Physical Chemistry , 1st ed.

for can be determined from atomic spectra. Since a specific value for is not

necessary in understanding the patternof the electronic states, its value is

not usually a matter of concern. (For carbon atoms,is about 1120 kJ/mol,

which is much larger than .)

15.10 Benzene and Aromaticity

The Hückel approximation is especially useful in understanding the chemical

stability of benzene, and by extension other aromatic compounds. Recall that

benzene (Figure 15.20) is more stable than expected for a “cyclohexatriene,”

and its chemistry is representative of an entire class of aromatic hydrocarbons

546 CHAPTER 15 Introduction to Electronic Spectroscopy and Structure

2 , (^3)
4 , (^5)
(^6)
(^1)
Figure 15.20 The orbitals of benzene. 1 and the degenerate pair  2 and  3 are occupied
in the ground electronic state (see Figure 15.21).