Unexpected trends in halogen-bond based noncovalent adducts

Unexpected trends in the strengths of halogen-bond based adducts of CY3I (Y = F, Cl, Br, I) with two typical Lewis bases (chloride and trimethylamine) show that the halogen-bond donor strength (Lewis acidity) of a compound R–X is not necessarily increased with higher electronegativity of the (carbon-based) group R