Assessing Parameter Suitability for the Strength Evaluation of Intramolecular Resonance Assisted Hydrogen Bonding in -Carbonyl Hydroquinones.

Intramolecular hydrogen bond (IMHB) interactions have attracted considerable attention due to their central role in molecular structure, chemical reactivity, and interactions of biologically active molecules. Precise correlations of the strength of IMHB's with experimental parameters are a key goal in order to model compounds for drug discovery. In this work, we carry out an experimental (NMR) and theoretical (DFT) study of the IMHB in a series of structurally similar -carbonyl hydroquinones. Geometrical parameters, as well as Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) parameters for IMHB were compared with experimental NMR data. Three DFT functionals were employed to calculated theoretical parameters: B3LYP, M06-2X, and ωB97XD. OH distance is the most suitable geometrical parameter to distinguish among similar IMHBs. Second order stabilization energies ΔE from NBO analysis and hydrogen bond energy (E) obtained from QTAIM analysis also properly distinguishes the order in strength of the studied IMHB. ΔE from NBO give values for the IMHB below 30 kcal/mol, while E from QTAIM analysis give values above 30 kcal/mol. In all cases, the calculated parameters using ωB97XD give the best correlations with experimental ¹H-NMR chemical shifts for the IMHB, with R² values around 0.89. Although the results show that these parameters correctly reflect the strength of the IMHB, when the weakest one is removed from the analysis, arguing experimental considerations, correlations improve significantly to values around 0.95 for R².