Probing Host–Guest Interactions of the Dual Anion Receptor Harmane with Halide and HSO₄^– Anions

Harmane is a polycyclic amine that can recognize F^- and HSO₄^- via the ═N-H or ≡N binding site. The active binding site depends on whether the solvent is protic or aprotic, but the underlying molecular mechanism remains unclear. As a first step toward obtaining such mechanisms in solutions, we investigated the interactions of harmane with halide anions (F^-, Cl^-, Br^-, and I^-) and HSO₄^- in the gas phase using negative ion photoelectron spectroscopy combined with theoretical calculations. The adiabatic/vertical detachment energies for deprotonated harmane and harmane·X^- (X = F, Cl, Br, I, and HSO₄) were determined to be 2.72/2.79, 3.25/3.38, 4.19/4.43, 4.35/4.40, 3.93/3.99, and 4.49/4.75 eV, respectively, with an uncertainty of ±0.05 eV. All the X^- anions were found to form hydrogen bonds with harmane through the ═N-H site. A nearly complete proton transfer was observed within the harmane·F^- complex anion. Larger halide anions in other harmane-halide complexes remain relatively intact. Four closely lying isomers of harmane·HSO₄^- were identified. The photodetachment locations of the harmane complex anions were also revealed by electronic state calculations and molecular orbital analyses. The current work lays out a foundation for future work on microsolvated clusters to probe how solvent molecules influence the harmane-anion binding motif.