelength, the light will promote an electron from its HOMO to its LUMO (from if/2 to 1A3). The molecule then is in an excited state (Section 1.2). The excitation of an electron from the HOMO to the LUMO is the basis of ultraviolet and visible spectroscopy (Section 8.9). 

Figure 14-7 A 7r-molecular-orbital description of 1,3-butadiene. Its four electrons are placed Into the two lowest tt (bonding) orbitals, tt, and tt2.

The TT electrons in 1,3-butadiene are delocalized over four sp carbons. In other words, there are four carbons in the rr system. A molecular orbital description of... 

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. 

Abstract A discussion on conservation of orbital symmetry and its application to select pericyclic reactions is presented. Initially, effort is made to explore the symmetry characteristics of the cr, cr, n and n molecular orbitals (MOs). This is followed by a description of the MOs and their symmetry characteristics for allyl cation, allyl radical, allyl anion, and 1,3-butadiene. This concept is applied to n2 + n2, n4 + it2 (Diels-Alder) and electrocyclic reactions. 

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