Knowledge Bank

Infrared spectra of the C-H stretching vibrations of the symmetric-top complexes, $HC=CH-CH2,HC=CD-NH3,HC=CC-CH--NH3,N=CH-NH3$, have been recorded using a color-center-laser electric-resonance optothermal spectrometer. Efforts to observe the N-H stretches were unsuccessful. The hydrogen-bonded C-H stretching modes of HC=CH--NH$3 and N=CH--NH$3 are strongly coupled to the hydrogen bond as evidenced by their large monomer red-shifts of 75 and $200cm-1$ and broad predissociation linewidths of 2000 ad 650-800 MHz, respectively. The complexation-induced asymmetry in $HC=CH=NH3$ is not sufficient to allow us to observe the local mode associated with the outer C-H stretch. However, isotopic substitution in $HC=CD-NH3$ shows that this mode is red shifted by less than $1cm-1$ from the monomer vibration. The narrow predissociation linewidths of this mode (7-12 MHz) are consistent with this small red shift. The weaker coupling of the C-H stretches in HC=CC=CH is completely quenched upon complexation with $NH3$. The outer C-H stretch is observed in the diacetylene (HC=CC=CH) complex, blue shifted by approximately $0.3cm-1$ from the infrared-active monomer C-H stretch at $3333.7cm-1$; the bound CH stretch is red shifted about the same amount as in $HC=CH--NH3$. These observations imply that the weak coupling of the local modes in diacetylene is significantly quenched upon complexation with $NH4$. The $HC=CH=CH-NH3$ predissociation linewidths are similar to those in $HC=CD--NH3$, even though the outer C-H stretch is now five bonds away from the hydrogen bond. Surprisingly, these results suggest that the length of the triple-bond backbone in acetylene chains does not significantly impede the rate of vibrational energy flow. Future efforts will be made to extend these studies to the triacelyne complex, $HC=CC=CC=CH-NH3$.