Cooperative effects and energy barriers to bromonium ion transfer in cyclic (BrX)n halogen-bonded clusters (X=F, OH, or NH2; n=3–5): An ab initio study

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I···I or I···X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I···F rings are generally more stable than the corresponding I···I cycles as the I···F bond is stronger than I···I even in the simple dimer. However, I···I rings are comparable in energy to I···Cl. It is possible to construct I···I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I···X rings is 6. Red shifts are observed in the I-X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I···I chains but diminishes when I···Cl bonds are present.

Halogen-bonded haloamine trimers - modelling the X3 synthon

Halogen-halogen bonded haloamine trimers serve as model systems for the X3 synthon present in numerous crystal structures and in two-dimensional self-assembled nanoarchitectures. Halogen bonds forming the synthon are often considered to display cooperativity. Synergy effects were previously found for halogen-halogen bonded bromoamine and iodoamine tetramers. In the present study comparison between haloamine cyclic trimers and tetramers is made. The cooperativity occurring in bromoamine and iodoamine clusters is significantly weaker in the case of the trimers. The present study demonstrates that the bromoamine and iodoamine trimers display weaker cooperativity due to a smaller number of synergy components in comparison to the corresponding tetramers of stronger cooperativity. Moreover, the halogen-halogen interactions in bromoamine and iodoamine dimers with the geometries of the corresponding trimers and tetramers are examined using energy decomposition analysis methods (supermolecular, canonical EDA and SAPT) and the Kohn-Sham molecular orbital model. The results of the analysis indicate that although the interaction energy values for the dimers of the different spatial arrangement are very close to each other, their origin is substantially different. For pairs with the geometry of the trimers orbital interactions and electrostatic attraction are both weaker than for the corresponding dimers with the geometry of the tetramers. This is especially important because both donor-acceptor interactions and electrostatic attraction were previously proven to be responsible for cooperative effects occurring in the bromoamine and iodoamine tetramers.