Abstract
In addition to the underlying basic concepts and early recognition of halogen bonding, this paper reviews the conflicting views that consistently appear in the area of noncovalent interactions and the ability of covalently bonded halogen atoms in molecules to participate in noncovalent interactions that contribute to packing in the solid-state. It may be relatively straightforward to identify Type-II halogen bonding between atoms using the conceptual framework of σ-hole theory, especially when the interaction is linear and is formed between the axial positive region (σ-hole) on the halogen in one monomer and a negative site on a second interacting monomer. A σ-hole is an electron density deficient region on the halogen atom X opposite to the R–X covalent bond, where R is the remainder part of the molecule. However, it is not trivial to do so when secondary interactions are involved as the directionality of the interaction is significantly affected. We show, by providing some specific examples, that halogen bonds do not always follow the strict Type-II topology, and the occurrence of Type-I and -III halogen-centered contacts in crystals is very difficult to predict. In many instances, Type-I halogen-centered contacts appear simultaneously with Type-II halogen bonds. We employed the Independent Gradient Model, a recently proposed electron density approach for probing strong and weak interactions in molecular domains, to show that this is a very useful tool in unraveling the chemistry of halogen-assisted noncovalent interactions, especially in the weak bonding regime. Wherever possible, we have attempted to connect some of these results with those reported previously. Though useful for studying interactions of reasonable strength, IUPAC’s proposed “less than the sum of the van der Waals radii” criterion should not always be assumed as a necessary and sufficient feature to reveal weakly bound interactions, since in many crystals the attractive interaction happens to occur between the midpoint of a bond, or the junction region, and a positive or negative site.
Highlights
There is much ongoing discussion on halogen-centered noncovalent interactions and many interesting papers [1,2,3] and reviews [4,5,6,7] have appeared
We aim to provide a brief overview of the advances of the underlying concepts of halogen bonding and halogen-centered noncovalent interactions including those involving fluorine
It is often asserted that halogen bonding shares many characteristics with hydrogen bonding [54]. If taken literally this is misleading: it is only the nature of some characteristics that are somehow similar to both types of bonding interactions, which is why halogen bonding is often considered to have a familial relationship with hydrogen bonding
Summary
There is much ongoing discussion on halogen-centered noncovalent interactions and many interesting papers [1,2,3] and reviews [4,5,6,7] have appeared. To be able to visualize a crystal structure in its entirety, not just look at selected intermolecular interactions which have been deemed to be important,” and of Nangia and Desiraju (quoted in [136]), who commented that “a detailed understanding of crystal packing and crystal design depends very substantially on viewing the molecule as an organic whole,” and have suggested to view molecules as “organic wholes”, thereby fundamentally altering the discussion of intermolecular interactions through the use of a variety of novel computational and graphical tools In this short review, we aim to provide a brief overview of the advances of the underlying concepts of halogen bonding and halogen-centered noncovalent interactions including those involving fluorine. Their presence cannot and should not be neglected if the nature of the intermolecular interactions at play and their characterizations are the central focus of discussion in attempting to arrive at an understanding of molecular packing and crystal formation
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