ULTRAVIOLET ABSOUI'TION ttl'lOCTUA 



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decreases with increasing); orbital energy. As a result, the transitions 

 induced by absorption in large conjugated systems, which are transitions 

 between high-level orbitals, re((uire less energy, and thus a longer wave 

 photon, than the transitions induced by absorption in smaller conjugated 

 systems. This deduction accounts generally for the increase in wave 

 length of the absorption peaks with increase in size of the conjugated 

 system. 



Absorption of radiation may also induce transitions from the highest 

 filled molecular orbital to some of the higher unfilled orbitals; these tran- 



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(C) 



Fi<i. 5-7. Illustration of the a and tt atomic orbitals of benzene and the fusion of the n 

 atomic orbitals to form the lowest energy molecular orbital, (a) a orbitals; ih) ir 

 atomic orbitals; (c) tt molecular orbitals. (Coulson, 1947.) 



sitions correspond to absorption bands at shorter wave length than those 

 of the bands just described. 



The excitation induced by absorption usually involves the transition 

 of an electron from a "bonding" to an "antibonding" orbital without 

 change of spin direction.- The antibonding orbital introduces an addi- 

 tional nodal plane into the function specifying the probability distribution 

 for the position of the electron. The absorption of double bonds or of 

 conjugated chains involves excitation of a tt electron, an electron of which 

 the probability distribution already contains a node in the plane of the 

 bond or chain. Hence excitation introduces a new node, which lies 

 between the atoms of the bond or two of the atoms of the chain. Thus 



^ Transitions involving a change in the net electron spin of the molecule, i.e., 

 "singlet-triplet transitions," are usually of low probability (Kasha, 1947). 



