title: Molecular polymorphism: microwave spectra, equilibrium structures, and an astronomical investigation of the HNCS isomeric family.
abstract: The rotational spectra of thioisocyanic acid (HNCS), and its three energetic isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution by a combination of chirped-pulse and Fabry-Pérot Fourier-transform microwave spectroscopy between 6 and 40 GHz in a pulsed-jet discharge expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid (HSNC), calculated here to be 35-37 kcal mol(-1) less stable than the ground state isomer HNCS, have been detected for the first time. Precise rotational, centrifugal distortion, and nitrogen hyperfine coupling constants have been determined for the normal and rare isotopic species of both molecules; all are in good agreement with theoretical predictions obtained at the coupled cluster level of theory. On the basis of isotopic spectroscopy, precise molecular structures have been derived for all four isomers by correcting experimental rotational constants for the effects of rotation-vibration interaction calculated theoretically. Formation and isomerization pathways have also been investigated; the high abundance of HSCN relative to ground state HNCS, and the detection of strong lines of SH using CH3CN and H2S, suggest that HSCN is preferentially produced by the radical-radical reaction HS + CN. A radio astronomical search for HSCN and its isomers has been undertaken toward the high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m Green Bank Telescope. While we find clear evidence for HSCN, only a tentative detection of HNCS is proposed, and there is no indication of HCNS or HSNC at the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations from a single-excitation temperature model, suggesting weak masing may be occurring in some transitions in this source.
authors.affiliation: National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Brett A
authors.initials: BA
authors.lastname: McGuire
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
authors.forename: Marie-Aline
authors.initials: MA
authors.lastname: Martin-Drumel
authors.affiliation: I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany.
authors.forename: Sven
authors.initials: S
authors.lastname: Thorwirth
authors.affiliation: I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany.
authors.forename: Sandra
authors.initials: S
authors.lastname: Brünken
authors.affiliation: Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: Valerio
authors.initials: V
authors.lastname: Lattanzi
authors.affiliation: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904, USA.
authors.forename: Justin L
authors.initials: JL
authors.lastname: Neill
authors.affiliation: Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: Silvia
authors.initials: S
authors.lastname: Spezzano
authors.affiliation: Aerodyne Research, Inc, Billerica, MA, USA.
authors.forename: Zhenhong
authors.initials: Z
authors.lastname: Yu
authors.affiliation: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904, USA.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Zaleski
authors.affiliation: National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Remijan
authors.affiliation: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904, USA.
authors.forename: Brooks H
authors.initials: BH
authors.lastname: Pate
authors.affiliation: National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA and School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA. mccarthy@cfa.harvard.edu.
authors.forename: Michael C
authors.initials: MC
authors.lastname: McCarthy