Origin of the diffuse vibrational signature of a cyclic intramolecular proton bond: Anharmonic analysis of protonated 1,8-disubstituted naphthalene ions

We analyze the structures and spectral signatures of the cyclic intramolecular proton bond, N–H⁺–A, A = O and F, formed when an excess proton is added to derivatives of the 1,8-disubstituted naphthalene scaffold. These compounds provide a quasi-rigid framework with which to study the spectral complexity often associated with the N–H⁺–A entity. Vibrational spectra were obtained by monitoring photodissociation of weakly bound H₂ adducts of the mass-selected ions cooled close to 10 K. Several bands across the 900–3500 cm⁻¹ spectral range were traced to involvement of the bridging proton by their telltale shifts upon selective H/D isotopic substitution at that position. We account for the complex patterns that occur near the expected locations of the NH stretching fundamentals in the context of background levels mixing with a “bright” zero-order state through cubic terms in the potential energy expansion. Thus, this system provides a detailed picture of one of the mechanisms behind the line broadening often displayed by embedded excess protons. It does so in a sufficiently sparse density of states regime that many discrete transitions are observed in the vicinity of the harmonic stretching transition involving displacement of the trapped proton.