Abstract
Periodic local vibrational modes were calculated with the rev-vdW-DF2 density functional to quantify the intrinsic strength of the X-I⋯OA-type halogen bonding (X = I or Cl; OA: carbonyl, ether and N-oxide groups) in 32 model systems originating from 20 molecular crystals. We found that the halogen bonding between the donor dihalogen X-I and the wide collection of acceptor molecules OA features considerable variations of the local stretching force constants (0.1–0.8 mdyn/Å) for I⋯O halogen bonds, demonstrating its powerful tunability in bond strength. Strong correlations between bond length and local stretching force constant were observed in crystals for both the donor X-I bonds and I⋯O halogen bonds, extending for the first time the generalized Badger’s rule to crystals. It is demonstrated that the halogen atom X controlling the electrostatic attraction between the -hole on atom I and the acceptor atom O dominates the intrinsic strength of I⋯O halogen bonds. Different oxygen-containing acceptor molecules OA and even subtle changes induced by substituents can tweak the (X-I) charge transfer character, which is the second important factor determining the I⋯O bond strength. In addition, the presence of the second halogen bond with atom X of the donor X-I bond in crystals can substantially weaken the target I⋯O halogen bond. In summary, this study performing the in situ measurement of halogen bonding strength in crystalline structures demonstrates the vast potential of the periodic local vibrational mode theory for characterizing and understanding non-covalent interactions in materials.
Highlights
Halogen bonding (D-X· · · Y) is one important type of non-covalent interaction between a donor halogen atom X (F, Cl, Br or I) and an electron-rich atom/group Y [1]
This work was focused on the X-I· · · OA halogen bonding in crystals, of which the structures were retrieved from two major databases collecting crystal structures — the Cambridge Structural Database (CSD) [113] and the Crystallography Open Database (COD) [114] by performing searches with the keywords “dihalogen”, “interhalogen”, “diiodine” or their synonyms. 62 unique crystal structures with halogen bonding, matching the search keywords were obtained and manually screened based on the following criteria:
In particular with regard to a more consistent and straightforward analysis, we limited our investigation to I2 and iodine monochloride (ICl) molecules with iodine atom as donor participating in the halogen bond
Summary
Halogen bonding (D-X· · · Y) is one important type of non-covalent interaction between a donor halogen atom X (F, Cl, Br or I) and an electron-rich atom/group Y (e.g., atoms with lone pair electrons including N, O, P and S) [1]. The underlying physical nature of this important proof results from the fact that k na equals the curvature of the potential energy surface (PES) in the direction of the bond stretching, determined via an infinitesimal change of the bond length and followed by the relaxation of all other atoms in the molecule [59,60] In this way, k na absorbs all electronic effects contributing to an individual chemical bond and serves as unique measure of the intrinsic strength of a chemical bond and/or weak chemical interaction based on vibrational spectroscopy. Important findings of this work are summarized in the Conclusions section and a future perspective is given
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