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 H-2 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. (C) 2013 AIP Publishing LLC.