Cyclic n Systems

Conjugated ring systems offer an alternative mode for the stabilization of a carbanion center. The most common situation is where deprotonation completes a cyclic n system leading to a highly stabilized, aromatic anion. The best known example is cyclopentadiene, which leads to a six-electron, aromatic ring after... 

The cyclopropenyl group, C3H3, is the simplest carbocycle with a delocalized n system and can serve as a prototype for this class of molecules. Let us see how the pn orbitals of the individual carbon atoms can be combined into MOs—or at least the immediate precursors of actual n MOs. Cyclic n systems will be discussed in general in Chapter 7, and this illustration is intended only to demonstrate the use of projection operators in making SALCs of AOs on different atoms. 

Figure 21-16 Calculated 77-molecular-orbital energies for N overlapping p orbitals in Mobius cyclic n systems...

The search in recent years for silicon compounds with multiple bonds or cyclic n-systems has renewed interest in siloles (66)77 and their mono- and di-anions (48 and 49), and led to the successful isolation of stable silole anions coordinated to various metal counter ions (Li+, Na+, K+)10a-c 78 - 86 and as complexes with ruthenium (e.g. 6a and 6b)10d. 

Fig. 4.22 A useful mnemonic for getting the simple Hiickel method pattern for cyclic n systems. Setting the radius of the circle at 2ipi, the energy separations from the nonbonding level can even be calculated by trigonometry...

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

See Fig. 20-1. The four C s and the heteroatom Z use sp2-hybridized atomic orbitals to form the a bonds. When Z is O or S, one of the unshared pairs of e s is in an sp2 HO. Each C has a p orbital with one electron and the heteroatom Z has a p orbital with two electrons. These five p orbitals are parallel to each other and overlap side-by-side to give a cyclic n system with six p electrons. These compounds are aromatic because six electrons fit Huckel s 4n -I- 2 rule, which is extended to include heteroatoms. 

The triene absorbs at a longer (275 nm). Since the cyclic n system of benzene has a lower enei than the linear 7t system of the triene, benzene absorbs radiation of shorter wavelength. 

Most organic compounds that show absorption in the visible or in the near-UV region have a linear or cyclic n system as the chromophoric system. Therefore, the results of the previous sections may be used and extended to discuss light absorption of all those compounds that can be derived from linear and cyclic hydrocarbons by including the influence of substituents in an appropriate way. (Cf. Michl, 1984.) A complete theory of substituent effects comprises all areas of organic chemistry. Here, only the fundamental concepts of the influence of inductive and mesomeric substituents will be considered. In order to simplify the discussion, substituent effects will be called inductive if in the HMO model they can be represented by a variation of the Coulomb integral of the substituted n center p. If they are due to an extension of the n system they will be called mesomeric. 

These molecular orbitals have the same energy in general, n molecular orbitals having the same number of nodes in cyclic n systems of hydrocarbons are degenerate. The total n molecular orbital diagram for cyclo-C3H3 can therefore be summarized as follows. 

Similar results are obtained for other cyclic n systems, as shown in Figure 2-11. In these diagrams, nodal planes are disposed symmetrically. For example, in cyclo-C4H4 the single node molecular orbitals bisect the molecule through opposite sides the nodal planes of these molecular orbitals are oriented at 90° angles to each other. The 2-node orbital for this molecule also has the nodal planes at 90° angles. 

Benzene. In the molecular orbital approach for benzene, each carbon in the ring is considered to use sp2 hybrid orbitals. These orbitals are involved in carbon-carbon (t bonding (from overlap of sp2 hybrids on adjacent carbons) and carbon-hydrogen a bonding (from overlap of an sp2 hybrid on each carbon with the Is orbital of hydrogen). This leaves on each carbon a p orbital not participating in the hybrids and available to participate in a cyclic n system. When these six p orbitals interact, six n molecular orbitals are formed, as illustrated in Figure 2-12. 

These compounds satisfy the criteria for aromaticity (planar, cyclic n system, and the Huckel number of 4n -I- 2 71 electrons) pyrrole, imidazole, indole, pyridine, 2-methylpyridine, pyrimidine, and purine. The systems with 6 7i electrons are pyrrole, imidazole, pyridine, 2-methylpyridine, and pyrimidine. The systems with 10 7i electrons are indole and purine. The other nitrogen heterocycles shown are not aromatic because they do not have cyclic 7i systems. 

Jhe perimeter model introduced by Platt (1949), reformulated in the LCAO Mo form by Moffitt (1954a), and extended by Gouterman (1%1), Heilbron-ner and Murrell (1963), and Michl (1978), has been very useful in understanding trends in the electronic spectra of cyclic n systems. It applies equally to singlet and triplet states and has provided the commonly used nomenclature for both. The following discussion is limited to the singlet states, which are more important in ordinary spectroscopy. 

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