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

The high-resolution (0.005 cm1) FTIR spectrum of CH2D35Cl has been investigated between 1160 and 1360 cm1, the region of the v4 (A') and v8 (A'') fundamentals. The v4 mode is expected to show an a/b-hybrid band, whereas v8 should generate a pure c-type envelope. The rovibrational levels (Dv0 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 v4 b-type component and the R branch of the v8 c-type band are missing. In addition, the v8 mode gives rise to a weak pseudo a-type component still deriving from interaction with v4. Because of closeness of the two vibrational states, several DKa=0 crossings occur between sublevels of different parity. The data set, composed of 3641 transitions of v4 and 2624 of v8, 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 centrifugal distortion parameters, and six coupling coefficients have been determined.