Knowledge Bank

The acetylenic C-H stretch of propargyl amine $(HC=CCH2NH2)$ has been measured using the electric resonance optothermal spectrometer at NIST. This region of the spectrum shows extensive vibrational mode couplings characterisitic of the intramolecular vibrational energy redistribution (IVR) process. Analysis of complex vibrational spectra in terms of the vibrational dynamics requires that the spectrum be fully assigned. In the case of propargyl amine this requires assignment not only of the rotational quantum numbers but also the nuclear spin species. Both singlet and triplet modifications of the molecule are possible due to the large amplitude inversion-internal rotation motion of the amine group. The barrier to this motion is sufficiently high that no splitting is observed in the ground vibrational state preventing nuclear spin assignment by combination differences or microwave double resonance. This final assignment was made using $10\mu m/3\mu m$ infrared-infrared double resonance. The IVR lifetime is about 500 ps for the acetylenic $C-H$ stretch and is comparable to the lifetime of structurally similar acetylenic compounds. There does appear to be a weak K dependence in the vibrational coupling. The measured density of states is significantly higher than the calculated density of states suggesting that the large amplitude part of the problem is not yet well established.