A DFT study on intramolecular hydrogen bonding in 2-substituted phenols: Conformations, enthalpies, and correlation with solute parameters

For a series of 61 2-substituted phenols, 2-X-PhOH, forming a total of 73 intramolecular hydrogen bonds, the intramolecular hydrogen bond enthalpy, DeltaH(intra-HB), has been determined by density functional theory (DFT) calculations on the B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level. The DeltaH(intra-HB) was defined as the enthalpy difference between the hydrogen-bonded (HB) form and the lowest-energy conformer in which the OH is rotated into the away position. The correlation of DeltaH(intra-HB) With geometrical factors such as r(OH), or r(OH(...)A), with A as the hydrogen bond accepting atom, was generally very poor, showing that none of these parameters can be used as an universal descriptor for the hydrogen bond strength. The relation between DeltaH(intra-HB) and v(O-H) (intra-HB) is also insignificant, in contrast with previous estimates. The data clearly demonstrate that the genuine DeltaH(intra-HB) of a phenolic compound cannot be unequivocally derived by simple rotation of the OH group into the away orientation, because additional steric and/or electronic 1,2 interactions may take place which are difficult or even impossible to be separated from the sole H-donor/acceptor interaction. Nevertheless, a good correlation has been found between computed and experimental liquid phase v(O-H)s obeying v(O-H)(DFT) = 1.0097nu(O-H)(exp) + 159.5. It could be established with the use of solute acidity, alpha(2)(H), and solvent basicity, pH, parameters, that the strength of an intramolecular hydrogen bond is noticeably fortified for electron withdrawing groups. Furthermore, it was found that with the proper non-HB geometries the bond dissociation enthalpy, BDE(O-H), in the series of 25 2-X-PhOHs correlates quite well with sigma(+), in the same way as has been found for 4-X-substituted phenols.