NH3 as a Strong H‑Bond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2‑Pyridone·(NH3) n , n = 1–3

Mass- and isomer-selected infrared spectra of 2-pyridone·(NH3) n clusters with n = 1–3 were measured in the NH and CH stretch fundamental region (2400–3700 cm–1) using infrared (IR) laser depletion spectroscopy combined with resonant two-photon ionization UV laser detection. The IR depletion spectra reveal three different H-bonding topologies of these clusters: The n = 1 and 2 clusters form ammonia bridges stretching from the N–H to the CO group of the cis-amide function of 2-pyridone (2PY), giving rise to intense and strongly red-shifted (2PY)NH and ammonia NH stretch bands. For n = 3, two isomers (3X and 3Y) are observed in the IR spectra: The spectrum of 3X is compatible with an ammonia-bridge structure like n = 2, with the third NH3 accepting an H-bond from C6–H of 2PY. The IR spectrum of 3Y exhibits a broad IR band in the 2500–3000 cm–1 range and is characteristic of a bifurcated double-bridged structure in which the first NH3 accepts an H-bond from the (2PY)NH and donates two H-bonds to the other two ammonias, both of which donate to the CO group of 2PY. This double-donor/double-bridge H-bonding pattern increases the acceptor strength of the first ammonia and dramatically lowers the (2PY)NH stretching frequency to ∼2700 cm–1. For all clusters the ammonia 2ν4 HNH bend overtones in the 3180–3320 cm–1 region gain intensity by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia NH stretches, which are red-shifted into the 3250–3350 cm–1 region. The experimental results are supported by optimized structures, vibrational frequencies, and IR intensities calculated using density-functional theory with the B3LYP and PW91 functionals, as well as with the more recent functionals B97-D and M06-2X, which are designed to include long-range dispersive interactions.