Evidence for nonrigid HCCN

Publisher Website
Google Scholar
Abstract
Rotational transitions from seven low‐lying vibrational states of HCCN and five low‐lying vibrational states of DCCN have been detected in the frequency range from 100 to 400 GHz with a sensitive millimeter‐wave spectrometer. The CCH bending states ν5±1, 2ν5±2, and 3ν5±3, and the CCN bending state ν4±1 have been assigned. In addition, transitions from three vibrational states in HCCN and one in DCCN with zero orbital angular momentum (l) were also detected. These states in all likelihood originate from the three lowest l=0 excited states, i.e., (ν4+ν5)−0, (ν4+ν5)+0, and 2ν05. Analysis of the high‐accuracy millimeter‐wave frequency data establishes that HCCN is not a near‐rigid bent molecule and intensity 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 ∼235 cm−1, estimated from intensity measurements and the ab initio calculations of Malmquist et al. [Theor. Chim. Acta 73, 155 (1988)], confirms that HCCN is quasilinear.