Quasi-atomic orbital analysis of halogen bonding interactions

A quasi-atomic orbital analysis of the halogen bonded NH3 & sdot; & sdot; & sdot;XF complexes (X = F, Cl, Br, and I) is performed to gain insight into the electronic properties associated with these sigma-hole interactions. It is shown that significant sharing of electrons between the nitrogen lone pair of the ammonia molecule and the XF molecule occurs, resulting in a weakening of the X-F bond. In addition, the N-X bond shows increasing covalent character as the size of the halogen atom X increases. While the Mulliken outer complex NH3 & sdot; & sdot; & sdot;XF appears to be overall the main species, the strength of the covalent interaction of the N-X bond becomes increasingly similar to that of the N-X bond in the [NH3X](+) cation as the size of X increases.