Tuning the C–X…π interaction of benzene–chloroacetylene complexes by aromatic substitutions

C-I···π Halogen Bonding Driven Supramolecular Helix of Bilateral N-Amidothioureas Bearing β-Turns

We report the first example of C-I···π halogen bonding driven supramolecular helix in highly dilute solution of micromolar concentration, using alanine based bilateral I-substituted N-amidothioureas that contain helical fragments, the β-turn structures. The halogen bonding interactions afford head-to-tail linkages that help to propagate the helicity of the helical fragments. In support of this action of the halogen bonding, chiral amplification was observed in the supramolecular helix formed in acetonitrile solution. The present finding provides alternative tools in the design of self-assembling macromolecules.

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⋅⋅⋅π 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⋅⋅⋅π interactions vary as a function of the relative geometries of the interacting pairs. Here we use the accurate CCSD(T) and SAPT2+3δMP2 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⋅⋅⋅π 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.

C-H···π interactions and the nature of the donor carbon atom

The influence of multiple substituents (F, CH3, NO2, CN, Cl, OH and NH2) on the C-H···π interaction in benzene-ethylene complex was investigated using the estimated CCSD(T) method and complete basis set limit. The results were compared with our earlier reported complexes of benzene-acetylene and benzene-methane, thus completing the sp, sp(2) and sp(3) series of C-H donors. The stabilization energy values for multiple fluoro-substituted benzene-ethylene complexes are found to be very close to those of the multiple fluoro-substituted benzene-methane complexes. Expectedly, the stabilization energies for the multiple methyl-substituted benzene-ethylene complexes lie between those of the multiple methyl-substituted benzene-methane and benzene-acetylene complexes. Energy decomposition analysis using the DFT-SAPT method predicts the dispersion energy to be dominant, similar to the benzene-methane complexes. For the symmetrically disubstituted complexes (-OH, -Cl, -NH2, -CN and -NO2), additional C-H···X interaction was observed, possibly due to the angular orientation of the ethylene molecule. Multidimensional correlation analysis between the electrostatic, dispersion and exchange-repulsion with the C-H···π interaction distance (r), Hammett constant (σ) and the molar refractivity (MR) revealed strong correlation between dispersion energy and the C-H···π interaction distance (r) as well as molar refractivity (MR).