By analysing the properties of the electron density in the structurally simple perhalogenated ethanes, X3C-CY3 (X, Y = F, Cl), a previously overlooked non-covalent attraction between halogens attached to opposite carbon atoms is found. Quantum chemical calculations extrapolated towards the full solution of the Schrödinger equation reveal the complex nature of the interaction. When at least one of the halogens is a chlorine, the strength of the interaction is comparable to that of hydrogen bonds. Further analysis shows that the bond character is quite different from standard non-covalent halogen bonds and hydrogen bonds; no bond critical points are found between the halogens, and the σ-holes of the halogens are not utilised for bonding. Thus, the nature of the intramolecular halogen⋯halogen bonding studied here appears to be of an unusually strong van der Waals type

We are grateful to Dr Eduard Matito, Dr Pedro Salvador, and Dr Raphael Berger for enlightening discussions. CSC - the Finnish IT Centre for Science hosted part of the calculations. This work has been supported by the Ministerio de Ciencia e Innovacion (MICINN projects JCI-2009-05953, CTQ2011-25086/BQU), the Academy of Finland (project 136079), MICINN and the European Fund for Regional Development (FEDER grant UNGI08-4E-003), and the DIUE of the Generalitat de Catalunya (project 2009SGR528)