92 



RADIATION BIOLOGY 



from 6100 to 7100 A. (These systems are too large for the weak n-g 

 transitions introduced by the aza nitrogen to be seen.) 



Kuhn showed that this shift of the 7r-transitions could be understood 

 by considering the symmetries of the orbitals, as sketched in Fig. 2-13. 

 The aza substitution attracts electrons. By perturbation theory, this 



Table 2-2. Positions of n-g Transitions in Different Systems" 



" Transitions probably ^A-'^IJ except where ^A-MF is indicated. Parentheses indi- 

 cate estimates or hidden and unidentified transitions; question marks indicate doubt- 

 ful assignments of observed bands. Wave lengths approximate (from Piatt, 1951a; 

 Braude, 1945; Braude ei al., 1947; Barany e( al., 1949, MConnell, 1952). 



'' The symbol x indicates that the group cannot go into a phenyl ring. 



lowers the energy of orbitals whose wave functions and electron densities 

 are large at the aza nitrogen, and it leaves unchanged the energy of those 

 whose wave functions are zero or small there. Since the aza atom is at 

 the center of a symmetrical molecule, all even orbitals will be lowered, 

 but odd ones will be unchanged, as shown by the short arrows in the 

 center of the figure. In these particular 16-electron systems, the /-level 

 is odd and unchanged and the g'-level is even and lowered, producing the 

 red shift of 1000 A observed. 



