VIBRATIONAL COUPLINGS AND ENERGY FLOW IN COMPLEXES OF HC=CH, HC=CD, HC=CC=C, AND N=CH WITH $NH_{3}$
Loading...
Date
1994
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
Infrared spectra of the C-H stretching vibrations of the symmetric-top complexes, $HC=CH-CH_{2}, HC=CD-NH_{3}, HC=CC-CH--NH_{3}, N = CH-NH_{3}$, 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 $200 cm^{-1}$ and broad predissociation linewidths of 2000 ad 650-800 MHz, respectively. The complexation-induced asymmetry in $HC=CH=NH_{3}$ is not sufficient to allow us to observe the local mode associated with the outer C-H stretch. However, isotopic substitution in $HC=CD-NH_{3}$ shows that this mode is red shifted by less than $1 cm^{-1}$ 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 $NH_{3}$. The outer C-H stretch is observed in the diacetylene (HC=CC=CH) complex, blue shifted by approximately $0.3 cm^{-1}$ from the infrared-active monomer C-H stretch at $3333.7 cm^{-1}$; the bound CH stretch is red shifted about the same amount as in $HC=CH--NH_{3}$. These observations imply that the weak coupling of the local modes in diacetylene is significantly quenched upon complexation with $NH_{4}$. The $HC=CH=CH-NH_{3}$ predissociation linewidths are similar to those in $HC=CD--NH_{3}$, 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, $HC=CC=CC=CH-NH_{3}$.
Description
Author Institution: Molecular Physics Division, National Institute of Standards and Technology; Molecular Physics Divisoin, National Institute of Standards and Technology