Knowledge Bank

The microwave spectra of $ClHC=C=CH2$, $BrHC=C=CH2$, and $IHC=C=CH2$ have been studied by a conventional Stark modulation spectrometer. The rotational constants in A, B, and C in a vibrational ground state were determined from both a- and b-type transitions frequencies. The nuclear quadrupole coupling constants $Xaa$, $Xbb$, $Xcc$, and $Xab$ for chlorine, bromine, and iodine have been determined from the hyperfine splittings. The molecular structure which reproduce the observed moments of inertia were obtained under the reasonable assumptions. Systematic changes among the structural parameters in the series of monohalogenated allenes, XHC=C=$CH2$ (X = H, F, Cl, Br, and l) were observed. The bond lengths and the quadrupole coupling constants for halogenated ethenes, allenes, and ethanes, revealed that the double bond in allenes has more single bond character than that in ethenes.