The hydrogen bond – practice and QTAIM theory

The direct proportionality between the energy (enthalpy) of the hydrogen bond and the intensification of the infrared stretching mode of the AH group on formation of the H-complex A–H⋯B [A. V. Iogansen, Spectrochim. Acta, Part A, 1999, 55, 1585] is compared empirically with the linear correlation of stabilization energies, in the range of 1–70 kJ mol−1, with the electron density of the H-bond critical point, which follows from the published computational results of the quantum theory of atoms in molecules (QTAIM). Very close similarity is seen between the increase in IR intensity on H-bond formation as ΔA1/2 ≡ AH1/2 − A01/2 (subscripts H and 0 relate to bound and free AH groups, respectively) and the electron density, ρ, in their relevance to the energy. Despite the fact that both parameters, ρ and ΔA1/2, are defined by the same wave functions of the electronic and vibrational states of a system, the direct interrelation between two parameters of different dimensionalities is not obvious. The suggestion is made that IR intensities in the spectra of H-complexes, in addition to the results of diffraction methods, can be a source of electron-density data as ρ(rc) (e a−3) = 10−2(ΔA1/2) (A, 10−4 cm mmol−1).