Stepwise Hydration Switches Electronic Structure in a Bifunctional Organocatalyst through Double Hydrogen Bonding

Abstract 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) is a widely used bifunctional organocatalyst; however, its microscopic behavior in hydrogen-bonding environments remains poorly understood. In this study, we employed broadband rotational spectroscopy to identify the stepwise microhydration of TBD with one to four water molecules, and investigated how solvent molecules progressively alter its structure and electronic properties. We found that the bifunctional character and rigid bicyclic skeleton of TBD govern the growth of the hydrogen-bond network, while simultaneously inducing significant geometric distortion in the TBD skeleton. Notably, the formation of double hydrogen bonds induces pronounced ring twisting and drives an electronic structural transition from a “π-localized” to a “π-delocalized” state. The water-induced electronic structure rearrangement is supported by experimental nuclear quadrupole coupling constants and theoretical natural population analysis. These results demonstrate that the basicity and reactivity of TBD are not inherent constants, but can be dynamically modulated by local hydrogen-bonding networks, offering new insights for the design of more efficient hydrogen-bond-assisted catalytic systems.