Naphthalene orbital picture

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Figure 15.12 An orbital picture and electrostatic potential map of naphthalene, showing that the ten tt electrons are fully delocalized throughout both rings.

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. 

Problem 10.13 Deduce an orbital picture (like Fig. 10-2) for naphthalene, a planar molecule with bond angles of 120°. ... 

PROBLEM 13.18 Draw schematic orbital pictures of naphthalene and isoquinoHne. Put in the Jt electrons as dots. It is not necessary to draw the carbon—hydrogen o bonds. 

Figure 15 14 (A) Orbital picture of naphthalene, showing its extended overlap of p orbitals.

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... 

Like naphthalene, anthracene and phenanthrene are classified as aromatic on the basis of their properties. Consideration of atomic orbitals follows the same pattern as for naphthalene, and leads to the same kind of picture a flat structure with partially overlapping n clouds lying above and below the plane of the molecule. 

The structure observed for the naphthalene group in [Li (TMED) 12-naphthalene can best be described in terms of a delocalized molecular orbital (MO) picture... 

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