Rotational spectra of SiCN, SiNC, and the SiCnH (n=2, 4-6) radicals

Three new silicon-carbon chains, SiC4H, SiC5H, and SiC6H, have been detecte d in a supersonic molecular beam by Fourier transform microwave spectroscop y. A detailed spectroscopic characterization of these and the previously de scribed chains SiCCH, SiCN, and SiNC is given here. All six radicals are li near chains with (2)Pi electronic ground states and all have resolvable hyp erfine structure in their lower rotational transitions; all except SiC5H ha ve resolved lambda-type doubling. Because transitions of SiCCH, SiCN, and S iNC were also detected by millimeter-wave absorption spectroscopy in both s pin components, for these the rotational, centrifugal distortion, and fine structure and hyperfine coupling constants were determined to high precisio n using the standard Hamiltonian for a molecule in a (2)Pi state. For SiC4H , SiC5H, and SiC6H, at least seven transitions in the lowest-energy fine st ructure component were measured between 7 and 30 GHz, and, at most, five sp ectroscopic constants were required to reproduce their spectra to a few par ts in 10(7). The hyperfine coupling constants of the SiCnH radicals are fai rly close to those of isovalent Cn+1H, indicating that the chemical bonding may be similar. The missing radical in the present sequence, SiC3H, may so on be found along with cyclic isomers of SiCCH and SiC4H. If SiC5H possesse s strong electronic transitions in the visible like isovalent C6H, its spec trum should be detectable by long path optical spectroscopy. (C) 2001 Ameri can Institute of Physics.