Thermodynamic characterization of three centered hydrogen bond using o-aromatic amides, oxalamates and bis-oxalamides as model compounds

The study of the thermodynamic properties and cooperativity involved in three centered hydrogen (THB) bond formation using aromatic ortho-A substituted amides, oxalamates and bis- oxalamides (A = H, OMe, F, CH2OH, NO2, COCH3) as model molecules is reported. ∆H° and ∆S° associated with disruption of intramolecular hydrogen bonding by solvent were estimated using temperature dependence data of the N-H chemical shift. The results suggest that the influence of the A group is more important when electron-withdrawing, increasing both the enthalpy and entropy with an important contribution from conformational changes. The data allowed the estimation of the Ph=NH + rotational barrier of 14.0 kJ mol -1 in the amide and 16.7- 18.0 kJ mol -1 in oxalyl moiety. Correlations between ∆H° and ∆S° with NH temperature gradients predicted an enthalpy change of 18.7(1.0) and 24.4(1.7) kJ mol -1 for the energy required to break a full THB bond (A⋅⋅⋅H⋅⋅⋅O=C) and entropy differences between the non hydrogen bonded and hydrogen bonded state of 42.0(4.7) and 61.9(11) J mol -1 K -1 in oxalamate and bis-oxalamide series, respectively, in agreement with the participation of cooperative effects.