Concise Physical Chemistry

c18 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

ELECTRONIC SPECTRA 293

9

4

e e 0

e e

e e

3

5

FIGURE 18.4 Electronic promotion in alkenes.

the square of the length in the denominator. The extended double-bond system in 1,3-

butadiene decreases the energy and frequency and, because of the inverse relationship

betweenνandλ, it increases the wavelength. Experimentally, this predicted shift to

longerλis verified.

It is true in general that extended conjugated double-bond systems absorb at

wavelengths that tend toward the longer wavelengths in the UV, and even into the

visible region of the electromagnetic spectrum. This can be seen in the orange color

of carotene from carrots and the red of lycopene, the principal natural coloring agent

in tomatoes. Both of these molecules have long conjugatedπelectron chains and

follow the qualitative trend predicted by the free electron model.

Double Bonds

0 2 4 6 8 10

Wavelength (nm)

100

150

200

250

300

350

400

450

500

FIGURE 18.5 Absorption wavelengths of conjugated polyalkenes. The wavelengths range

from the far ultraviolet well into the visible region. (Taken in part from Streitwieser, 1961.)