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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 SiCn, n = 3 and 5-8, were detected by Fourier transform microwave spectroscopy in a supersonic molecular beam among the products of a gas discharge through silane and diacetylene; SiC3 was detected as well by conventional millimeter-wave absorption spectroscopy in a large glow discharge through silane and acetylene. The electronic ground state was found to be X1Σ+ for those with even n and X3Σ- for those with odd n, in agreement with recent ab initio calculations. The rotational and centrifugal distortion constants of all five molecules were determined to high accuracy, as were the spin-spin and spin-rotation coupling constants for triplet SiC3, SiC5, and SiC7. A fairly steep increase in the magnitude of the spin-spin constant with chain length, similar to that found previously for the Cn and CnO chains, was observed for the present silicon carbides. Of the five new silicon carbides, SiC3 is of particular astronomical interest because radio emission lines of the isomeric ground state, a compact rhomboidal ring, have recently been detected in IRC+10216 and because this chain is calculated to possess a large dipole moment of 4.8 D and to lie only 5 kcal mol-1 above ground. From the laboratory data, the most intense transitions of SiC3 can now be calculated to better than 1 km s-1 in equivalent radial velocity at frequencies up to 300 GHz.