Using equilibrium isotope effects to detect intramolecular OH/OH hydrogen bonds: structural and solvent effects

Area of Science:

• Nuclear Magnetic Resonance Spectroscopy
• Physical Organic Chemistry
• Supramolecular Chemistry

Background:

• Intramolecular hydrogen bonds in diols influence their chemical properties.
• Equilibrium isotope shifts in NMR spectroscopy are sensitive to hydrogen bonding environments.
• Solvent effects significantly impact hydrogen bond strength and NMR chemical shifts.

Purpose of the Study:

• To investigate the influence of solvent polarity and hydrogen-bonding capability on equilibrium isotope shifts in partially deuterated diols.
• To elucidate the relationship between hydrogen bond geometry and the sign and magnitude of NMR isotope shifts.
• To explore the utility of different deuterated solvents for configurational assignment and hydrogen bond detection.

Main Methods:

• Comparative (1)H NMR spectroscopy of partially deuterated 1,3- and 1,4-diols.
• Utilizing various hydrogen-bond-accepting (DMSO-d(6), acetone-d(6), THF-d(8)) and apolar (CD(2)Cl(2), benzene-d(6)) deuterated solvents.
• Employing pyridine-d(5) as a solvent to amplify isotope shifts.

Main Results:

• Observed sign inversion of equilibrium isotope shifts in hydrogen-bond-accepting solvents, attributed to competitive intermolecular hydrogen bonding and limiting chemical shifts.
• Quantified deuterium's preference for intermolecular solvent hydrogen bonds (10.9 +/- 0.5 cal/mol in DMSO-d(6) for 1,4-diol).
• Demonstrated pyridine-d(5)'s ability to amplify positive isotope shifts (up to 3x) for configurational assignment (2,4-pentanediol) and detecting intramolecular hydrogen bonds (cyclodextrins).
• Identified negative (upfield) isotope shifts in apolar solvents, with larger shifts observed in benzene-d(6), useful for configurational analysis.

Conclusions:

• The geometry of intramolecular hydrogen bonds dictates the sign of equilibrium isotope shifts, modulated by solvent interactions.
• Deuterium exhibits a preference for forming intermolecular hydrogen bonds with solvent molecules.
• Deuterated pyridine and apolar solvents serve as valuable tools for NMR-based structural elucidation, including configurational assignment and the study of hydrogen bonding networks.