Raman and infrared frequency shifts proceeding from ionization of perhalo-p-benzoquinones to radical anions

Abstract

The vibrational assignment of the fundamental modes of 2,3,5,6‐tetrachloro‐ and 2,3,5,6‐tetrabromo‐p‐benzoquinone (chloranil and bromanil, respectively) radical anions is reported and compared with that of the neutral molecules. For this purpose the single crystal Raman and infrared spectra of bromanil are also given and interpreted. The absence of the vibronic effects present in the infrared spectra of some free radical salts is demonstrated, and the vibrational spectra of the potassium salts of both radical ions are shown to be interpretable in terms of substantially unperturbed molecular structures even in the case of crystalline samples. The vibrational assignments and their normal coordinate analysis allow one to obtain most of the relevant information on the frequency shifts proceeding from the addition of an extra electron to the perhalo‐p‐benzoquinone structure.