The rQ1 branch of carbodiimide, HNCNH, at 1.1 THz Original Research Article

The rQ1 branch of the skew-chain molecule HNCNH has been measured at 1.1 THz using a frequency and phase stabilized and continuously tunable backward wave oscillator with a frequency accuracy better than 100 kHz for unblended lines. The newly assigned transitions were added to the existing data base and then fitted to constants of the Watson Hamiltonian in S-reduction. The rQ1-branch spectrum is complex because it consists of the superposition of four subbranches resulting from an internal torsional motion through a trans- and a cis-torsional potential energy barrier of nearly equal height (∼2070 cm−1) and the resolved Ka doubling. As discovered previously, the Ka=2 energy levels in HNCNH are inverted, as in the H2S2 and D2S2 molecules. This is a consequence of the dominance of the centrifugal-distortion splitting over the pure inertial asymmetry splitting in these accidentally nearly symmetric tops. The contributions of the two effects to the energy levels and to the rQ1-branch transitions have been analysed as well as the J dependence of the torsional splitting.