Estimating the energy of intramolecular bifurcated (three-centered) hydrogen bond by X-ray, IR and 1H NMR spectroscopy, and QTAIM calculations

Abstract

The structure of a series of the 2-substituted pyrroles containing the intramolecular NH⋯OC hydrogen bond and the 2,5-disubstituted pyrroles having the three-centered intramolecular CO⋯H(N)⋯OC hydrogen bond was studied by X-ray, IR, NMR spectroscopy, and quantum chemical calculations, including calculations in terms of Quantum Theory of Atoms in Molecules (QTAIM). The X-ray diffraction data show that the intramolecular rH … O and RN … O distances increase upon conversion from a two-centered hydrogen bond to a three-centered hydrogen bond, indicating a weakening of the last interaction. The topological parameters of the hydrogen bond obtained from the QTAIM calculations (electron density ρBCP and potential energy density V at the critical point of the hydrogen bond) made it possible to quantify the hydrogen bond energy in the studied compounds. The energy of intramolecular NH⋯OC hydrogen bond in the 2-substituted pyrroles is in the range from 5.5 to 6.0 kcal mol−1 (moderate hydrogen bond), whereas the energy of the components of the three-centered intramolecular CO⋯H(N)⋯OC hydrogen bond in the 2,5-disubstituted pyrroles is reduced to 2.5–3.5 kcal mol−1 (weak hydrogen bond). For weak components of the three-centered hydrogen bond, the principle of the additivity of effects on the spectral parameters influenced by the hydrogen bond (change in the vibration frequency of the NH bond, ΔνN‒H, and the chemical shift of bridge proton, ΔδNH) was proposed. For this reason, the ½ΔνN-H and ½ΔδNH values also were used for estimating the energy of the two-centered components of the three-centered CO⋯H(N)⋯OC hydrogen bond. These estimates are in good agreement with those obtained from the ρBCP and V parameters. Consequently, the ½ΔνN-H and ½ΔδNH parameters can be used as an energy descriptor for two-centered components in the case of a symmetric intramolecular three-centered hydrogen bond.