TD11		15min11:28

A. R. HIGHT WALKER, R. OMRON, G. T. FRASER, R. D. SUENRAM, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899; G. HILPERT, Institut für Angewandte Physik, Universität Bonn, Wegelerstraße 8, D-53115 Bonn, Germany.

Microwave spectra of NCCCH-NH₃, CH₃CCH-NH₃, and NCCCH-OH₂ have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes, NCCCH-NH₃ and CH₃CCH-NH₃, are found to have symmetric-top structures with the acetylenic proton hydrogen-bonded to the nitrogen of the NH₃. The data for CH₃CCH-NH₃ are 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 water complex has a dynamical C_2v structure, 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\space and 10.8 N/m, 2.322 Å\space and 6.0 N/m, and 2.125 Å\space 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 NH₃ and H₂O. The NH₃ bending and weak-bond stretching force constants for CH₃CCH-NH₃ are 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 the bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH₃CCH-NH₃ the antialigned monomer dipole moments leads to a repulsive dipole-dipole interaction energy, while in NCH-NH₃ and NCCCH-NH₃ the aligned dipoles give an attractive interaction.