Quantum structure of the intermolecular proton bond

A proton shared between two closed-shell molecules, [A.H+.B], constitutes a ubiquitous soft binding motif in biological processes. The vibrational transitions associated with the shared proton, which provide a direct probe of this interaction, have been extensively studied in the condensed phase but have yielded only limited detailed information because of their diffuse character. We exploited recent advances in gas-phase ion spectroscopy to identify sharp spectral features that can be assigned to both the shared proton and the two tethered molecules in a survey of 18 cold, isolated [A.H+.B] ions. These data yield a picture of the intermolecular proton bond at a microscopic scale, facilitating analysis of its properties within the context of a floppy polyatomic molecule.