1H NMR Studies of Intramolecular OH/OH Hydrogen Bonds via Titratable Isotope Shifts.

Methanol titrations of partially deuterated 1,4- and 1,3-diols dissolved in nonpolar solvents such as CD2Cl2 and benzene-d6 have provided 1H NMR measurements of OH/OD isotope shifts, diagnostic for intact intramolecular hydrogen bonds, under conditions of increasing protic solvent concentration. 1,4- and 1,3-diols with conformationally favored intramolecular OH/OH hydrogen bonds can be titrated to constant isotope shift values, albeit with variable sign, in the presence of excess methanol equivalents, providing evidence for intact intramolecular hydrogen bonds under these conditions. Conversely, the isotope shift in a 1,3-diol with a conformationally labile intramolecular hydrogen bond titrated to zero when in the presence of excess equivalents methanol, consistent with intramolecular hydrogen bond rupture under these conditions. Additionally, the titration behavior of hydroxyl chemical shifts in diols and protected derivatives has revealed significant OH/OD isotope shifts in the absence of chemical shift differences (δOHin = δOHout) that are necessary for an equilibrium isotope effect, lending evidence for an intrinsic contribution to the isotope effect. OH/OD isotope shift titration thus provides a means for understanding the origins of these isotope effects and for probing the intact or nonintact nature of intramolecular OH/OH hydrogen bonds in response to intermolecular hydrogen bonds provided by a protic solvent.