Halogen Bonding and Other Iodine Interactions in Crystals of Dihydrothiazolo(oxazino)quinolinium Oligoiodides from the Electron-Density Viewpoint

The spatial organization of electron density in dihydrothiazolo(oxazino)quinolinium crystals with oligoiodide anions of various structures has been studied on the basis of 3D periodic KohnSham calculations. The combination of QTAIMC and the analysis of one-electron potential and electrostatic potential has revealed the significant differences between halogen bonds (Type II interactions) and van der Waals (Type I) interactions for iodine atoms in crystalline environment. The traces of s-holes in electrostatic potential on the zero-flux interatomic surfaces of iodine moieties are the distinctive feature of halogen bonding; they do not appear in the weak van der Waals I...I interactions at all. The analysis of superposition of the gradient fields of the electron density and electrostatic potential has allowed detection of the strong electron redistribution along the oligoiodide chain [I-3...I-I...I-3]; the electron density is shifted from I-3 moiety to the cation via iodine molecule I-2 as a mediator. The quantitative relationship between the experimentally measured dissociation energy D-e(I-I/I...I) and the kinetic energy density at the bond critical point in the whole range of observed iodine interactions has been established.