Infrared and microwave spectra of the acetylene–ammonia and carbonyl sulfide–ammonia complexes: a comparative study of a weak C–H⋯N hydrogen bond and an S⋯N bond

Abstract

We report a combined high resolution infrared and microwave spectroscopic investigation of the acetylene–ammonia and carbonyl sulfide–ammonia complexes using a pulsed slit-nozzle multipass absorption spectrometer based on a quantum cascade laser and a pulsed nozzle beam Fourier transform microwave spectrometer, respectively. The ro-vibrational transitions of the acetylene–ammonia complex have been measured at 6 μm in the vicinity of the ν₄ band of ammonia for the first time. The previously reported pure rotational transitions have been extended to higher J and K values with ¹⁴N nuclear quadrupole hyperfine components detected and analyzed. The spectral analysis reveals that acetylene binds to ammonia through a C–H⋯N weak hydrogen bond to form a C_3v symmetric top, consistent with the previous microwave [Fraser et al., J. Chem. Phys., 1984, 80, 1423] and infrared spectroscopic study at 3 μm [Hilpert et al., J. Chem. Phys., 1996, 105, 6183]. A parallel study has also been carried out for the carbonyl sulfide–ammonia complex whose pure rotational and ro-vibrational spectra at 6 μm have been detected and analyzed for the first time. The spectral and the subsequent structural analyses, in conjunction with the corresponding ab initio calculation, indicate that the OCS–NH₃ complex assumes C_3v symmetry with S pointing to N of NH₃, in contrast to the T-shaped geometries obtained for the isoelectronic N₂O–NH₃ and CO₂–NH₃ complexes.