180 



RADIATION inOLOGY 



ENVIRONMENTAL FACTORS INFLUENCING ABSORPTION 



In general, a variation of any factor that influen(;e^s the electronic (^on- 

 fi}z;in'ation of the absorbing molecules, either uniformly or with a statistical 

 (listrii)ution, will affect the absorption spectrum (Sheppard, 1942). In 

 addition, certain factors may alter the technical conditions of the absorp- 

 tion measurement and thereby affect the spectrum. 



SOLVENT 



The choice of solvent can influence the position, width, and intensity of 

 absorption bands. Changes in position are to some degree correlated 



4 8 



I 6 



V = 900 1000 HOC 1200 1300 1400 



I/X= 30,000 33,330 36,670 40,000 43,330 46,660 



A = 3333 3000 2727 2500 2308 2143 



1500 1600 f 



50,000 cm'' 

 2000 A 



Fig. 5-10. The absorption spectrum of phthalic anhydride. I, in hexane; IT, in 

 alcohol. {Mcnrzcl 1927.) 



with the dielectric constant of the solvent according to Kundt's rule which 

 states that, with increasing dispersion of the solvent, the absorption 

 maximum is shifted toward longer wave lengths. Although Kundt's rule 

 is generally valid for nonpolar solvents, there are serious deviations with 

 polar solvents (Sheppard, 1942). 



This shift may be interpreted as indicative of the increased role played 

 by ionized structures in resonance stabilization of the excited state in 

 media of high dielectric constant (Wheland, 1944) or in terms of the 

 influence of the reaction field of the oscillating dipole on the electric field 



