FTIR spectra of ammonia clusters in noble gas matrices

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Abstract
FTIR spectra of ammonia have been studied from 200 to 5200 cm−1 over a wide range of concentration and temperature conditions in solid neon, argon, and nitrogen matrices. Dimer bands appear between monomer and higher aggregate absorptions and exhibit intermediate growth behavior on sample annealing and concentration changes. Comparison of spectra in solid argon at 5 and 12 K shows unrelaxed monomer absorptions at 12 K, which almost completely vanish at 5 K without any difference in the dimer spectrum; this indicates that the NH3 submolecules are relaxed in the matrix-isolated dimer. One antisymmetric and two symmetric N–H stretching modes were observed for the dimer, which follow the 14NH3–15NH3 and NH3–ND3 shifts of their monomer analogs. The dimer N–H stretching modes are intensified by fivefold relative to the dimer umbrella bending mode as compared to the same relative monomer band intensities, which is diagnostic of the hydrogen bonding interaction. The matrix dimer spectra show that one N–H bond from one submolecule and two N–H bonds from the other submolecule are involved in nonlinear, intermolecular hydrogen bonding, and that these inequivalent weakly bonded NH3 submolecules form an asymmetric cyclic dimer structure. The matrix dimer spectra further indicate that classical one-hydrogen bond structures cannot be correct. Finally, trimer and higher clusters probably have similar structures, built from the dimer, based on similar infrared spectra.