1H NMR study of the hetero‐association of non‐symmetrical diols with pyridine; GIAO/DFT calculations on diols

The ¹H NMR titration method is used to investigate the association of non‐symmetrical 1,2‐ and 1,3‐diols with pyridine in benzene. These diols give well‐defined titration curves for the two non‐equivalent OH protons, but it is not possible to determine individual association constants. Only the sum of the first association constants for the two protons and the product of the first and second association constants are accessible. The sum is significantly higher than that of the association constants of the corresponding primary and secondary alcohols, but close to an estimate based on symmetrical diols. The product leads to second association constants similar to those found for symmetrical diols. The sum of the chemical shifts of the associated and non‐associated OH protons in either 1:1 pyridine complex is higher than that of the shifts in the free diol and the 2:1 complex. These features are consistent with small cooperative effects, amounting to an average increase in the reaction free energy of 1.3 kJ mol^?1 compared to monohydric alcohols. Infrared (IR) spectra of non‐symmetrical diols and quantum mechanical (QM) calculation of the energies and ¹H NMR shifts of the OH protons in several conformers of propan‐1,2‐ and butan‐1,3‐diols indicate that both the primary and the secondary OH groups act as “donor” or “acceptor” in the free molecule. Gauche interactions in propan‐1,2‐diol enhance chemical shifts considerably less than does the intramolecular hydrogen bond in butan‐1,3‐diol. Copyright © 2011 John Wiley & Sons, Ltd.