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1996-TD-11.jpg 138.1Kb JPEG image Thumbnail of ROTATIONAL SPECTRA OF $CH_{3}CCH-NH_{3}$, $NCCCH-NH_{3}$, AND $NCCCH-OH_{2}$

Creators: Hight Walker, A. R.; Omron, R.; Fraser, G. T.; Suenram, R. D.; Hilpert, G.
Issue Date: 1996
Publisher: Ohio State University
Abstract: Microwave spectra of $NCCCH-NH_{3}$, $CH_{3}CCH-NH_{3}$ and $NCCCH-OH_{2}$ have been recorded using a pulsed-nozzle Fourier transform microwave spectrometer. The complexes, $NCCCH-NH_{3}$ and $CH_{3}CCH-NH_{3}$ are found to have symmetric-top structures with the acetylenic proton hydrogen-bonded to the nitrogen of the $NH_{3}$. The data for $CH_{3}CCH-NH_{3}$ are further consistent with free or nearly free internal rotation of the methyl top against the ammonia top. For $NCCCH-OH_{2}$, the acetylenic proton is hydrogen-bonded to the oxygen of the water. The water complex has a dynamical $C_{2}v$ 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 \AA and 10.8 N/m, 2.322 \AA and 6.0 N/m, and 2.125 \AA and 9.6 N/m, for $NCCCH-NH_{3}$, $CH_{3}CCH-NH_{3}$, and $NCCCH-OH_{2}$, respectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanide and acetylene complexes of $NH_{3}$ and $H_{2}O$. The $NH_{3}$ bending and weak-bond stretching force constants for $CH_{3}CCH-NH_{3}$ are less than those found in $NCCCH-NH_{3}$, $NCH-NH_{3}$, and $HCCH-NH_{3}$, suggesting that the hydrogen bonding interection is particularly weak in $CH_{3}CCH-NH_{3}$. The weakness of the bond is particularly a consequence of the orientation of the monomer electric dipole moments in the complex. In $CH_{3}CCH-NH_{3}$ the antialigned monomer dipole moments leads to a repulsive dipole-dipole interaction energy, while in $NCH-NH_{3}$ and $NCCCH-NH_{3}$ the aligned dipoles give an attractive interaction.
Description: Author Institution: Optical Technology Division, National Institute of Standards and Technology; Institut f\""{u}r Angewandle Physik, Universit\""{a}t Bonn
Other Identifiers: 1996-TD-11
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