14.2 : 1/8

14.2 Absorption Spectraof Alkenes and Aromatics

• spectral data for linear polyenes• spectral data for linear, fused aromatics• spectral data for non-linear, fused aromatics• spectral data for linear polyphenyls• quite often the absorption spectrum of a new compound can be estimated by comparison to known analogs

Linear Polyenes

14.2 : 2/8

120,000328decapentaene5?304octatetraene4

35,000268hexatriene321,000217butadiene2

?163ethylene1εmax (M-1 cm-1)λmax (nm)namen

CH CHn

• as the number of double bonds increases, the long wavelength absorption shifts to higher values (called a red-shift)• the molar absorptivity increases as the molecular orbital size increases• to anticipate the spectrum, use the number of conjugated doublebonds, i.e. CH2=CH-CH2-CH=CH2 has a spectrum closer to ethylene than butadiene.

Linear Fused Aromatics (1)

14.2 : 3/8

10,000471tetracene

10,000357anthracene

320315naphthalene

220255benzene

εmax (M-1 cm-1)λmax (nm)namestructure

• as the number of fused rings increases, the long wavelength absorption shifts to higher values• the long wavelength transition is forbidden in benzene and naphthalene, but allowed in anthracene and tetracene• to anticipate the spectrum use the number of conjugated double bonds, i.e. diphenylmethane has a spectrum that resembles toluene

Linear Fused Aromatics (2)

14.2 : 4/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951.

14.2 : 5/8

Non-Linear Fused Aromatics (1)

37,000437perylene

120370pyrene

800360chrysene

1703703,4-benzo

phenanthrene

ε0-0 (M-1 cm-1)0-0 band (nm)namestructure

• the 0-0 band appears at lower wavelengths than would be predicted by the number of fused rings (379 for anthracene and 479 for tetracene)• the first three have band positions similar to anthracene and molar absorptivities similar to naphthalene• perylene has properties between anthracene and tetracene

Non-Linear Fused Aromatics (2)

14.2 : 6/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951.

Linear Polyphenyls (1)

14.2 : 7/8

4,000294340p-quaterphenyl4

3,300276320p-terphenyl3

1,600248288biphenyl2

220255264benzene1

εmax (M-1 cm-1)λmax(nm)0-0 band (nm)namen

n

• as the number of conjugated rings increases, the 0-0 band shifts to higher wavelengths• the increase in wavelength is not as fast as the polyenes or linear aromatics because of the bond between the rings is twisted• the spectrum is featureless because thermally induced oscillation about the twist angle adds width to the vibronic bands• the molar absorptivity increases because the number of double bonds is increasing

Linear Polyphenyls (2)

14.2 : 8/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951.