Overlooked Difference between Hydrogen Bonds of Equal Strength Formed between Catechol and an Oxygen or Nitrogen Base. Experiments and DFT Calculations

The IR spectrum of catechol in CCl4 shows two fairly sharp O-H stretching bands of roughly equal absorbance at 3615.0 and 3569.6 cm-1 due, respectively, to the "free" OH and the intramolecularly H-bonded OH groups. Intermolecular H-bond formation between the "free" OH and a hydrogen bond acceptor (HBA) decreases its stretching frequency by several hundred wavenumbers and simultaneously decreases the frequency of the intramolecularly H-bonded OH by a few tens of wavenumbers. The magnitude of these frequency shifts, inter and intra, respectively, are very well reproduced by DFT calculations. As would be expected, the magnitudes of inter and intra increase as the HB accepting ability of the HBA increases as quantified, on a relative scale, by the HBA's values (Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521). However, plots of experimental, or calculated, frequency shifts versus reveal that inter and intra are ca. 40% larger for a nitrogen atom HBA than for an oxygen atom HBA having equal HBA activity. We hypothesize that for HBAs of equal strength, i.e., of equal , the H-bond in (O-H- - -O)inter is shorter and, hence, intrinsically stronger than the H-bond in the (O-H- - -N)inter. However, we further hypothesize that there is more charge separation in the H-bond to N because N is a better proton acceptor than O. Hence, it is the greater Coulombic attraction in (O-H- - -N)inter which strengthens this H-bond and compensates for its greater length. Theoretical calculations lend support to these hypotheses.