MOLECULAR ROTATIONS AND LOCAL MODES

Rotational energy level structures of stretching vibrational states ha ve been investigated in the XH(2), XH(3), and XH(4) type symmetrical h ydrides. Transformations from standard vibration-rotation normal coord inate Hamiltonians are made to internal coordinate representations whi ch explicitly give the terms responsible for rotational coupling betwe en the different local mode states. It is shown that the local mode re lations between the vibration-rotation parameters as given by Lehmann [J. Chem. Phys. 95, 2361 (1991)] and by Halonen and Robiette [J. Chem. Phys. 84, 6861 (1986)] make these Hamiltonians diagonal in the local mode basis. The effective vibration-rotation parameters of overtones a re then proved to obey the local mode relations closer and closer as t he vibrational excitation increases. A simple vibrational model accoun ts well for the vibrational dependencies of vibration-rotation constan ts in the case of SiH4, GeH4, and SnH4.