Intramolecular halogen bonds in 1,2-aryldiyne molecules: a theoretical study

Intramolecular halogen bonds have been the subject of several current experimental and theoretical studies. In this work, intramolecular halogen bonds in a series of 1,2-aryldiyne molecules were investigated using density functional theory calculations at the M06-2x level of theory. For comparison, some dimeric complexes between halogenated aryldiynes and quinolinyl compounds were also considered. The calculated interatomic distances and interaction angles of intramolecular halogen bonds compare fairly well with those determined experimentally, and the triangle motifs retain almost perfectly planar in all the studied molecules. Many of the well-known properties of conventional halogen bonds are reproduced in intramolecular halogen bonds: the interaction strength tends to increase with the enlargement of the atomic radius of halogens (I > Br > Cl); the attachment of electron-withdrawing moieties to halogens leads to much stronger intramolecular halogen bonds; the X···N (quinolinyl) interactions are stronger than the X···O (carbonyl) halogen bonds. On the basis of the shorter interatomic distances and the larger values of electron densities at the bond critical points, intramolecular halogen bonds become stronger in strength than corresponding intermolecular halogen bonds. However, these interactions have similar structural, energetic, atoms in molecules (AIM), and noncovalent interaction index (NCI) characteristics to traditional halogen bonds. Therefore, these interactions can be recognized as halogen bonds that are primarily electrostatic in nature. Particularly, the formation of intramolecular halogen bonds gives rise to the essential coplanarity of the molecules, whereas the two subunits in the dimeric complexes deviate from planarity to a large degree. In addition, a small number of crystal structures containing intramolecular halogen bonds were retrieved from the Cambridge Structural Database (CSD), to provide more insights into these interactions in crystals. This work not only will extend the knowledge of noncovalent interactions involving halogens as electrophilic centers but also could be very useful in molecular design and synthetic chemistry.