Ab initio variational predictions for understanding highly congested spectra: rovibrational assignment of 108 new methane sub-bands in the icosad range (6280-7800 cm-1)

A detailed study of methane spectra in the highly congested icosad range of 6280-7800 cm(-1) has been performed using global variational calculations derived from accurate ab initio potential energy and dipole moment surfaces. About 13 000 (CH4)-C-12 lines of the WKLMC line lists recorded at 80 and 296 K using very sensitive laser techniques (DAS, CRDS) have been rovibrationally assigned from first principles predictions. Overall, a total of 7436 upper energy levels were determined. Among the 20 bands and the 134 sub-levels contained in the icosad system, 20 and 108 have been identified for the first time, respectively. The assigned transitions represent 98% of the sum of the experimental intensity at 80 K. This work demonstrates for the first time how accurate first principles global calculations allow assigning complicated spectra of a molecule with more than 4 atoms.