Anomalous Absorption and Photolysis of Iodobenzene

Abstract

IODOBENZENE, as a vapour or in solution, is unstable to light, like the alkyl iodides, and lends itself to the same sort of photolytic study. Moreover, we have found that the presence of nitric oxide accelerates the decomposition, as is the case with methyl iodide¹. The alkyl iodides show continuous absorption spectra in the near ultra-violet ; but it would not be expected from preliminary considerations that iodobenzene would do so. Benzene and its simpler derivatives have been subjected to a great number of absorption measurements. The bands in the 2600 A. region found in benzene are considered to be due to a forbidden transition determined by symmetry considerations². Substitution of a hydrogen atom by a fluorine, chlorine or bromine atom alters the symmetry, the band system becomes stronger and shifts towards the red end of the spectrum. The red shifts³ in the case of fluoro-, chloro- and bromo-benzene for the ν(0—0) band are respectively -271, -1,037 and -1,093 cm.^-1. We might have expected the ν(0—0) band of the iodo-derivative to have displayed a red shift not very much larger than -1,093 cm.^-1 ; but substitution of the hydrogen by the iodine produces an anomalous result. There is still absorption in the 2600 A. region ; but the discrete bands have disappeared, and only a continuous absorption is observed⁴.