90 



RADIATION BIOLOGY 



The spectra of vitamin A and the carotenes (Fig. 2-1) are obviously 

 standard examples of alkyl-substituted all-trans polyenes. 



Conjugated Hetero Replacement. The situation is different if we replace 

 some of the carbon atoms in the conjugated system by conjugated hetero 

 atoms, ^N, =N, or =0. These atoms have nonbonding electrons or 

 n-electrons in high n-orbitals, as shown in Fig. 2-11. These orbitals may 

 even lie above the highest x-orbitals, or /-orbitals, in small conjugated 

 systems, although the / shell in a large enough conjugated system will 

 rise above the ri-energy, which changes httle with the size of the system. 



50,000 



B^' 



Fig. 2-12. Change of n-g (n) and f-g (tt) transition frequencies with increasing size of 

 a conjugated system. Figures in columns give extinction values. (From Piatt, 

 195Ja.) 



The longest-wave-length transition in a small system will then be of 

 n-g type. With increasing size of the conjugated system the gr-orbital 

 will move, but not the n, so that the transition frequency will move to 

 the red only about half as fast as the f-g transition, and will be concealed 

 by the latter in large systems, as shown in Figs. 2-11 and 12. This is 

 one of the main identif,ying properties of 7i-g transitions. 



The symbol . . . n'^ must be added to the 7r-configuration symbols in 

 such molecules, as shown in Fig. 2-7 and Table 2-1. The ground state 

 becomes . . . dH-pn-\ iha J-g excited configuration, . . . d~e-fn'^g; etc., 

 each with the same states as before. But we must now add the con- 

 figuration from n-g excitation, . . . d''ej-ng. This gives singlet and 

 triplet states that we shall call ^U and H^ when the transition from 

 ground is forbidden and 'ir and ^W when it is allowed. Two of these 

 states are indicated in Fig. 2-7, and some typical absorption intensities 



