Making use of CEPA-1, CCSD and CCSD(T) calculations with relatively large basis sets and taking the major anharmonicity effects into account, predictions are made for the following 12 molecules which are of interest to interstellar cloud chemistry: HC;

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The millimeter-wave rotational spectra of four isotopic species of the cumulene carbene propadienylidene (H 2 13 CCC, H 2 C 13 CC, H 2 CC 13 C, and D 2 CCC) were measured and the same set of rotational and centrifugal distortion constants used to describe the normal species [Vrtilek et al., Astrophys. J. Lett. 364, L53 (1990)] were determined, allowing the ro and rs structures to be derived. Vibration-rotation coupling constants calculated ab initio in the CEPA-l approximation were combined with the experimental rotational constants for the four isotopic species and the normal species to yield the equilibrium geometry: re (HC(l)) =1.083±0.OOl A, r e (C(l)C(2»)=1.3283±0.0005 A, r e (C(2)C(3») = 1.291 ±0.001 A, and L (HC(l)H) = 117.6±0.2°. The calculated spectroscopic properties may aid forthcoming high-resolution IR spectroscopy of H 2 CCC.

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