A study of intramolecular hydrogen bonds C-H⋯X (X = N, O) within the theory of the electron localization function

Electron localization functions (ELFs) are calculated at the hydrogen bond critical points for a number of molecules having the intramolecular hydrogen bond C-H⋯X (X = N, O). It is shown that the values of the core and valence electrons bifurcation index (CVBI) are positive in all cases. This enables the characterization of the studied hydrogen bonds as weak interactions of mainly electrostatic nature. For stronger intramolecular hydrogen bonds N-H⋯O with a significant contribution of covalence to the bonding, the bifurcation index is negative. A relationship between the parameters calculated within the theory of the electron localization function (ELF) and the quantum theory “Atoms in Molecules” (QTAIM) is observed. In the 1H NMR spectra, the resonance shift of the bridging hydrogen atom, induced by a hydrogen bond is much smaller for the hydrogen bonds having a positive bifurcation index than that for the hydrogen bonds with the negative bifurcation index, and the interatomic H⋯X distance is much longer in the former case. In the IR spectra the positive bifurcation index corresponds to a blue shift of the stretching vibration of the covalent bond of the hydrogen donor due to the hydrogen bonding and the negative bifurcation index corresponds to the red shift of this band.