BENTLEY GLASS 827 



plane; (7) fi — > rr* singlet-singlet transitions usually occupy the 

 longest wavelengths, leing even lower in energy than the tt -» vr* 

 transitions of N-heterocyclic molecules, although the latter have lower 

 ionization potentials; (8) the n -^ rr* lowest singlet-singlet fluores- 

 cence is cjuenched strongly; (9) a characteristic coupling of a parti- 

 ctilar molecular vibration with the electronic excitation often occurs, 

 as in pyridines and pyrimidines from a particular angular distortion 

 of the ring; and (10) radical-like behavior owing to the solitary highly 

 localized electron left in the Ji orbital after the n — > tt* promotion 

 of its mate. 



The N-heterocyclic molecules form a group of particular interest 

 to biologists. Among those sttidied so far, pyridine was first to reveal 

 n -^ TT* transitions. Pyridine manifests sharp spectral lines in the 

 2700-2900 A region in hydrocarbon solvent. These lines are absent 

 in the benzene spectrum and shift strongly to the blue in ethanol as 

 solvent. Even more striking is the sharpening of the main part of the 

 band at 2500 A upon changing from isopentane as solvent to ethanol; 

 this must be due to an n -^ tt* transition in the tt ^ n* region of 

 the spectriun. Diazines have n -^ tt* bands completely distinct from 

 the TT -^ TT* transition, e.g., pyridazine at 3500 A and pyrazine at 

 about 3300 A. Both n -^ tt* bands shift markedly in changing solvent 

 from hydrocarbon to water. The tt — > 77* bands in these molecules 

 undergo respectively a small blue shift and a small red shift under 

 the same circumstances. Other N-heterocyclic molecules exhibit only 

 a long wavelength shoulder or tail on the absorption curve at 3500- 

 3800 A, which may be identified as arising from an n ^ tt* transition 

 by virtue of its disappearance when the solvent is changed from hy- 

 drocarbon to Avater. Such molecules include quinoxaline, phenazine, 

 and quinoline. In respect to quinoline, the blue shift that accom- 

 panies the change of solvent from hydrocarbon to ethanol is scarcely 

 observable; but there is a dramatic change in emission at low tem- 

 peratures in a rigid ethanol-ether-isopentane glass solvent, and this 

 represents, according to Kasha, an actual interchange of the n,7r* 

 and 77,77* electronic states. 



Pyrimidine in hydrocarbon solvent has a distinct n -^ 77* band at 

 approximately 3000 A, shifting strongly tow^ard the blue in water. 

 The 77 ^ 77* band at 2450 A shifts but slightly. Chemical substitution 

 on the biologically important 2 and 6 positions of the ring pushes the 

 n ^ 77* and 77 -» tt* regions together, yet without altogether abolish- 

 ing the evidence of the former. Purines, too, in hydrocarbon solvent 

 have an n ^ 77* shoulder at about 3000 A which strongly shifts toward 



