Intramolecular hydrogen bonds of gossypol imine derivatives

Evaluation of intramolecular hydrogen bond energies of twenty gossypol imine derivatives was carried out using ¹H NMR spectroscopy and quantum chemistry methods. Gossypol imine derivatives contain various intramolecular hydrogen bonds: O-H⋯O, N-H⋯O, O-H⋯N, C-H⋯O and O-H⋯π. The existence of dienamine and diimine tautomeric forms causes some differences in the intramolecular hydrogen bonding of these compounds. It was found that the position of the O-H group proton signals in the ¹H NMR spectra made it possible to provide an initial estimation of the hydrogen bond energies and the degree of proton involvement in non-covalent interactions. Using geometric characteristics, hydrogen bond length, distance between electronegative atoms involved in the hydrogen bonding, and hydrogen bond angle, made it possible to compare the energies of all intramolecular hydrogen bonds of the studied gossypol imine derivatives in the gas phase. It was shown that the strengths of the intramolecular hydrogen bonds C(6)O-H⋯OC(7) are different for compounds in dienamine and diimine tautomeric forms, which may be a key factor affecting the tautomeric equilibrium of this class of compounds.