High resolution FTIR Spectroscopy of CH2D35Cl: analysis of the nearly degenerate v4 and v8 levels

The high-resolution (0.005 cm−1) FTIR spectrum of CH2D35Cl has been investigated between 1160 and 1360 cm−1, the region of the ν4 (A′) and ν8 (A″) fundamentals. The ν4 mode is expected to show an a/b-hybrid band, whereas ν8 should generate a pure c-type envelope. The rovibrational levels (Δν0 for the deperturbed band centres is 0.66 cm−1 only) interact through a-and b-type Coriolis resonances. As a result, the P branch of the ν4 b-type component and the R branch of the ν8 c-type band are missing. In addition, the ν8 mode gives rise to a weak pseudo a-type component still deriving from interaction with ν4. Because of closeness of the two vibrational states, several ΔKa = 0 crossings occur between sublevels of different parity. The data set, composed of 3641 transitions of ν4 and 2624 of ν8, was fitted using Watson's A-reduction Hamiltonian in the Ir representation implemented with first and higher order a- and b-Coriolis coupling terms. Accurate upper state molecular constants, including all quartic centri...