The frequency of a vibrational mode depends on the reduced mass characteristic of the mode, and the restoring force, driving the molecule towards the equlibrium configuration. If such reduced mass is large and/or restoring force is small, the eigenfrequency of the mode becomes low. In certain cases, the transition frequencies of vibration modes in question lies in sub-teraherz region. Vibrational spectroscopy of such low frequency vibrational modes provides a unique opportunity to explore the details of the potential surfaces of the molecules and intermolecular interaction between moieties comprising weakly bound complexes, such as van der Waals molecule. For the purpose of searching for and studying, of such low frequency vibrational modes, a new spectrometer operating in the submillimeter wave region has been designed. The apparatus combines a previously developed FASSST (FAst Scan Submillimeter wave Spectroscopic Technique) technique with a pulsed supersonic jet sample. We have demonstrated that the apparatus is capable of performing the search for the previously unobserved weak transitions by observation of the transitions in Ar-CO and low abundance isotopomers of CO.
A series of van der Waals complexes Rg-ND3 (Rg = Ne, Ar, Kr) have been studied using the newly developed technique. In particular, a vibrational-torsional-rotation band Σ 00 - Π 10 has been observed in all of the complexes involved, including 2 isotopomers of Ne-ND3 and 4 isotopomers of Kr-ND3. The two nuclear spin components corresponding to the different inversion states of ammonia, were observed, and the effect of complexation on the inversional motion of ammonia has been studied.
Another kind of a large amplitude motion, pseudorotation, in heterocyclic five-membered rings, such as tetrahydrofuran (THF) and 1,3-dioxolane (DOX), has been studied. One pseudororotational band in THF and two bands in DOX were observed, which have not been reported previously. The observation of the new bands in both molecules allowed us to determine the symmetries and the energies of the lowest pseudorotational states, and to derive the functional form of the barrier to pseudorotation. An analytical and computational formalism for the description of lightly hindered pseudorotation characteristics has been developed. A discussion of the analytical capabilities, and potential further improvements to the experimental technique is given.