Predictions for methane spectra from potential energy and dipole moment surfaces: Isotopic shifts and comparative study of 13CH4 and 12CH4

In this paper, we focus on the calculations of dipole transition intensities and rovibrational spectra for (CH4)-C-13 and (CH4)-C-12. Global variational calculations of methane spectra were performed using our recent potential energy and dipole moment surfaces, combined with the tensor formalism derived in previous works. Isotopic vibrational band center shifts due to the C-12 -> C-13 substitution were calculated up to the tetradecad range and compared with experimental values. These shifts were found to be quite irregular, but their variational predictions were very accurate, of the order similar to 0.01 cm(-1), and could thus be used for a precise calculation of (CH4)-C-13 line positions. Rovibrational line intensities computed from the ab initio dipole moment surfaces agree very well with those available in the HITRAN 2008 database, at least for strong and medium lines. For both isotopologues, the first comparative study of theoretical spectra with J(max) = 20 is presented up to the octad range. Our preliminary results suggest that numerous (CH4)-C-13 bands which remain still unassigned could be identified and modeled using the proposed approach. (C) 2013 Elsevier Inc. All rights reserved.