Spectroscopic investigation of proton bonding at sub-kelvin temperatures

The proton bond is a pivotal chemical motif in many areas of science and technology. Its quantum chemical description is remarkably challenged by nuclear and charge delocalization effects and the fluxional perturbation that it induces on molecular substrates. This work seeks insights into proton bonding at sub-kelvin temperatures. In this way, intrinsic features of the proton bond are exposed, essentially free from thermal fluctuations of the molecular frame. To this end, a proton is bound within the molecular ring cavity provided by the 12-crown-4 ether. The resulting ion is isolated in a He-droplet at ∼0.4 K, where it is interrogated by infrared laser spectroscopy. The recorded spectrum features narrow vibrational bands, consistent with a robust proton bond bridging ether sites across the cavity of the essentially frozen crown ether. The potential energy surface sustaining the proton bond is broad and markedly anharmonic. In consequence, common modeling methods within the harmonic approximation fail to capture the observed band positions, whose accurate description seems to be even beyond perturbative anharmonic approaches. Calculations show that at elevated temperatures, the crown ether backbone is highly fluxional and that the distance between the oxygen atoms fluctuates in time, modulating the potential that the proton or deuteron is exposed to, and yielding dynamic inhomogeneous broadening and blue shifts with respect to the cryogenic spectra. These observations call for novel computational developments, for which the vibrational signatures outlined in this work should provide a valuable benchmark.