Rotational Spectra of the Nitrogen-Sulfur Carbon Chains NCnS, n = 1–7

M. C. McCarthy, S. Mohamed, V. D. Gordon (currently with UT Austin), P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, and Division of Engineering & Applied Sciences, Harvard University, 29 Oxford Street, Cambridge,MA02138.-- A. L. Cooksy is with the Department of Chemistry, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182.Seven carbon chain radicals NCnS where n = 1-7 have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. Although NCCS is found to have a bent structure and an asymmetric top spectrum, the five longer chains have linear heavy-atom backbones, and like the isovalent HCnS chains, the electronic ground states of each alternate with even and odd numbers of carbon atoms: NC4S and NC6S have 2∏1/2 ground states, while NC3S, NC5S, and NC7S have 2∏3/2 ground states. In addition, the lowest-rotational transitions of the NCS radical have been detected in both fine-structure levels in the same supersonic molecular beam source, allowing a precise determination of the ground state hyperfine coupling constants. The centimeter-wave spectra of NC3S and the four longer chains have been fully characterized, and spectroscopic constants, including those that describe the lambda-type doubling and hyperfine structure from the nitrogen nucleus, have been determined to high precision. A complete account of the centimeter-wave spectrum of NCCS will be given by Nakajima et al. Predicted properties from UQCISD/cc-pVDZ ab initio calculations are also reported for the NCnS chains up to NC7S. All six new chains are highly polar and all are plausible candidates for astronomical detection because CnS chains up to C3S and CnN chains up to C5Nare detected in both interstellar and circumstellar sources. Subject headings: ISM: molecules — line: identification — methods: laboratory — molecular data — molecular processes — radio lines: ISMChemistr