STRUCTURE OF PROPADIENYLIDENE, H2CCC

The millimeter-wave rotational spectra of four isotopic species of the cumulene carbene propadienylidene (H2CCC)-C-13, (H2CCC)-C-13, (H2CCC)-C-13, and D2CCC) 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 r0 and r(s) structures to be derived. Vibration-rotation coupling constants calculated ab initio in the CEPA-1 approximation were combined with the experimental rotational constants for the four isotopic species and the normal species to yield the equilibrium geometry: r(e)(HC(1)) = 1.083+/-0.001 angstrom, r(e)(C(1)C(2)) = 1.3283+/-0.0005 angstrom, r(e)(C(2)C(3)) = 1.291+/-0.001 angstrom, and angle (HC(1)H) = 117.6+/-0.2-degrees. The calculated spectroscopic properties may aid forthcoming high-resolution IR spectroscopy of H2CCC.