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Abstract
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.
Highlights
The pioneering study on three centered hydrogen bonding using aromatic amides, oxalamates and bis-oxalamides was developed by Martínez-Martínez,[1] in his PhD dissertation under the supervision of Professor Rosalinda Contreras
The method adjusts the data to a sigmoidal curve by Non Linear Fitting Procedures (NLFP) (Figure 1d), from which δHB and δSB values are predicted
The linear equations predict an enthalpy change of 18.7(1.0) and 24.4(1.7) kJ mol-1 for the energy required to break a full three centered hydrogen bond (THB) (|∆δΝΗ/∆T| = 0) in oxalamate and bis-oxalamide series, respectively
Summary
The pioneering study on three centered hydrogen bonding using aromatic amides, oxalamates and bis-oxalamides was developed by Martínez-Martínez,[1] in his PhD dissertation under the supervision of Professor Rosalinda Contreras. To maintain the influence of the A group constant and appreciate the energetics related with three centered hydrogen bonding (THB) and cooperativity effects, the quotient ∆YHB = ∆Yona-c /∆Yona (Y = H, S) is proposed; where ∆Yona-c is the estimated enthalpy or entropy for each one of 1a-c to 7a-c and ∆Yona is the enthalpy or entropy for each amide na as reference compound, ∆HHB values for 2b are ∆H°(2b)/∆H°(2a) = 9.68/9.50 = 1.0, for three centered hydrogen bonds, and for 2c, ∆H°(2c)/∆H°(2a) = 12.92/9.50 = 1.4 (Table 2), for cooperativity effects.
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