Striking impact of solvent polarity on the strength of hydrogen-bonded complexes: A nexus between theory and Experiment

Abstract

The binding free energy of hydrogen-bondedcomplexes is generally inversely proportional to thesolvent dielectric constant. This occurs because thesolvent-accessible surface area of the complex is alwayssmaller than that of the individual subsystems, leadingto a reduction in solvation energy. The present studyexplores the potential for stabilizing hydrogen-bondedcomplexes in a solvent with higher polarity. Contrary tothe established understanding, we have demonstratedthat the hydrogen-bonded complex(CH3CH2COOH···2,4,6-trimethylpyridine) can be betterstabilized in a solvent with higher polarity. In this case, asignificant charge transfer between the subsystemsresults in an increased dipole moment of the complex,leading to its stabilization in a more polar solvent. Theexpected inverse relationship between binding freeenergy and solvent dielectric constant is observed whenthe charge transfer between the subsystems is low. Thus,the magnitude of the charge transfer between subsys-tems is possibly the key factor in determining thestabilization or destabilization of H-bonded complexesin different solvents. Here, we present a comprehensivestudy that combines experimental and theoretical ap-proaches, including nuclear magnetic resonance (NMR),infrared (IR) spectroscopies and quantum chemicalcalculations to validate the findings.