Supersonic jet FTIR and quantum chemical investigations of ammonia/acetylene clusters

Abstract

The vibrational dynamics of ammonia/acetylene clusters was characterized experimentally and theoretically. IR spectra ranging from 3500 to 600 cm⁻¹ were recorded using a ragout-jet FTIR spectrometer. Four dimer absorption bands, centred at 3214, 1961, 1000, and 795 cm⁻¹, are assigned to the bonded CH stretching, CC stretching, NH₃ umbrella bending, and bonded CH bending modes, respectively. Resonance between the bonded CH stretching fundamental and a “dark state” is discussed in the context of the observation of a second dimer absorption band. Two additional bands at 3192 and 3167 cm⁻¹ are tentatively assigned to the cyclic 1:2 trimer. The spectroscopic investigations are accompanied by ab initio calculations at the CCSD(T) and MP2 level using large basis sets. Anharmonicity effects are considered for the totally symmetric vibrations and anharmonicity constants for the stretching vibrations of the HCCH moiety in different environments are discussed in detail.