Exploring the (Very Flat) Potential Energy Landscape of R-Br•••π Interactions with Accurate CCSD(T) and SAPT Techniques

Halogen bonds involving an aromatic moiety as an acceptor, otherwise known as R-X center dot center dot center dot pi interactions, have increasingly been recognized as being important in materials and in protein-ligand complexes. These types of interactions have been the subject of many recent investigations, but little is known about the ways in which the strengths of R-X center dot center dot center dot pi interactions vary as a function of the relative geometries of the interacting pairs. Here we use the accurate CCSD(T) and SAPT2+3 delta MP2 methods to investigate the potential energy landscapes for systems of HBr, HCCBr, and NCBr complexed with benzene. It is found that only the separation between the complexed molecules have a strong effect on interaction strength while other geometric parameters, such as tilting and shifting R-Br center dot center dot center dot pi donor relative to the benzene plane, affect these interactions only mildly. Importantly, it is found that the C-6v (T-shaped) configuration is not the global minimum for any of the dimers investigated.