The rotational spectrum of rhomboidal SiC3

Rhomboidal SiC3, a planar ring with C-2v symmetry and a transannular C-C bo nd, was detected at centimeter wavelengths in a pulsed supersonic molecular beam with a Fourier transform microwave (FTM) spectrometer, and was subseq uently observed in a low-pressure dc glow discharge with a free-space milli meter-wave absorption spectrometer. The rotational spectrum of SiC3 is char acterized by large harmonic defects and large splitting of the K-type doubl ets. Lines in the centimeter-wave band were very strong, allowing the singl y substituted isotopic species to be observed in natural abundance. Measure ments of the normal and five isotopically substituted species with the FTM spectrometer provided conclusive evidence for the identification and yielde d an experimental zero-point (r(0)) structure. Forty-six transitions betwee n 11 and 286 GHz with K(a)less than or equal to 6 were measured in the main isotopic species. Three rotational and nine centrifugal distortion constan ts in Watson's A-reduced Hamiltonian reproduce the observed spectrum to wit hin a few parts in 10(7) and allow the most intense transitions up to 300 G Hz to be calculated with high accuracy. The spectroscopic constants confirm that SiC3 is a fairly rigid molecule: the inertial defect is comparable to those of well-known planar rings and the centrifugal distortion constants are comparable to molecules of similar size. The number of SiC3 molecules i n our supersonic molecular beam in each gas pulse is at least 3x10(11), so large that electronic transitions may be readily detectable by laser spectr oscopy. (C) 1999 American Institute of Physics. [S0021-9606(99)00330-X].