The a-type rotational spectrum of isocyanic acid,HNCO, in the excited bending states

Rotational transitions of HNCO in the v₄ = 1, v₅ = 1, and v₆ = 1 vibrational states have been measured. The assignment of the a-type ^qR_K and ^qQ₁ branches has been made with the help of a qualitative discussion of the vibration-rotation interactions. Effective rotational and centrifugal distortion constants have been determined precisely for each vibrational K_a-rotational state, up to K_a = 4 for the lowest excited state and K_a = 3 for the other two excited states. The K_a dependence of the effective rotational constants B and D was observed to be quite anomalous for some of the transitions because of the a-type Coriolis interactions and accidental b-type Coriolis resonances. From a discussion of the selection rules and the effect on B and D of the interactions, the first excited state of the out-of-plane vibration, ν₆, has been assigned definitely to the second lowest excited vibrational state of HNCO.