ROTATIONAL SPECTRA OF CH3CCH-NH3, NCCCH-NH3, AND NCCCH-OH2

Microwave spectra of NCCCH-NH3, CH3CCH-NH3, and NCCCH-OH2 have been re corded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes NCCCH-NH3 and CH3CCH-NH3 are found to have symmetric-to p structures with the acetylenic proton hydrogen bonded to the nitroge n of the NH3. The data for CH3CCH-NH3 are further consistent with free or nearly free internal rotation of the methyl top against the ammoni a top. For NCCCH-OH2, the acetylenic proton is hydrogen bonded to the oxygen of the water. The complex has a dynamical C-2v structure, as ev idenced by the presence of two nuclear-spin modifications of the compl ex. The hydrogen bond lengths and hydrogen-bond stretching force const ants are 2.212 Angstrom and 10.8 N/m, 2.322 Angstrom and 6.0 N/m, and 2.125 Angstrom and 9.6 N/m for NCCCH-NH3, CH3CCH-NH3, and NCCCH-OH2, r espectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanid e and acetylene complexes of NH3 and H2O. The NH3 bending and weak-bon d stretching force constants for CH3CCH-NH3 are less than those found in NCCCH-NH3, NCH-NH3, and HCCH-NH3, suggesting that the hydrogen bond ing interaction is particularly weak in CH3CCH-NH3. The weakness of th is hydrogen bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH3CCH-NH3 the anti aligned monomer dipole moments lead to a repulsive dipole-dipole inter action energy, while in NCH-NH3 and NCCCH-NH3 the aligned dipoles give an attraction interaction. (C) 1996 Academic Press, Inc.