170 



RADIATION BIOLOGY 



I'or a (limbic bond, in (he cxcitod .state, there is a greater proliahility of the 

 electron hciiifj; on one or llic other atom of the bond than of ils Ix-inj!; in 

 the interatomic region. The probability distributions and associated 



/ + 



+ 



+ 



+ 



c -< 



+ 



yc 



' 1 1 



/ \ 



c -< 



>- c 



- - I + 



+ 



! + 1 - 



>-c 



A 



>C' 



V + 



:a 



+ 



•c-^ 



+ )! - 



>-c 



li 

 I \ 



I / 

 w 



II 



X 



- )1 + 1! - 



c -< 





c -< 



- ; + 



Fig. 5-8. A schematic representation of the wave functions describing the four molecxi- 

 hir orbitals that may be synthesized by Hnear combinations of the four tt atomic 

 orbitals of butadiene CHo=CH— CH=CH.,. On and outside the dotted Hues the 

 wave functions are practically zero. Within the dotted lines the wave functions have 

 finite values, with sign as indicated, and thus these are the regions with an appreciable 

 probability for the presence of an electron (the pluses and minuses refer only to the 

 sign of the electronic wave function and not to the charge). (Reproduced from 

 Chemical Aspects of Light, by E. J . Bowen, copyright, 1940, by Oxford University Press.) 



./^v 



^n/ V 





> \ 



/^s 



V 



) (, 



.-s 



1 ( 



-* I- 



V 



Fig. 5-9. The molecidar orbitals of anthracene. (Bmren, 1950.) 



nodal planes for the molecular orbitals of a simple conjugated chain 

 (butadiene) are shown in Fig. 5-8 (Bowen, 1940). Absorption induces a 

 transition of an electron from orbital 2 to orbital 3. 



In aromatic molecules the new nodal plane may intersect the molecular 



