Laboratory detection of five new linear silicon carbides: SiC3, SiC5, SiC6, SiC7, and SiC8

The rotational spectra of five highly polar linear silicon-carbon chains Si Cn, n = 3 and 5-8, were detected by Fourier transform microwave spectroscop y in a supersonic molecular beam among the products of a gas discharge thro ugh silane and diacetylene; SiC3 was detected as well by conventional milli meter-wave absorption spectroscopy in a large glow discharge through silane and acetylene. The electronic ground state was found to be X(1)Sigma(+) fo r those with even n and X(3)Sigma(-) for those with odd n, in agreement wit h recent ab initio calculations. The rotational and centrifugal distortion constants of all five molecules were determined to high accuracy, as were t he spin-spin and spin-rotation coupling constants for triplet SiC3, SiC5, a nd SiC7. A fairly steep increase in the magnitude of the spin-spin constant with chain length, similar to that found previously for the C-n and CnO ch ains, was observed for the present silicon carbides. Of the five new silico n carbides, SiC3 is of particular astronomical interest because radio emiss ion lines of the isomeric ground state, a compact rhomboidal ring, have rec ently been detected in IRC+10216 and because this chain is calculated to po ssess a large dipole moment of 4.8 D and to lie only 5 kcal mol(-1) above g round. From the laboratory data, the most intense transitions of SiC3 can n ow be calculated to better than 1 km s(-1) in equivalent radial velocity at frequencies up to 300 GHz.