Anion Binding of One-, Two-, and Three-Armed Thiourea Receptors Examined via Photoelectron Spectroscopy and Quantum Computations

Benzene rings substituted with 1-3 thiourea containing arms (1-3) were examined by photoelectron spectroscopy and density functional theory computations. Their conjugate bases and chloride, acetate, and dihydrogen phosphate anion clusters are reported. The resulting vertical and adiabatic detachment energies span 3.93-5.82 eV (VDE) and 3.65-5.10 (ADE) for the deprotonated species and 4.88-5.97 eV (VDE) and 4.45-5.60 eV (ADE) for the anion complexes. These results reveal the stabilizing effects of multiple hydrogen bonds and anionic host-guest interactions in the gas phase. Previously measured equilibrium binding constants in aqueous dimethyl sulfoxide for all three thioureas are compared to the present results, and cooperative binding is uniformly observed in the gas phase but only for one case (i.e., 3·H₂PO₄^-) in solution.