First Laboratory Detection of N13CO- and Semiexperimental Equilibrium Structure of the NCO- Anion.

The cyanate anion (NCO-) is a species of considerable astrophysical relevance. It is widely believed to be embedded in interstellar ices present in young stellar objects but has not yet been detected in the dense gas of the interstellar medium. Here we report highly accurate laboratory measurements of the rotational spectrum of the N13CO- isotopologue at submillimeter wavelengths along with the detection of three additional lines of the parent isotopologue up to 437.4 GHz. With this new data, the rotational spectrum of both isotopologues can be predicted to better 0.25 km s-1 in equivalent radial velocity up to 1 THz, more than adequate for an astronomical search in any source. Moreover, a semiexperimental equilibrium structure of the anion is derived by combining the experimental ground-state rotational constants of the two isotopologues with theoretical vibrational corrections, obtained by using the coupled-cluster method with inclusion of single and double excitations and perturbative inclusion of triple excitations (CCSD(T)). The estimated accuracy of the two bond distances is on the order of 5 × 10-4 Å: a comparison to the values obtained by geometry optimization with the CCSD(T) method and the use of a composite scheme, including additivity and basis-set extrapolation techniques, reveals that this theoretical procedure is very accurate.

Zeeman effect in sulfur monoxide: A tool to probe magnetic fields in star forming regions.

CONTEXT: Magnetic fields play a fundamental role in star formation processes and the best method to evaluate their intensity is to measure the Zeeman effect of atomic and molecular lines. However, a direct measurement of the Zeeman spectral pattern from interstellar molecular species is challenging due to the high sensitivity and high spectral resolution required. So far, the Zeeman effect has been detected unambiguously in star forming regions for very few non-masing species, such as OH and CN. AIMS: We decided to investigate the suitability of sulfur monoxide (SO), which is one of the most abundant species in star forming regions, for probing the intensity of magnetic fields via the Zeeman effect. METHODS: We investigated the Zeeman effect for several rotational transitions of SO in the (sub-)mm spectral regions by using a frequency-modulated, computer-controlled spectrometer, and by applying a magnetic field parallel to the radiation propagation (i.e., perpendicular to the oscillating magnetic field of the radiation). To support the experimental determination of the g factors of SO, a systematic quantum-chemical investigation of these parameters for both SO and O2 has been carried out. RESULTS: An effective experimental-computational strategy for providing accurate g factors as well as for identifying the rotational transitions showing the strongest Zeeman effect has been presented. Revised g factors have been obtained from a large number of SO rotational transitions between 86 and 389 GHz. In particular, the rotational transitions showing the largest Zeeman shifts are: N, J = 2, 2 ← 1, 1 (86.1 GHz), N, J = 4, 3 ← 3, 2 (159.0 GHz), N, J = 1, 1 ← 0, 1 (286.3 GHz), N, J = 2, 2 ← 1, 2 (309.5 GHz), and N, J = 2, 1 ← 1, 0 (329.4 GHz). Our investigation supports SO as a good candidate for probing magnetic fields in high-density star forming regions.

Laboratory measurements and astronomical search for the HSO radical.

CONTEXT: Despite the fact that many sulfur-bearing molecules, ranging from simple diatomic species up to astronomical complex molecules, have been detected in the interstellar medium, the sulfur chemistry in space is largely unknown and a depletion in the abundance of S-containing species has been observed in the cold, dense interstellar medium (ISM). The chemical form of the missing sulfur has yet to be identified. AIMS: For these reasons, in view of the fact that there is a large abundance of triatomic species harbouring sulfur, oxygen, and hydrogen, we decided to investigate the HSO radical in the laboratory to try its astronomical detection. METHODS: High-resolution measurements of the rotational spectrum of the HSO radical were carried out within a frequency range well up into the THz region. Subsequently, a rigorous search for HSO in the two most studied high-mass star-forming regions, Orion KL and Sagittarius (Sgr) B2, and in the cold dark cloud Barnard 1 (B1-b) was performed. RESULTS: The frequency coverage and the spectral resolution of our measurements allowed us to improve and extend the existing dataset of spectroscopic parameters, thus enabling accurate frequency predictions up to the THz range. These were used to derive the synthetic spectrum of HSO, by means of the MADEX code, according to the physical parameters of the astronomical source under consideration. For all sources investigated, the lack of HSO lines above the confusion limit of the data is evident. CONCLUSIONS: The derived upper limit to the abundance of HSO clearly indicates that this molecule does not achieve significant abundances in either the gas phase or in the ice mantles of dust grains.

Molecular polymorphism: microwave spectra, equilibrium structures, and an astronomical investigation of the HNCS isomeric family.

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.

Detection of nitrogen-protonated nitrous oxide (HNNO+) by rotational spectroscopy.

