The energy of the intramolecular hydrogen bond in chloro-substituted N-methyl-salicylidene imines

By using a thermodynamic cycle, the steric effects connected with chelate ring formation are estimated to be up to 5 kcal/mol, much higher than in related Mannich bases (∼1 kcal/mol) which do not form resonance assisted hydrogen bonds. Accounting these effects the “real” value of the energy of hydrogen bond formation was estimated to be 15 kcal/mol which increases with growing number of chlorine atoms up to 16.5 kcal/mol for 4,5,6-trichloro substitution.