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 = 13 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 C-6-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 similar to 2700 cm(-1). For all clusters the ammonia 2 nu(4) 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.