ULTRAVIOLET ABSORPTION SPECTRA 



171 



WAVE LENGTH, A 

 3250 3000 2750 2500 



2250 



3.0 - 



2.5 ■ 



2 - 



o 

 o 



ABSORPTION AND CHEMICAL CONSTITUTION 

 EMPIRICAL CORRELATIONS 



The empirical correlation of the spectral position of absorption bands 

 in the ultraviolet with certain chemical structures was begun about 1885 

 with the work of Hartley (1885) and has been steadily continued and 

 expanded as improvements in technique have simplified the task of 

 measuring absorption spectra (Braude, 1945; Lewis and Calvin, 1939; 

 Ferguson, 1948; Erode, 1943; Jones, 1943). The broader long-recog- 

 nized empirical correlations have now been given a theoretical basis by 

 the development of approximate 

 wave-mechanical methods of calcu- 

 lation of electronic-energy levels in 

 complex molecules. 



These correlations early indi- 

 cated that the absorption bands 

 of compounds, composed exclu- 

 sively of saturated linkages, oc- 

 curred generally below 2000 A, 

 usually in the vacuum ultraviolet 

 below 1850 A. The long-wave 

 arms of such peaks extended above 

 2000 A, increasing in amplitude 

 with increase in size of the molecule. 

 Because many spectrographs and 

 spectrophotometers do not record 

 below 2000-2200 A, these long- 

 wave limbs of bands of saturated 

 compounds, rising in absorption 

 with decreasing wave length, are 

 often referred to as "end" 

 absorption. 



Compounds with single unsatu- 

 rated bonds such as C'=C, C=0, 

 or C=N were found to have ab- 

 sorption bands, usually weak, in 

 the region 1900-3000 A, the actual 

 wave length being dependent on the adjacent parts of the molecule. 



Strong absorption bands in the region 2000-4000 A are always corre- 

 lated with molecular structures containing chains or rings of conjugated 

 double bonds; in general, the larger the conjugated structure, the stronger 

 is the absorption and the longer the wave length of the maximum absorp- 

 tion. Ring structures with conjugated double bonds, as in aromatic 

 compounds, often possess particularly high absorption. 



0.5 - 



30,000 



35,000 



40,000 



45,000 



Fig. 5-4. Ultraviolet absorption spectra 

 of some .simple derivatives of benzene. 

 I, aniline; II, phenol; III, chlorobenzene ; 

 IV, benzene; all in heptane. (Adapted 

 from Wolf and Herold, 1931.) 



