A Theoretical Evaluation of the pK(HB) and Delta H-HB(circle minus) Hydrogen-Bond Scales of Nitrogen Bases - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Chemistry - A European Journal Année : 2008

A Theoretical Evaluation of the pK(HB) and Delta H-HB(circle minus) Hydrogen-Bond Scales of Nitrogen Bases

Résumé

The experimental pK(HB) hydrogen-bond (HB) basicity scale and the corresponding Delta H-HB(circle minus) enthalpic scale of nitrogen compounds are extended and analysed in light of simple theoretical descriptors using the B3LYP density functional method and it medium-size basis set (6-31+G(d,p)). The selected training set includes 59 monofunctional unhindered nitrogen bases for which homogeneous and accurate experimental pK(HB) and Delta H-HB(circle minus) data have been determined by means of the association equilibrium of the bases with a reference hydrogen-bond acid, 4-fluorophenol, in CCl4. The three hybridisation states encountered in the nitrogen atom, sp, sp(2) and sp(3), are equally represented in this data set. A proper estimation of their experimental enthalpy (Delta H-HB(circle minus)) is directly attainable from the theoretical enthalpy of the complexation reaction with hydrogen fluoride (Delta H-circle minus(HF)). However, a second parameter is required to calculate with good accuracy the experimental free energy of association represented by pK(HB). About 99% of the variance of the pK(HB) scale is described by a bilinear equation using the minimum electrostatic potential (V-s.min) of the monomer in addition to the interaction energy (D-0((HF))). The equations are tested for an external set of 99 additional compounds including very different nitrogen bases such as ortho-substituted pyridines, polyazines and azoles. Theoretical calculations give a reliable estimation of hydrogen-bond basicity provided that the populations of the different isomers of the bases are taken into account by using the Boltzmann law, and that a specific halogen-bond interaction with the solvent CCl4 is considered for polybasic molecules. The pK(HB) scale can thus be extended to important classes of species experimentally inaccessible in CCl4, to polynitrogen compounds and to molecules of biological significance.

Domaines

Chimie

Dates et versions

hal-02143965 , version 1 (29-05-2019)

Identifiants

Citer

Francois Besseau, Jérôme Graton, Michel Berthelot. A Theoretical Evaluation of the pK(HB) and Delta H-HB(circle minus) Hydrogen-Bond Scales of Nitrogen Bases. Chemistry - A European Journal, 2008, 14 (34), pp.10656-10669. ⟨10.1002/chem.200800977⟩. ⟨hal-02143965⟩
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