MICHAEL KASHA 



57 



seems to disai^pcai in Avater (Fig. 16) . The tt ^ tt* absorptions, 

 which are naphthalene-like, are relatively unshifted with this solvent 

 change. 



Although in aqueous solution a resolved n -^ tt* shoulder is no 

 longer present, the shape of the absorption curve of quinoxaline in 

 water at about 3600 suggests that the n -» tt* absorption region is 

 still present, even though blue-shifted relative to hydrocarbon solvents. 

 We shall find this a characteristic subtlety of N-heterocyclic spectro- 

 scopy. 



Phenazine (12a) absorption spectra in hydrocarbon solvents show 

 only a long-wavelength tail, which blue-shifts upon changing the 

 solvent to ethanol, instead of a distinct resolved shoulder. This point 

 was noted also by Badger and Walker (1). Since even in ethanol 

 solvents the molecule is still non-fluorescent, having a strong phos- 

 phorescence only, the n -> it* transition is probably still the longest 

 'wavelength absorption (12a) . 



-1 



50,000 



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WAVELENGTH , A 



Fig. 1(). Near ultraviolet absorption curves ol quinoxaline at room temperature, 

 after Hirt, King, and Cavagnol (13). Curve a, in cyclohevane; curve b, in water. 



