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147 



jverlappin 

 it electron 

 above the 

 plane of 

 the atoms 



Structural r, 



formu la with 

 conjugated 

 double bonds q 



// 



\ // 



n-T excitation 



2 s electrons 

 an unshared pair 

 (non bonding) 



Figure 6-10. The n-7r Electronic Transition (schematic). 



the number of unshared electrons). When excited, however, the promoted 

 electron, now in a formerly empty ir orbital, is unpaired; S = 3, and the 

 molecule is said to be in a "triplet 1 ' state. Triplet states are important be- 

 cause they sometimes retain the extra energy, without radiating it, for rela- 

 tively long periods of time. Thus molecules in the triplet state sometimes 

 have time to collide with others which are similarly excited, and the total 

 energy of the collision may be sufficient to cause the isomeric or mutation 

 reaction. 



Based on the work of M. Kasha, Reid 10 has listed a few types of molecules 

 (containing N, O, P, S) whose n — it transition and the subsequent triplet 

 states probably are energy carriers in biological processes: 



Amides 

 Aldehydes and 



ketones 

 Amides 

 Quinones 

 Thioketones 



Pyridines 

 Diazines and 



triazines 

 Azo- and 



diazo-compounds 

 Nitroso-compounds 

 Pyrimidines 



possibly 



Carbonates 



Nitrates 



Nitro-compounds 



The mechanism of some isomeric reactions in which a triplet excited state 

 is an intermediate is now fairly well understood. For large macromolecules, 

 however, pertinent information remains for the future. Nevertheless the 

 direction and importance of such work is now clear. 



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There are now four types of experiment which support the contention thai 

 genetic information is carried by the nucleic acids, DNA and RNA. There 

 is still little direct evidence from any species higher than virus or bacterium. 



