A Theoretical Model for the Rotation and Vibration of Symmetrical Triatomic Molecules with Strong Coupling Between the Local Stretching Modes

Abstract

The present paper reports the derivation of a new theoretical model describing the rotation and vibration of a triatomic molecule. The new method is based on the MORBID (Morse Oscillatory Rigid Bender Internal Dynamics) method (P. Jensen, J. Mol. Spectrosc.128, 478-501 (1988); P. Jensen, J. Chem. Soc. Faraday Trans. 284, 1315-1340 (1988)), but is especially adapted for symmetrical triatomic molecules for which the masses of the "outer" atoms are comparable to or larger than that of the central atom. The new approach employs symmetrized bond length coordinates r1 + r3 and r1 − r3 for describing the stretching motion, whereas the original MORBID approach uses the "local mode" coordinates r1 and r3. We use the new approach to calculate the rotation-vibration energies for C3 on the basis of an ab initio calculation by P. Jensen, C. McMichael Rohlfing, and J. Almlöf (J. Chem. Phys.97, 3399-3411 (1992)) and compare our results with energies calculated by these authors by means of the original MORBID approach. We show that she choice of the analytical function used to represent the potential energy function has a significant influence on the calculated rotation-vibration energies.