Molecular orbitals of naphthalene

Fig. 2. Orbital energies (eV) and self-consistent field molecular orbitals of naphthalene.

Fig. 3-7. Energy levels of the 7r-molecular orbitals of naphthalene (LCAO) and ground state occupancy by TT-electrons.

Esr spectra are subject to exchange effects in the same way as nmr spectra. A specific example is provided by electron exchange between sodium naphthalenide and naphthalene. Naphthalene has a set of ten 77-molecular orbitals, similar to the six 7r-molecular orbitals of benzene (Figure 21-5). The ten naphthalene it electrons fill the lower five of these orbitals. In a solvent such as 1,2-dimethoxyethane, which solvates small metal ions well, naphthalene accepts an electron from a sodium atom and forms sodium naphthalenide, a radical anion ... 

Table 7.1.6. The Hiickel energies and wavefunctions of the jr molecular orbitals in naphthalene...

The other kind of addition compound, exemplified by sodium-naphthalene, cannot be regarded as a substitution product, as it is formed by the addition of one or more electrons to the lowest vacant molecular orbital of an aromatic hydrocarbon. Aromatic hydrocarbons containing two or more aromatic rings, joined (biphenyl, terphenyls), conjugated (1,4-diphenylbutadiene), or fused (naphthalene, anthracene), react with alkali metal without loss of hydrogen. Tliese addition compounds are all strongly coloured, and their formation is greatly facilitated in basic solvents such as tetrahydrofuran or 1,2-dimethoxyethane. 

Replacing one carbon atom of naphthalene with an a2omethene linkage creates the isomeric heterocycles 1- and 2-a2anaphthalene. Better known by their trivial names quinoline [91-22-5] (1) and isoquinoline [119-65-3] (2), these compounds have been the subject of extensive investigation since their extraction from coal tar in the nineteenth century. The variety of studies cover fields as diverse as molecular orbital theory and corrosion prevention. There is also a vast patent Hterature. The best assurance of continuing interest is the frequency with which quinoline and isoquinoline stmctures occur in alkaloids (qv) and pharmaceuticals (qv), for example, quinine [130-95-0] and morphine [57-27-2] (see Alkaloids). 

One way to anticipate the favored product is to consider the shape of naphthalene s best electron-donor orbital, the highest-occupied molecular orbital (HOMO). Display the HOMO in naphthalene and identify the sites most suitable for electrophilic attack. Which substitution product is predicted by an orbital-control mechanism Ts this the experimental result ... 

The delocalization of molecular orbitals lies at the heart of modern chemistry. The concept that the tt orbitals of benzene or naphthalene cover the entire carbon skeleton promoted the successful understanding of conjugated molecules. The work of R. Hoffmann and others has proven that in saturated molecules [Pg.1]

Naphthalene undergoes electrophihc substitutions at the a rather than p position. The Hueckel molecular orbital calculations show that all the carbons have the same jt electron density 1.0. This is not in agreement with the theory of organic reactions based on the Coulombic interaction that electrophilic attack occurs on the most negatively charged atom. Fukui [7] proposed the frontier orbital theory for the discrepancy between the theory and the experimental observation. The importance of... 

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

The upper part of Fig. 26 (see p. 114/115) contains several examples of localized 7r orbitals of pure type 7t 2, the prototype being naphthalene. It is seen that, even in fairly asymmetric molecular situations, the localized orbitals are still of quite pure type 7r 2. Really strong asymmetry is seen in the two molecules at the bottom of the figure, which show orbitals of the type 7r 22. The type 7t 2 would result from type ir 2 if all the contours from the right side of the orbital were pushed over to the left side. On the last figure (see Fig. 27,p. 116/117), there are further examples of pure and deformed type 7r 2. It can be seen that in sufficiently asymmetric molecular situations this type of orbital, which extends over four minor atoms, can have quite irregular forms. 

HOMO) and the subjacent HOMO - 1 depicted in Chart 3. As such, the centrosymmetric structures above accord with the optimum overlap of the naphthalene HOMO and HOMO - 1 of au and bIu symmetry, respectively, with the degenerate pair of tropylium e i lowest unoccupied molecular orbitals (LUMOs) of the same gross symmetry. On the basis of similar considerations of orbital symmetry, the EDA complexes of tropylium with neither benzene nor anthracene donors would be centrosymmetric. 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

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