In Cat3 the negative and neutral ligands are secondary aliphatic amines and pyridines, respectively, where the ligands are arranged as in Cat1 (Figure ). As with Cat1, the ligand that makes the single largest contribution is negative ligand 4, which increases the protonation energy from 0.1 to 19.0 kcal/mol when replaced with NH - 2 . Interestingly the following three substitutions all lower the protonation energy.