Vibration-rotation interactions and local modes in tetrahedral molecules

Abstract

Vibration-rotation coupling in tetrahedral molecules is treated by using tensorial formalism in the T d point group and by expressing the vibrational part of the rovibrational operators in terms of “local” bond stretching coordinates and momenta. Due to the X -H stretching anharmonicity any realistic calculation of the rotational structures of the excited stretching states of an X H 4 molecule is much simpler by this approach than by the customary description of the X -H stretches in terms of ν 1 and ν 3 normal modes. The observed first-order Coriolis coupling coefficients of several excited states of CH 4 and SiH 4 are very well accounted for by considering the harmonic interaction with the ν 4 bending mode. A very high resolution spectrum of the 2 ν 3 band of SiH 4 is interpreted from P (4) to R (3) (i.3., about 40 transitions) with a rms deviation of 3 × 10 −3 cm −1 by using only two adjustable parameters in the effective vibration-rotation Hamiltonian.