The ν12-ν9 band of ethane: A global frequency analysis of data from the four lowest vibrational states

The ν12-ν9 difference band of ethane in the region around 385 cm−1 has been recorded with a Bruker IFS-120 HR Fourier transform spectrometer at a resolution of 0.003 cm−1 to characterize interactions between the torsional stacks of symmetric rocking mode ν12 with its analog motion the anti-symmetric rocking mode ν9. This spectrum is analyzed with data from the ν3 and ν9 fundamentals, the ν9+ ν4-ν4 hot band and torsional transitions from the ν4,2ν4-ν4, and 3ν4 bands. The combined data set includes more than 4800 frequencies, and was fitted to within experimental accuracy using a 67-parameter model Hamiltonian. The first determination of molecular parameters for the ν12 state from a high resolution spectrum has been made. The selection rules for the ν12-ν9 band, the development of the coupling terms between ν9 and ν12 states and the possible extension of the model to treat the bands of ethane in the 1300 to 1600 cm−1 region are discussed.