The rotational spectrum of nitrogen-protonated nitrous oxide (HNNO(+)), an isomer whose existence was first inferred from kinetic studies more than 30 years ago, has now been detected by Fourier transform microwave spectroscopy, guided by new high-level coupled-cluster calculations of its molecular structure. From high-resolution measurements of the hyperfine splitting in its fundamental rotational transition, the rotational constant (B + C)/2 and the quadrupole tensor element χaa(N) for both nitrogen atoms have been precisely determined. The derived constants agree well with quantum-chemical calculations here and others in the literature. The χaa(N) values for the two isomers of protonated nitrous oxide are qualitatively consistent with the valence bond description of H-N═N(+)═O for the electronic structure of the nitrogen-protonated form and N≡N(+)-O-H for the oxygen-protonated form. HNNO(+) is found to be 2-4 times less abundant than NNOH(+) under a range of experimental conditions, as might be expected because this metastable isomer is known to be only ∼6 kcal mol(-1) less stable than ground-state NNOH(+) from kinetic measurements by Ferguson and co-workers.

Detection of protonated vinyl cyanide, CH2CHCNH+, a prototypical branched nitrile cation.

The rotational spectrum of protonated vinyl cyanide, CH2CHCNH(+), a prototypical branched nitrile species and likely intermediate in astronomical sources and in the planetary atmosphere of Titan, has been detected in a pulsed-discharge supersonic molecular beam by means of Fourier transform microwave spectroscopy. Fifteen lines arising from 11 a-type rotational transitions have been observed between 9 and 46 GHz, several with partially resolved nitrogen hyperfine structure. From this data set, the leading spectroscopic constants, including all three rotational constants, have been determined to high accuracy. The agreement between experimental rotational constants and those calculated at the CCSD(T) level of theory is of order 0.1%. An even better estimate was obtained through empirical scaling using calculated and experimental rotational constants of isoelectronic vinyl acetylene. Measurement of a small nitrogen quadrupole coupling constant in protonated vinyl cyanide is consistent with a quadruply bound nitrogen atom and a H(+)-N≡C-R type structure. Because vinyl cyanide is abundant in molecule-rich astronomical sources and possesses a high proton affinity, and because protonated vinyl cyanide is unreactive with hydrogen and other well-known interstellar species, this cation is an excellent candidate for astronomical detection. The present work suggests that other organic molecules containing the nitrile group and closely related species such as protonated vinyl acetylene can probably be detected with the same instrumentation.

On the molecular structure of HOOO.

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H(2)O and O(2) heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H(2) and O(2) as precursor gases. Using equal mixtures of normal and (18)O(2), it has been possible to detect three new isotopic species, H(18)OOO, HO(18)O(18)O, and H(18)O(18)O(18)O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O(2) → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r(0)) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 Å), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 Å), and a more acute [angle]HOO angle (92.4°) when compared to equilibrium (r(e)) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (r(e) (emp)) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.

Two Isomers of Protonated Isocyanic Acid: Evidence for an Ion-Molecule Pathway for HNCO ↔ HOCN Isomerization.

Ion-molecule reactions are thought to play a crucial role in the formation of metastable isomers, but relatively few protonated intermediates beyond HNCH(+) have been characterized at high spectral resolution. We present here laboratory measurements of the rotational spectra of protonated isocyanic acid in two isomeric forms, the ground state H2NCO(+) with C2v symmetry and a low-lying bent chain HNCOH(+), guided by coupled cluster calculations of their molecular structure. Somewhat surprisingly, HNCOH(+) is found to be more abundant than H2NCO(+), even though this metastable isomer is calculated to lie approximately 15-20 kcal/mol higher in energy. In the same way that HCNH(+) serves as a key intermediate in ion-molecule reactions that form HNC via dissociative electron recombination in cold dense interstellar molecular clouds, HNCOH(+) may play an analogous role in the conversion of HNCO to HOCN.

Spatial distributions and interstellar reaction processes.

Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical-radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope.

Silicon Oxysulfide, OSiS: Rotational Spectrum, Quantum-Chemical Calculations, and Equilibrium Structure.

Silicon oxysulfide, OSiS, and seven of its minor isotopic species have been characterized for the first time in the gas phase at high spectral resolution by means of Fourier transform microwave spectroscopy. The equilibrium structure of OSiS has been determined from the experimental data using calculated vibration-rotation interaction constants. The structural parameters (rO-Si = 1.5064 Å and rSi-S = 1.9133 Å) are in very good agreement with values from high-level quantum chemical calculations using coupled-cluster techniques together with sophisticated additivity and extrapolation schemes. The bond distances in OSiS are very short in comparison with those in SiO and SiS. This unexpected finding is explained by the partial charges calculated for OSiS via a natural population analysis. The results suggest that electrostatic effects rather than multiple bonding are the key factors in determining bonding in this triatomic molecule. The data presented provide the spectroscopic information needed for radio astronomical searches for OSiS.

Laboratory detection of protonated SO2 in two isomeric forms.

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO(2), is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO(+) is an excellent candidate for radioastronomical detection.