The stability of hydrogen-bonded ion-pair complex unexpectedly increases with increasing solvent polarity

Abstract

The generally observed decrease of the electrostatic energy in the complex with increasing solvent polarity has led to the assumption that the stabilityof thecomplexeswithion-pairhydrogenbonds decreaseswith increasingsolventpolarity.Besides, the smaller solvent-accessible surfacearea (SASA) of the complex in comparisonwith the isolated subsystems resultsinasmallersolvationenergyofthelatter, leading to a destabilization of the complex in the solvent compared to the gas phase. In our study, which combinesNuclearMagneticResonance, InfraredSpec troscopy experiments, quantumchemical calculations, andmoleculardynamics(MD)simulations,wequestion thegeneral validityof this statement.Wedemonstrate that thebinding freeenergyof the ion-pairhydrogen bondedcomplexbetween2-fluoropropionicacidandn butylamine(CH3CHFCOO…NH3But+) increaseswith increasedsolventpolarity.Thisphenomenonisrational ized by a substantial charge transfer between the subsystemsthatconstitutetheion-pairhydrogen-bonded complex. This unexpected finding introduces a new perspectivetoourunderstandingofsolvationdynamics, emphasizingtheinterplaybetweensolventpolarityand molecularstabilitywithinhydrogen-bondedsystems.