Rotational Spectra of Sulfur-Carbon Chains. I. The Radicals C and 4S, C5S, C6S, C7S, C8S, C9S

V. D. Gordon (currently with UT Austin), M. C. McCarthy, A. J. Apponi, and P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138The linear carbon-chain radicals C6S, C7S, C8S, and C9S have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy, and measurements of the previously studied chains C4S and C5S have been extended using the same technique. The electronic ground state, as predicted, is found to be a triplet for those with an even number of carbon atoms and a singlet for those with an odd number. The microwave spectra of all six chains have been fully characterized, and spectroscopic constants, including fine-structure constants where applicable, have been determined to high precision. Transitions from the J=N and J=N-1 fine-structure levels of C4S have been detected for the first time, and the lengths of the individual bonds of C5S have been determined from its isotopic species. The carbon-sulfur chains here are all highly polar, calculated to possess dipole moments in the 4-7 D range; all are plausible candidates for astronomical detection. The spectroscopic constants determined from this work allow transitions up to 40 GHz to be calculated with an uncertainty of less than 1 km s -1 for each chain. Subject headings : ISM: molecules -- line : identiÐcation -- molecular data -- molecular processes -- radio lines : ISMChemistr

Structures of the linear silicon carbides SiC 4 and SiC 6 : Isotopic substitution and Ab Initio theory

V. D. Gordon,a) E. S. Nathan, A. J. Apponi, M. C. McCarthy, and P. Thaddeus are with Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138 -- P. Botschwina is with the Institut fu¨r Physikalische Chemie der Universita¨t Go¨ttingen, Tammannstr, 6. D-37077 Go¨ttingen, GermanyThe structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6 , a bending mode calculated to lie about 205 cm -1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American Institute of Physics. @S0021-9606~00!01537-3#Chemistr

Laboratory detection of HC6N, A carbon chain with a triplet electronic ground state

V. D. Gordon (currently with UT Austin), M. C. McCarthy, A. J. Apponi, AND P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA 02138; and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138A linear triplet isomer of HC6N has been detected by Fourier transform microwave spectroscopy in a supersonic molecular beam. A total of 85 hyperfine components from six rotational transitions between 8 and 18 GHz were measured to an uncertainty of 5 kHz; a similar set of transitions were detected for the 15N isotopic species, produced using an isotopically enriched precursor gas sample. The spectroscopic constants for both species, including the fine and hyperfine coupling constants, were determined to very high accuracy, and these allow calculation of the radio spectrum to a fraction of 1 km s-1 in equivalent radial velocity. Triplet HC6N is a highly polar, low-lying isomer ; measurements show it to be about 10 times more abundant than a ring-chain isomer recently detected with the same spectrometer. Subject headings : ISM: molecules -- line : identification -- methods: laboratory -- molecular data -- molecular processes -- radio lines : ISMChemistr

Rotational Spectra of Sulfur Carbon Chains II: HC5S, HC6S, HC7S, & HC8S, and H2C4S, H2C5S, H2C6S, & H2C7S

V. D. Gordon (currently with UT Austin), M. C. McCarthy, A. J. Apponi, and P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA02138; and Division of Engineering and Applied Sciences, Harvard UniversityThe linear carbon chain radicals HC5S, HC6S, HC7S, and HC8S and the asymmetric top chains H2C4S, H2C5S, H2C6S, and H2C7S have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. Like the shorter HCnS chains, the electronic ground states of the four longer chains here alternate with even and odd number of carbon atoms: HC5S and HC7S have 2P1/2 ground states, while HC6S and HC8S have 2P3/2 ground states. Each of the four new thiocumulene chains possesses a rotational spectrum characteristic of a molecule with C2v symmetry, in agreement with that observed for the two shorter chains H2C2S and H2C3S. The microwave spectra of all eight chains have been fully characterized, and spectroscopic constants, including fine and hyperfine structure constants where applicable, have been determined to high precision. The eight new chains are probably all highly polar and all are plausible candidates for astronomical detection. For the radical chains, the spectroscopic constants determined from the present work allow the astronomically most relevant lines to be calculated with an uncertainty of less than 1 km s 1 up to 40 GHz; for the asymmetric top chains, the same transitions can be calculated with an uncertainty of less than 0.5 km s 1 up to 55 GHz. Subject headings: ISM: molecules — line: identification — molecular data — molecular processes — radio lines: ISMChemistr

The 1 mm spectrum of VY Canis Majoris: Chemistry in an O-rich envelope

We present preliminary results of an unbiased spectral survey at 1 mm of the oxygen-rich supergiant, VY CMa. A number of exotic molecules have been detected, including NaC1 and PO, and a relatively rich organic chemistry is observed. Results of the survey will be compared with carbon-rich stars

Laboratory Detection of Two New C5H2 Isomers

Vernita Gordon (currently with UT Austin) -- All authors are with the Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138. – M.C. McCarthy, A. J. Apponi, and P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138.Two new isomers of the C5H2 molecule have been detected in the laboratory, and their microwave spectra have been characterized to high accuracy. Both are good candidates for radio astronomical detection. Like the two isomers previously detected in the laboratory, both are closed-shell carbenes, and both are extremely polar and fairly stable. The first, calculated to lie 0.73 eV above isomer 1, the C5H2 ring-chain ground state, has a bent carbon chain backbone with single, double, and triple bonds; the second, calculated to lie only 0.19 eV higher in energy, is a new type of ring chain formed by adding a pair of doubly bonded carbon atoms to the top of the C3H2 ring, which is one of the most abundant interstellar molecules. Subject headings: ISM: molecules—line: identification—molecular data—molecular processes— radio lines: ISMPhysic

The 1 mm spectrum of VY Canis Majoris: Chemistry in an O-rich envelope

We present preliminary results of an unbiased spectral survey at 1 mm of the oxygen-rich supergiant, VY CMa. A number of exotic molecules have been detected, including NaC1 and PO, and a relatively rich organic chemistry is observed. Results of the survey will be compared with carbon-rich stars

Rotational spectrum and theoretical structure of the carbene HC 4 N

M. C. McCarthy,a) A. J. Apponi, V. D. Gordon (currently with UT Austin), C. A. Gottlieb, and P. Thaddeus are with the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138 and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 -- T. Daniel Crawford and John F. Stanton are with the Institute for Theoretical Chemistry, Departments of Chemistry and Biochemistry, The University of Texas,Following a high-level coupled cluster calculation, the rotational spectrum of the bent HC4N singlet carbene was detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants agree with those calculated ab initio to better than 0.5%. Like the isoelectronic C5H2 carbene of similar structure, HC4N was found to have fairly large centrifugal distortion and a large inertial defect. The calculated dipole moment of HC4N is 2.95 D. © 1999 American Institute of Physics. @S0021-9606~99!00339-6#Chemistr