The permanent electric dipole moment of CH_2D_2: FIR spectroscopy, centrifugal distortion effects and quantum Monte Carlo calculations with 9-dimensional analytical dipole moment and potential functions of methane

摘要

The pure rotational spectrum in the far-infrared between 30 and 170 cm~(-1) and its absolute intensity has been measured for CH_2D_2 in the vibrational ground state by high-resolution interferometric Fourier transform techniques. The analysis of the integrated cross-sections in the essentially water-free spectrum results in an accurate value for the permanent, vibrationally induced ground state electric dipole moment of CH_2D_2 |μ_0| = (6.40 ± 0.33) x 10~(-3) D. The influence of centrifugal effects on intensities and on the determination of the permanent dipole moment was investigated. Although centrifugal effects are important for the explanation of single band profiles, they appear to be of little relevance for the resulting permanent dipole moment. A new, more general 9-dimensional dipole moment function for methane is derived from ab initio calculations and experimental band strength information of CHD_3. Quantum Monte Carlo calculations using this function and a new, more general 9-dimensional analytical, anharmonic potential function for methane yield a semi-theoretical estimate μ_0~z = -(7.8 + 0.5) x 10~(-3) D for CH_2D_2.