THE (R)Q(1) BRANCH OF CARBODIIMIDE, HNCNH, AT 1.1 THZ

The (r)Q(1) branch of the skew-chain molecule HNCNH has been measured at 1.1 THz using a frequency and phase stabilized and continuously tun able backward wave oscillator with a frequency accuracy better than 10 0 kHz for unblended lines. The newly assigned transitions were added t o the existing data base and then fitted to constants of the Watson Ha miltonian in S-reduction. The (r)Q(1)-branch spectrum is complex becau se it consists of the superposition of four subbranches resulting from an internal torsional motion through a trans-and a cis-torsional pote ntial energy barrier of nearly equal height(similar to 2070 cm(-1)) an d the resolved K-a doubling. As discovered previously, the K-a = 2 ene rgy levels in HNCNH are inverted, as in the H2S2 and D2S2 molecules. T his is a consequence of the dominance of the centrifugal-distortion sp litting over the pure inertial asymmetry splitting in these accidental ly nearly symmetric tops, The contributions of the two effects to the energy levels and to the (r)Q(1)-branch transitions have been analysed as well as the J dependence of the torsional splitting. (C) 1997 Else vier Science B.V.