Figure 4.

Relationship between local stretching force constant $k_n^a$ and bond length r for Cl-I⋯O (green) and I-I⋯O (purple) halogen bonds, respectively. Data points are shown with 4 shapes based on the acceptor molecule type, see Figure 1. An exponential function in the form of $k_n^a=a·exp(b·r)$ was used to fit all 34 data points with $R^2=0.923$. Two data points are identified as outliers (encircled by blue hexagons) and $R^2=0.974$ is obtained for fitting the remaining 32 data points. The blue dashed curve shows the updated fitting function excluding two outliers.