Rotational energy level structure in the local mode limit

Abstract

The nature of the rotational energy level structure of the stretching vibrational states in XY2, XY3, and XY4 molecules (belonging to the C2v, C3v, and Td point groups, respectively) has been investigated in a local mode limit with no kinetic or potential energy copling between the bond oscillators and with the fundamental stretching frequencies much larger than the bending frequencies. The Coriolis effects between the rotational levels in the stretching vibrational states disappear in this limiting case, but the H22 resonance terms are seen to be important owing to the multiple vibrational degeneracies. Simple relationships are obtained between the coefficients of the vibrationally diagonal and off-diagonal H22 terms and consequently the number of independent molecular parameters is reduced. The model is compared with experimental data for XHn hydrides, and particularly in the molecules with a heavy central mass the major contribution to the H22 coefficients is well explained.