THE PERMANENT ELECTRIC-DIPOLE MOMENT OF CH2D2 - FIR SPECTROSCOPY, CENTRIFUGAL-DISTORTION EFFECTS AND QUANTUM MONTE-CARLO CALCULATIONS WITH 9-DIMENSIONAL ANALYTICAL DIPOLE-MOMENT AND POTENTIAL FUNCTIONS OF METHANE

The pure rotational spectrum in the far-infrared between 30 and 170 cm (-1) and its absolute intensity has been measured for CH2D2 in the vib rational ground state by high-resolution interferometric Fourier trans form techniques. The analysis of the integrated cross-sections in the essentially water-free spectrum results in an accurate value for the p ermanent, vibrationally induced ground state electric dipole moment of CH2D2\mu(0)\ = (6.40 +/- 0.33) x 10(-3) D. The influence of centrifug al effects on intensities and on the determination of the permanent di pole moment was investigated. Although centrifugal effects are importa nt for the explanation of single band profiles, they appear to be of l ittle relevance for the resulting permanent dipole moment. A new, more general 9-dimensional dipole moment function for methane is derived f rom ab initio calculations and experimental band strength information of CHD3. Quantum Monte Carlo calculations using this function and a ne w, more general 9-dimensional analytical, anharmonic potential functio n for methane yield a semi-theoretical estimate mu(0)(z) = -(7.8 +/- 0 .5) x 10(-3) D for CH2D2.