PURE ROTATIONAL SPECTRA OF HNCS IN THE FAR-INFRARED REGION - THE 3 BENDING EXCITED-STATES

The pure b-type rotational spectra of HNCS in the excited bending vibr ational states v(4) = 1, v(5) = 1, and v(6) = 1 were measured in the f ar infrared region. A large number of forbidden transitions have been observed between the ground state and the v(5) (NCS-in-plane bend) = 1 state and between the excited bending states, which allowed us to det ermine precisely the rovibrational energy levels of the excited states without measuring the spectra in the infrared. Effective molecular pa rameters which reproduce the observed line positions within the experi mental uncertainty have been determined in a global analysis. This glo bal analysis allowed us to make exact predictions for additional forbi dden transitions in the millimeter-wave region. They occur between the two bending states, the v(4) (HNC) and v(6) (NCS-out-of-plane) bendin g modes. Strong a-type Coriolis interaction causes these forbidden tra nsitions and it is discussed by introducing a Pade formulation to desc ribe the K-a rotational levels. (C) 1997 Academic Press.