Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine-dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations

Abstract

Articles you may be interested in Energetics and dipole moment of transition metal monoxides by quantum Monte CarloA full nine-dimensional potential-energy surface for hydrogen molecule-water collisions A nine-dimensional perturbative treatment of the vibrations of methane and its isotopomersThe pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD 3 and CH 3 D, and the integrated band strength of the N=5 CH-stretching overtone of CHD 3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (liLbl=(5.69±O.14)XIO-3 D for CHD 3 , 1fL51=(5.57±O.10)XIO-~ D for CH 3 D), consistent with previous experimental data. The integrated N = 5 overtone cross section is found to be (O.828±O.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH 4 within the cbromophore model for the CH cbromoph,ore in CHD 3 . 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 expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrodinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants 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 vibrationally averaged permanent . dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is fL5= -(6.6±0.4)XIO-3 D for CHD 3 (with positive z coordinate for the H atom) and ,u~=(6.8±O.5)XIO-3 D forCH 3 D (with positive z coordinate for 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 is C--H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.3588

Global Analytical Potential Energy Surfaces for High‐resolution Molecular Spectroscopy and Reaction Dynamics

“…State-of-the-art calculations of vibrational eigenstates of tetra-atomic and larger molecules yield deviations on the order of typically 1-5 cm −1 in the midinfrared, and 5-50 cm −1 in the near-infrared to visible range of overtone transitions and combination bands. The quality of bound vibrational eigenfunctions may be assessed by comparison of integrated absorption cross sections or transition dipole moments, which also critically depends on the quality of the dipole moment surface; given that experimental values are accurate to within 20-30%, comparison of transition dipole moments evolutions from fundamental to overtones are often considered to be satisfactory, if orders of magnitude match (Ha et al 1990, Hollenstein et al 1994, 1995, Signorell et al 1996, Marquardt et al 2003a, Yurchenko et al 2005b. The agreement may be much better, though, for fundamental transitions (Rosmus et al 2008).…”

An improved third order dipole moment surface for methane

“…Values obtained from our new fit are compared to those of [29]. Note, that some of the latter were slightly wrong due to a "copy-paste" error and are [48]. The sign convention for the normal coordinates is that of Gray and Robiette [15], so the signs of the derivatives including q 2a and q 2b of Loete [46] have been changed accordingly, (see also Tab.…”

“…In the same vein, a simultaneous determination of force constants and dipole moment first and (some) second derivatives of methane was performed by Mourbat et al [47]. First order derivatives were calculated ab initio by Hollenstein et al at the second order Moller-Plesset, double zeta plus polarization, MP2/DZP, level [48]. The same group combined ab initio and experimental band strength from CHD 3 to derive a nine dimensional DMS for methane [49].…”

High‐Resolution Fourier Transform Infrared Spectroscopy

“…This is, in turn, related to a long-standing question concerning the CH-bond dipole moment in methane (Hollenstein et al 1994, Signorell et al 1996 and literature cited therein. Figure 15(a) shows the FIR spectrum of the symmetric top isotopomers CH 3 D and CHD 3 measured in a light pipe cell of 2.71 m path length (Quack and Suhm 1990), specially developed for the IR showing pressure broadening.…”