Dear Colleagues,

This Special Issue of Crystals reflects the very significant role that crystallography has played in the recognizing the existence of halogen bonding and in arriving at an understanding of what had sometimes been described as an enigma.  While halogen bonds had already been observed in the 19th century, a major advance was the series of crystallographic studies by Hassel et al. in the 1960s. They characterized complexes between covalently-bonded halogen atoms and oxygen/nitrogen Lewis bases, i.e., attractive interactions between two ostensibly negative sites.

Some years later, surveys of crystal structures by Murray-Rust et al. revealed close contacts, direction-dependent, between halogen atoms and both electrophiles and nucleophiles. The explanation came in 1992 with the surprising observation by Brinck et al. that covalently-bonded halogen atoms can have regions of positive electrostatic potential along the extensions of the bonds, and negative potentials on their lateral sides. These positive regions were labeled σ-holes by Clark et al. in 2007. Between 2007 and 2009, Murray et al. showed that covalently-bonded atoms of Groups IV-VI can also have positive σ- holes. This accounts for Parthasarathy et al. finding, in surveys of divalent sulfide crystals, that the close contacts of the sulfurs follow patterns analogous to those of halogens.

The importance of halogen bonding and other σ-hole interactions (which includes hydrogen bonding) in biological systems and in areas such as the design of new materials is now well established and continues to increase. This Special Issue is intended to present an overview of current activity.  While the emphasis is upon such interactions in crystals, related theoretical and computational analyses have played a key part in the development of the field, which this Special Issue should reflect.

Prof. Dr. Peter Politzer
Prof. Dr. Jane S. Murray
Guest Editors

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• halogen bonding
• σ-hole bonding
• electrostatic potentials
• nature of noncovalent interactions