The millimeterhyphen;wave rotational spectra of four isotopic species of the cumulene carbene propadienylidene (H213CCC, H2C13CC, H2CC13C, and D2CCC) were measured and the same set of rotational and centrifugal distortion constants used to describe the normal species lsqb;Vrtileketal., Astrophys. J. Lett.364, L53 (1990)rsqb; were determined, allowing ther0andrsstructures to be derived. Vibrationndash;rotation coupling constants calculatedabinitioin the CEPAhyphen;1 approximation were combined with the experimental rotational constants for the four isotopic species and the normal species to yield the equilibrium geometry:re(HC(1))=1.083plusmn;0.001 Aring;,re(C(1)C(2))=1.3283plusmn;0.0005 Aring;,re(C(2)C(3))=1.291plusmn;0.001 Aring;, and angupr; (HC(1)H)=117.6plusmn;0.2deg;. The calculated spectroscopic properties may aid forthcoming highhyphen;resolution IR spectroscopy of H2CCC.
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