Halogen-substitution effect on the optical absorption bands of uracil.

The ultraviolet absorption spectra of uracil and its 5-halogenated derivatives have been in­ vestigated in regard to the electron attracting properties of the substituents. It could be shown that the position of the two absorption bands is proportional to the inverse of the electronegativity; the extinction coefficients are a linear function of the electron affinities. In this way, the red shift obtained upon substitution with halogens can be explained. Also, the decrease in absorbance of the absorption bands at A >250 nm, occuring concomitant­ ly, is understandable. The increase in absorbance with increasing electron affinity, as observed in the case of the absorption bands at 2 < 2 50 nm, due to a higher pi —pi* excitation.