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KELLY J. HIGGINS, ZHENHONG YU AND WILLIAM KLEMPERER, Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138.
The excited E internal rotor state of He-CH3F is studied using microwave spectroscopy and ab initio calculations. Bound state calculations on a high-level ab initio intermolecular potential surface predict a T-shaped ground state and two low-lying excited states with the He localized either at the C end (``linear'') or the F end (``anti-linear'') of the C-F bond, similar to that found for the A internal rotor state presented previously. There are several distinct differences between the A and E states. Whereas the ground state rotational transitions of the A state could be fit with a standard asymmetric rotor Hamiltonian, the E state exhibits a much more complex energy level structure due to the internal rotation of the CH3F. In addition, the E state is calculated to be bound by 11.709 cm-1, 0.249 cm-1 greater than the A state relative to their respective dissociation products, K = 1 or K = 0 CH3F, and the ground state to linear state energy gap is predicted to be 4.148 cm-1, 0.156 cm-1 greater than in the A state. At present six rotational transitions between five different energy levels have been observed and assigned with the aid of double resonance experiments and the ab initio calculations.