| Abstract | 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, Δυ inter and Δυintra, respectively, are very well reproduced by DFT calculations. As would be expected, the magnitudes of Δυinter and Δυintra, increase as the HB accepting ability of the HBA increases as quantified, on a relative scale, by the HBA's β2H values (Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521). However, plots of experimental, or calculated, frequency shifts versus β2H reveal that Δυinter and Δυintra 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 β2H, 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. |
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