32 LIGHT AND LIFE 



II. Definition of t? ^ tt* Transitions 



A. Empirical Classifications (Burawoy's K and R bands) 



Burawoy (4, 4a) developed a very general classification of electronic 

 absorption bands in polyatomic molecules based on empirical classi- 

 fication of band displacements due to solvent effects and substitution 

 effects. According to Burawoy, K-bands (konjugierte) arose from 

 conjugated electronic systems, and could be characterized by (a) 

 high intensity, (b) red-shifts (bathochromic) in solvents of higher 

 dielectric constant, and (c) red-shifts upon the introduction of con- 

 jugative electron-donating substituents ( — NHo, — Ni?2» — OH, — SH) 

 on the molecule. This broad group of electronic transitions is now 

 associated with orbital promotions of TT-electrons in conjugated mole- 

 cules, or 77 — > 77* transitions (17) . 



According to Burawoy, R-bands (radikal-artige) were associated 

 with a radical-like state asserted to be present in the ground state for 

 certain molecular groupings. These bands were characterized em- 

 pirically by (a) low intrinsic intensity (e < 2000) , (b) blue-shifts 

 (hypsochromic) in solvents of higher dielectric constant, (c) dis- 

 appearance of the absorption bands in acid media, and (d) blue- 

 shifts upon introduction of conjugative electron-donating substituents 

 ( — NHo, —NR.,, — OH, — SH) on the molecule. It is now known 

 that Burawoy's interpretation of the origin of 'R'-bands is incorrect, 

 and that electronic transitions corresponding to this latter set of 

 empirical classifications arise in most cases from the promotion of 

 "lone-pair" electrons (n) to antibonding Tr-orbitals in polyatomic 

 molecules (u -^ 77* transitions, 17). 



Nevertheless, Burawoy's empirical classification is a very useful 



H2 

 H2C CH2 



CH, — C,v^ ^C>. /3-Cyclocitral 



^ ^C^ (CH3)2 



one, and was applied by him to many compounds containing carbonyl 

 groups (aldehydes, ketones ) and thiocarbonyl groups (thioketoncs) , 

 as well as azo-, nitro-, nitroso-, and nitrite-groups. Fig. 1 shows ab- 



