Linearity of halogen bonds

Directionality and linearity definition

A systematic series of hydrogen and halogen bonded complexes: B···HCCH, B···HCl and B···ClF (B = Lewis base) were investigated in terms of geometry and interaction energies, with excellent agreement to experimental data. SAPT calculations also gave insights into differences in the underlying nature of hydrogen and halogen bonds, especially in terms of deviation from collinearity.

The diagram above indicates both the directionality (φ) and the deviation from linearity (θ). The paper clearly shows that quantum mechanical exchange repulsion acts as an effective barrier when halogen bonds are forced away from collinearity.

On the directionality and non-linearity of halogen and hydrogen bonds. J. G. Hill and A. C. Legon, Phys. Chem. Chem. Phys., 2015, 17, 858.