A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

The covalent nature of strong N−Br⋅⋅⋅N halogen bonds in a cocrystal ( 2 ) of N ‐bromosuccinimide ( NBS ) with 3,5‐dimethylpyridine ( lut ) was determined from X‐ray charge density studies and compared to a weak N−Br⋅⋅⋅O halogen bond in pure crystalline NBS ( 1 ) and a covalent bond in bis(3‐methylpyridine)bromonium cation (in its perchlorate salt ( 3 ). In 2 , the donor N−Br bond is elongated by 0.0954 Å, while the Br⋅⋅⋅acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å −3 along the Br⋅⋅⋅N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å −3 for the Br⋅⋅⋅O contact in 1 , and approximately 0.7 e Å −3 in both N−Br bonds of the bromonium cation in 3 . A calculation of the natural bond order charges of the contact atoms, and the σ*(N1−Br) population of NBS as a function of distance between NBS and lut , have shown that charge transfer becomes significant at a Br⋅⋅⋅N distance below about 3 Å.