MICHAEL KASHA 



43 



The orbital order lor the vr-orbitals of pyrimidine is not the same 

 as that lor pyridine, in so far as the nodal planes of the tto and tts 

 and also the 77-4* and tt.-,* pairs are concerned; this explains the dif- 

 ferent symmetry classifications of these pairs of orbitals for the two 

 molecides, as recorded in Fig. 8 (in pyrimidine, e.g., the TT-orbital with 

 a nodal plane containing the z-axis is lower in energy than the 

 TT-orbital with a nodal plane perpendicular to the z-axis, cf. 772 and 

 77-;{ of Fig. 4) . The orbital order for the pyrimidine Tr-orbitals was 

 taken from the work of McWeeny and Peacock. In both pyridine 



n( 



Hb 



Fig. 9. Schematic contour diagrams of the ?!-orl3itals of pyrimidine. 



and pyrimidine that orbital lies lowest, of otherwise equivalent pairs, 

 which residts in the greatest electron density on the more electro- 

 negative N-atoms versus C-atoms. 



For purine, with only two symmetry operations, it is immediately 

 evident that the Tr-orbitals are all antisymmetric with respect to the 

 o"/, reflection plane, and therefore all Tr-orbitals of purine transform 

 as a" species. On the other hand, all 77-orbitals of purine are sym- 

 metric with respect to the reflection, therefore are classified as a'. 

 There are nine ring atoms with 2prr atomic orbitals which build up 

 the molecidar vr-orbitals upon molecular formation: there are con- 

 sequently 9 independent molecular vr-orbitals for the purine molecule. 

 In the next section the electronic configurations and states of the 



