The ν12+ν6−ν6 and ν11−ν12 bands of 12CH313CH3: A frequency analysis including data from the four lowest vibrational states

•A global fit of the lowest four vibrational states of 12CH3-13CH3 has been carried out.•The first determination of molecular parameters for the ν11 state has been made.•Partition functions between 100 and 320 K and lower state energies are calculated.•Results provided are used in the modeling of planetary atmospheres.

High-resolution infrared spectra of the ν12+ν6−ν6 (around 820 cm−1) and ν11−ν12 (around 370 cm−1) bands of 12CH313CH3 were measured. Frequencies from these bands together with data from the ν12 and ν5 fundamentals and the torsional bands were analyzed in a 4-state fit to determine the torsion mediated Coriolis and Fermi interactions. As compared to normal ethane this lower symmetry isotopologue shows more complicated vibrational couplings. The combined data set includes more than 6800 frequencies and was fitted to within experimental accuracy using a 78-parameter Hamiltonian. The first determination of molecular parameters for the ν11 state of 12CH313CH3 has been made and using this Hamiltonian we provide lower state energies and partition functions between 100 and 320 K in increment of 10 K for planetary data analysis.