The Halogen Bond in Weakly Bonded Complexes and the Consequences for Aromaticity and Spin-Orbit Coupling

• Published in: Molecules (Basel, Switzerland) Switzerland), 2023-01, Vol.28 (2), p.772
• Main Authors: Cunha, Ana V, Havenith, Remco W A, van Gog, Jari, De Vleeschouwer, Freija, De Proft, Frank, Herrebout, Wouter
• Format: Article
• Language: English
• Subjects: Aromatic compounds; Aromaticity; Chemical bonds; density functional theory; Electrostatic properties; energy decomposition analysis; halogen bonds; Halogens - chemistry; Hybridization; Hydrogen bonds; Iodine; ring current analysis; Ring currents; spin-orbit coupling; Spin-orbit interactions; Static Electricity
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules28020772

The halogen bond complexes CF3X⋯Y and C2F3X⋯Y, with Y = furan, thiophene, selenophene and X = Cl, Br, I, have been studied by using DFT and CCSD(T) in order to understand which factors govern the interaction between the halogen atom X and the aromatic ring. We found that PBE0-dDsC/QZ4P gives an adequate description of the interaction energies in these complexes, compared to CCSD(T) and experimental results. The interaction between the halogen atom X and the π-bonds in perpendicular orientation is stronger than the interaction with the in-plane lone pairs of the heteroatom of the aromatic cycle. The strength of the interaction follows the trend Cl < Br < I; the chalcogenide in the aromatic ring nor the hybridization of the C−X bond play a decisive role. The energy decomposition analysis shows that the interaction energy is dominated by all three contributions, viz., the electrostatic, orbital, and dispersion interactions: not one factor dominates the interaction energy. The aromaticity of the ring is undisturbed upon halogen bond formation: the π-ring current remains equally strong and diatropic in the complex as it is for the free aromatic ring. However, the spin-orbit coupling between the singlet and triplet π→π* states is increased upon halogen bond formation and a faster intersystem crossing between these states is therefore expected.