Knowledge Bank

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 $C2v$ 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 $NCCCH-NH3$, $CH3CCH-NH3$, 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 interection is particularly weak in $CH3CCH-NH3$. The weakness of the bond is particularly 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.