Rotational spectral lines of the H2CSi molecule are observed in the millimeter-wave and submillimeter-wave regions. The molecule is produced in a glow-discharge plasma of a gaseous mixture of SiH4 and CO. Rotational constants and centrifugal distortion constants have precisely been determined from observed frequencies of 32 a-type transitions with J=5–4 to J=12–11 and Ka=0 to Ka=4. From the observed inertial defect, the vibrational frequency of the CH2 rocking mode is estimated to be as low as 331 cm−1, which is consistent with the large quartic centrifugal distortion constants, DJK and d2. Higher-order centrifugal distortion constants up to the octic terms are necessary to obtain a good fit between the observed and calculated frequencies within experimental uncertainties. The low vibrational frequency and the necessity of the higher-order centrifugal distortion terms indicate a floppy nature of the CH2 rocking mode. A preliminary radioastronomical search for H2CSi in space has been carried out toward a few astronomical sources without success.

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The rotational spectra of silacyclopropenylidene, c-C2H2Si, and its isotopic species (C2H229Si, C2D2Si, 13C2H2Si) were observed in the frequency region of 220–400 GHz by using a source-modulated microwave spectrometer combined with a free space absorption cell. c-C2H2Si was produced in the cell by discharging a mixture of SiH4, C2H2, and He. Least-squares analyses of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants for the normal and its isotopic species. From the observed rotational constants, the rs structure was determined: rs (C=C) = 1.3458 Å, rs (C—Si) = 1.8200 Å rs (C—H) = 1.0795 Å, and [Formula: see text]. (1 Å = 10−10 m.)