MICHAEL KASHA 



59 



3000 A in hydrocarbon solvent, and strongly blue-shifts in water. 

 The IT -^ IT* region at 2150 A is but slightly shifted. Even in 4 iV 

 sulfuric acid some n -^ tt* absorption remains as a shoulder. 



Fig. 18 compares pyrimidine and 2,6-dichloropyrimidine (40), in 

 which the substituents are in the positions corresponding to those of 

 the biologically important pyrimidines. The effect of substitution is 



45.000 



WAVENUMBERS , CM 

 40,000 



-I 



35,000 



2100 



2400 2700 



WAVE LENGTH 



3000 



o 

 A 



Fig. 18. Near ultraviolet absorption curves of pyrimidine in water (curve a), and 

 2, 6-dichloropyrimidine in methanol (curve b), at room temperature. After Uber 

 and Winters (40). 



invariably to push together the n -^ tt* and tt -^ it* absorption regions 

 of such a molecule. However, it is to be noted (curve b. Fig. 18) 

 that even in the spectrum of the disubstituted pyrimidine in water, 

 there is a long-wavelength shoulder at 2800-2900 which must be due 

 to the n -^ tt* absorption band. 



An analogous situation is found for 9-methylpurine (25, 25a) , Fig. 

 19. Note that the n -^ tt* shoulder of this molecule in hydrocarbon 



