MICHAEL KASHA 45 



For forrnaldeliyde, in which only the C-O o--orbitals, the Tr-orbitals, 

 and only the highest //-orbital are considered as being important in 

 presently observable nltraviolet electronic transitions, the lowest four 

 electronic state configurations are given in the top portion of Fig. 10. 

 Only singlet states are considered here, as indicated by the superscript 

 prefix on the general state symbol r. The molecular state synunetry 

 would in general replace r, when established. The three different 



STATE CONFIGURATION SYMMETRY SELECTION RULE, 



POLARIZATION 



Formaldehyde (Cgv ) 



Fig. 10. Slate symmetries and electronic dipole transition selection 

 rules for .several heteromolecules. 



types of orbital promotion, ?? -^ tt*, n —> a*, and tt -> tt* are indicated 

 by corresponding electronic configurations as shown in the second 

 column of Fig. 10. The subscript suffix on the state symbol is used 

 as a convenient index of the orbital promotion type. The derivation 

 of the overall state synunetry will be given below. 



For pyridine, in which only the six 77-orbitals and the w-orbital are 

 involved in near ultraviolet spectra, two different low-energy n -^ tt* 

 and TT —> TT* promotions are considered, each leading to different 

 state symmetries, as will be described below. 



For purine, with its lower symmetry, all n -^ tt* promotions lead 

 to excited configurations and states of the same symmetry, and all 



