EVIDENCE FOR NONRIGID HCCN

Rotational transitions from seven low-lying vibrational states of HCCN and five low-lying vibrational states of DCCN have been detected in t he frequency range from 100 to 400 GHz with (+/-1) a sensitive millime ter-wave spectrometer. The CCH bending states nu(5)(+/-1), 2 nu(5)(+/- 2), and 3 nu(5)(+/-3), and the CCN bending state nu(4)(+/-1) have been assigned. In addition, transitions from three vibrational states in H CCN and one in DCCN with zero orbital angular momentum (l) were also d etected. These states in all likelihood originate from the three lowes t l=0 excited states, i.e., (nu(4)+nu(5)>(-0) (nu(4) + nu(5))(+0), and 2 nu(5)(0). Analysis of the high-accuracy millimeter-wave frequency d ata establishes that HCCN is not a near-rigid bent molecule and intens ity measurements confirm that the CCH bending states are much lower in energy than in typical well-behaved linear molecules. The low barrier to linearity in HCCN and DCCN of similar to 235 cm(-1), estimated fro m intensity measurements and the ab initio calculations of Malmquist e t al. [Theor. Chim. Acta 73, 155 (1988)], confirms that HCCN is quasil inear. (C) 1995 American Institute of Physics.