| Unconventional non-covalent interactions such as halogen, chalcogen, and tetrel bonds are gaining interest in several domains including but not limited to drug design, as well as novel catalyst design. Non-covalent interactions are known as weak forces of interactions when they are considered on an individual basis, but on a molecular basis, these effects become important such that their prevalence can be seen in the construction of large biomolecules like proteins, DNA and RNA. In this work, the fundamental aspects of these interactions are looked upon using ab initio and Density Functional Theory (DFT). An essential aspect of chalcogen bonds involving Sulfur as donor atom with nitrogen, oxygen and pi-system as electron sources was examined. These bonds are strong with binding energy that varies from a minimum of 3 kcal/mol in some pi-system to 19 kcal/mol in primary amine systems. Decomposition of the total interaction energy reveals that the induction energy constitutes more than half of the total interaction energy. The shortness and strength of some of the chalcogen bond interactions suggest these interactions may in some cases be described as weak covalent bonds. A comparative study of pi-hole tetrel bonding with sigma-hole halogen bonds in complexes of XCN (X = F, Cl, Br, I) and ammonia shows that the pi-hole geometry if favored for X = F, and the sigma-hole structure is preferred for the heavier halogens. Also, the potential use of these non-covalent interactions in organic catalysis was explored. The energy barrier of the Aza-Diels-Alder reaction is substantially lowered by the introduction of an imidazolium catalyst with either a Hydrogen or halogen (X) atom in the 2-position. |
