Rotational Spectra of CH3CCH–NH3, NCCCH–NH3, and NCCCH–OH2

Abstract

Microwave spectra of NCCCH–NH3, CH3CCH–NH3, and NCCCH–OH2have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes NCCCH–NH3and CH3CCH–NH3are found to have symmetric-top structures with the acetylenic proton hydrogen bonded to the nitrogen of the NH3. The data for CH3CCH–NH3are further consistent with free or nearly free internal rotation of the methyl top against the ammonia top. For NCCCH–OH2, the acetylenic proton is hydrogen bonded to the oxygen of the water. The complex has a dynamicalC2vstructure, as evidenced by the presence of two nuclear-spin modifications of the complex. The hydrogen bond lengths and hydrogen-bond stretching force constants are 2.212 Å and 10.8 N/m, 2.322 Å and 6.0 N/m, and 2.125 Å and 9.6 N/m for NCCCH–NH3, CH3CCH–NH3, and NCCCH–OH2, respectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanide and acetylene complexes of NH3and H2O. The NH3bending and weak-bond stretching force constants for CH3CCH–NH3are less than those found in NCCCH–NH3, NCH–NH3, and HCCH–NH3, suggesting that the hydrogen bonding interaction is particularly weak in CH3CCH–NH3. The weakness of this hydrogen bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH3CCH–NH3the antialigned monomer dipole moments lead to a repulsive dipole–dipole interaction energy, while in NCH–NH3and NCCCH–NH3the aligned dipoles give an attraction interaction.