Mössbauer effect of 129I in n–σ and π–σ charge-transfer complexes of iodine in the frozen solution

Abstract

The Mössbauer effect of 129I was applied to investigate charge-transfer complexes of the n–σ and π–σ types in the carbon disulfide frozen solution at 16 K. In the case of n–σ type complexes such as triethylamine–iodine and pyridine–iodine, two chemical species of iodine, which correspond to the bridging and terminal iodines, were observed in the Mössbauer spectra. It is elucidated that a charge-transfer bond is described as a mixture of a four-electron three-center bond and a covalent bond involving dσpσ hybrid orbitals in the bridging iodine. On the other hand, for the π–σ type complexes such as benzene–iodine and methylated benzene–iodine the Mössbauer spectra show only one chemical species of iodine. The acceptor orbital of iodine is thought to be described by an spd hybrid orbital. That is, a small amount of s and d characters contributes to the formation of the charge-transfer bond. From the Mössbauer parameters the charge densities and the charge distributions of iodine in these complexes were obtained.