Linear Free Energy Relationship in Ion Thermochemistry

We report a linear correlation between the proton affinity of 4-substituted pyridines and the hydride affinity of comparably substituted methyl cations that spans almost 30 and 120 kcal/mol, respectively. The linear correlation holds for cations bearing substituents that can stabilize an adjacent charge by a resonance interaction; cations with nonconjugating substituents fall off of the line. The cyanomethyl radical, for which a sharp, vertical ionization with IPad[•CH2CN] = 10.30 ± 0.04 eV was observed, is found to belong to the class of resonance-stabilized cations, contrary to earlier suggestions.