DIPOLE-MOMENT FUNCTION AND EQUILIBRIUM STRUCTURE OF METHANE IN AN ANALYTICAL, ANHARMONIC 9-DIMENSIONAL POTENTIAL SURFACE RELATED TO EXPERIMENTAL ROTATIONAL-CONSTANTS AND TRANSITION MOMENTS BY QUANTUM MONTE-CARLO CALCULATIONS

The pure rotational spectrum in the far-infrared and its absolute inte nsity in the vibrational ground state of CHD3 and CH3D, and the integr ated band strength of the N=5 CH-stretching overtone of CHD, in the ne ar infrared to visible were measured by high-resolution interferometri c Fourier transform techniques. The far-infrared data result in perman ent electric dipole moments (\mu(0)(z)\=(5.69+/-0.14)x10(-3) D for CHD 3, \mu(0)(z)\=(5.57+/-0.10)x10(-3) D for CH3D), consistent with previo us experimental data. The integrated N=5 overtone cross section is fou nd to be (0.828+/-0.068) fm(2). The overtone data are used, together w ith previous data, to derive a new, nine-dimensional, isotopically inv ariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expect ation values on a new, fully anharmonic potential surface was performe d using the solution of the rovibrational Schrodinger equation by diff usion quantum Monte Carlo methods. The results for the rotational cons tants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is r(e)=108.6 pm. The calculated value for the vibr ationally averaged permanent dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function co nsistent with the analysis of the overtone bands, is mu(0)(z) =(6.8+/- 0.5)X10(-3) D for CHD3 (with positive z coordinate for the H atom) and mu(0)(z)=(6.8+/-0.5)X10(-3) D for CH3D (with positive z coordinate fo r the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane i s C--H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.