Rotational energy level structure of stretching vibrational states in some small symmetrical molecules

Abstract

The vibrational dependence of various vibration–rotation parameters in XY2, XY3, and XY4 molecules with C2v, C3v, and Td point groups has been investigated between normal mode and local mode limits. The results at these two limiting cases differ drastically from each other. The rotational constants of the two lowest vibrational states in each overtone manifold (v=v1+v3=constant) are equal for v≥2 in the local mode limit. The effective Coriolis constants between these vibrational pairs disappear but the vibrationally off-diagonal H22 resonance terms remain important as the local mode limit is approached. The vibrational dependence of the rotational constants of the stretching vibrational states in the symmetrically substituted acetylenes and in normal silane has been analyzed. It is shown that it is necessary to couple levels by quartic anharmonic resonance terms in the normal coordinate space in order to explain the vibrational behavior of the rotational constants.