Molecular orbitals conjugated systems

Visualizing benzene as a resonance hybrid of two Kekule structures cannot fully explain the unusual stability of the aromatic ring. As we have seen with other conjugated systems, molecular orbital theory provides the key to understanding aromaticity and predicting which compounds will have the stability of an aromatic system. 

In the discussion of the Kramer s doublets we have dealt entirely with the properties of the atomic orbitals of the heme iron. In a different approach the electronic states are expressed in terms of molecular orbitals. This is particularly interesting for aromatic radicals because the unpaired electron spreads out over the entire conjugated system of -orbitals. 

Figure 12.7 A summary of the molecular orbitals and relative energies for linear conjugated pi systems of 1 to 6 p orbitals. The phase of the ends of a linear pi system molecular orbital alternates as the energy of the MO increases. The fundamentai (lowest MO) is always symmetric.

Molecular Orbitals of Alkenes and Conjugated Polyene Systems Molecular Orbitals of Conjugated Ions or Radicals... 

Salem L 1966 Molecular Orbital Theory of Conjugated Systems (Reading, MA Benjamin)... 

In spite of the success of this method it was later felt that the calculation of the charge distribution in conjugated r-systems should be put on a less empirical basis. To achieve this, a modified Huckel Molecular Orbital (HMO) approach (Section 7.4) was developed. Again, the charge distribution in the r-skeleton is first calculated by the PEOE method. 

The PEOE method in conjunction with a modified Hiickel Molecular Orbital (HMO) method allows charge calculation in conjugated r-systems. 

Salem, L., 1966. The Molecular Orbital Theory of Conjugated Systems. Benjamin, New York. Saunders, M., 1987. 7. Amer. Chem. Soc. 109, 3150. 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

Hiickel s calculations on planar conjugated systems were extensively exploited, and I refer you once again to Streitwieser s classic book. Molecular Orbital Theory for Organic Chemists. What few calculations that had been done at that time on the (T framework had used the method of linear combination of bond orbitals. 

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.

Molecular Orbitals and Pericyclic Reactions of Conjugated Pi Systems... 

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). 

McWeeny, R., Proc. Roy. Soc. [London) A237, 355, (ii) "The density matrix in self-consistent field theory. II. Applications in the molecular orbital theory of conjugated systems."... 

The se the orie s are inevitablj based upem analyse s of the interactions and transformations of molecular orbitals, and consequently the accurate construction and re presentation of molecular orbitals has become essential, furthermore, although the forms of molecular orbitals in diatomics and of delocalized tt orbitals in conjugated systems are familiar, a general, non-computational method for determining the qualitative nature of or and t orbitals in arbitrary molecules has been lacking. 

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