Isotopic substitution shifts in methane and vibrational band assignment in the 5560–6200 cm−1 region

The absorption spectra of the 12CH4 and 13CH4 molecules have been recorded and assigned in the 5560–6200 cm−1 region. The effects of isotopic substitution for 12C by 13C on the methane vibrational energy levels have been calculated from an ab initio potential energy surface and compared with experiment. Comparison of the results obtained for two isotopic species allows us to confirm the vibrational assignment for the strongest bands of 12CH4 in this region. Good agreement of ab initio calculations with observed energy levels has been demonstrated. A list of the assigned 13CH4 lines valuable in atmospheric applications is reported.