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 bisoxalamides (A = H, OMe, F, CH₂OH, NO ₂, COCH₃) as model molecules is reported. ΔH° and ΔS° 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 ΔH° and ΔS° 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.