Infrared spectroscopy of H-bonded bridges stretched across the cis-amide group:: II.: Ammonia and mixed ammonia/water bridges

Clusters of two model cis amides, oxindole and 3,4-dihydro-2(IH)-quinolinone, containing one and two ammonia molecules have been studied in the IR hydride stretch region using resonant ion-dip IR spectroscopy. The spectra confirm that ammonia is able to form hydrogen-bonded bridges across the adjacent amide N-H and C=O sites in a manner very similar to that of water. Such bridged structures require that ammonia assume the role of a hydrogen bond donor. Further similarities of the hydrogen bonding capabilities of ammonia and water have been revealed by investigations of ternary clusters containing an amide, one ammonia, and one water molecule. Experimentally, two species are observed having IR spectra consistent with a hydrogen-bonded bridge structure. The two species differ only in the relative positions of the ammonia and water molecules within the bridge. These experimental results are well supported by optimized structures, vibrational frequencies, and IR intensities calculated using density functional theory with the Becke3LYP functional. Additionally, the characteristic features of the hydride stretch fundamentals in a hydrogen-bond-donating ammonia molecule can be readily understood using a simple model for the coupled NH oscillators in which the hydrogen-bonded NH has its force constant lowered and its dipole derivative increased, much like in other hydrogen-bonded XH groups.