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We present the first high spectral resolution mid-infrared survey in the Orion BN/KL region, covering 7.2–28.3 μ m. With SOFIA/EXES, we target the enigmatic source Orion IRc2. While this is in the most prolifically studied massive star-forming region, longer wavelengths and molecular emission lines dominated previous spectral surveys. The mid-infrared observations in this work access different components and molecular species in unprecedented detail. We unambiguously identify two new kinematic components, both chemically rich with multiple molecular absorption lines. The "blue clump" has v LSR = −7.1 ± 0.7 km s−1, and the "red clump" has 1.4 ± 0.5 km s−1. While the blue and red clumps have similar temperatures and line widths, molecular species in the blue clump have higher column densities. They are both likely linked to pure rotational H2 emission also covered by this survey. This work provides evidence for the scenario that the blue and red clumps are distinct components unrelated to the classic components in the Orion BN/KL region. Comparison to spectroscopic surveys toward other infrared targets in the region show that the blue clump is clearly extended. We analyze, compare, and present in-depth findings on the physical conditions of C2H2, 13CCH2, CH4, CS, H2O, HCN, H 13 CN , HNC, NH3, and SO2 absorption lines and an H2 emission line associated with the blue and red clumps. We also provide limited analysis of H2O and SiO molecular emission lines toward Orion IRc2 and the atomic forbidden transitions [Fe ii ], [S i ], [S iii ], and [Ne ii ]. [ABSTRACT FROM AUTHOR]

The interstellar presence of protonated nitrous oxide has been suspected for some time. Using established high-accuracy quantum chemical techniques, spectroscopic constants and fundamental vibrational frequencies are provided for the lower energy O-protonated isomer of this cation and its deuterated isotopologue. The vibrationally-averaged B0 and C0 rotational constants are within 6 MHz of their experimental values and the DJ quartic distortion constants agree with experiment to within 3%. The known gas phase O-H stretch of NNOH+ is 3330.91 cm-1, and the vibrational configuration interaction computed result is 3330.9 cm-1. Other spectroscopic constants are also provided, as are the rest of the fundamental vibrational frequencies for NNOH+ and its deuterated isotopologue. This high-accuracy data should serve to better inform future observational or experimental studies of the rovibrational bands of protonated nitrous oxide in the interstellar medium and the laboratory. [ABSTRACT FROM AUTHOR]

Recent work on cyclopropenylidene, c-C 3 H 2 , sought to improve on past ab initio results with a complete basis set (CBS) quartic force field method capable of predicting fundamental bands to within 5 cm−1 accuracy of gas-phase spectroscopy experiments for semirigid molecules. It was suggested that a problematic coupling between n-body correlation and 1-particle basis set effects prevented a proper application of this method for the out-of-plane bending vibrations of c-C 3 H 2 . We establish here that the actual source of this anomalous behaviour is numerical instability in the out-of-plane bending quartic force constants. It is found that the use of the normal equations to solve for the quartic force field, as well as a bending coordinate grid which is too compact, leads to errors of ca. 20 cm−1 in ν 5 and ν 9 . Our revised analysis shows ν 5 and ν 9 to be 980 cm−1 and 777 cm−1, respectively. [ABSTRACT FROM AUTHOR]

In the interstellar medium (ISM) there are many regions where the formation of molecules is kinetically driven rather than thermochemically, which can lead to the formation of many isomers even though some may be fairly higher in energy relative to the molecular global minimum. Recent laboratory experiments where noble gas cations are reacted with pyrimidine favored the formation of C3H3N+, but the molecular structure(s) of this fragment was not determined. Microscopic reversibility means that pyrimidine could form under interstellar conditions should the required C3H3N+ reactant be detected in the ISM. Hence C3H3N+ could be a strong candidate for involvement in the formation of heterocyclic biomolecules such as pyrimidine in the ISM. In this study, we have investigated the low energy isomers of the acrylonitrile ion (C3H3N+) using density functional theory as well as high levels of ab initio theory, namely, the singles and doubles coupled-cluster theory that includes a perturbational correction for connected triple excitations, denoted as CCSD(T). An automated stochastic search procedure, Kick, has been employed to find isomers on the ground state doublet potential energy surface. Several new structures, along with all the previously reported minima, have been found. The global minimum H2CCCNH+ is energetically much lower than either H2CC(H)CN+, the acrylonitrile ion, or HCC(H)NCH+, the most likely intermediate of the reaction between HCCH+ and HCN. These isomers are connected to the global minimum via several transition states and intermediates. The results indicate that not only the global minimum but also several higher energy isomers of the C3H3N+ ion could be important in interstellar pyrimidine formation. The isomeric molecules have the necessary CCNC backbone needed for the reaction with HCN to form the cyclic pyrimidine framework. The structural and rotational parameters of all the isomers studied in this work have been predicted at the CCSD(T) level of theory with the anticipation that it will expedite their laboratory as well as astronomical identification. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH3 molecule. The singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations and the internally contracted averaged coupled-pair functional electronic structure methods have been used in conjunction with very large correlation-consistent basis sets, including diffuse functions. Extrapolation to the one-particle basis set limit was performed and core correlation and scalar relativistic contributions were included directly, while the diagonal Born–Oppenheimer correction was added. Our best purely ab initio PES, denoted “mixed,” is constructed from two PESs which differ in whether the ic-ACPF higher-order correlation correction was added or not. Rovibrational transition energies computed from the mixed PES agree well with experiment and the best previous theoretical studies, but most importantly the quality does not deteriorate even up to 10 300 cm-1 above the zero-point energy (ZPE). The mixed PES was improved further by empirical refinement using the most reliable J=0–2 rovibrational transitions in the HITRAN 2004 database. Agreement between high-resolution experiment and rovibrational transition energies computed from our refined PES for J=0–6 is excellent. Indeed, the root mean square (rms) error for 13 HITRAN 2004 bands for J=0–2 is 0.023 cm-1 and that for each band is always ≤=0.06 cm-1. For J=3–5 the rms error is always ≤=0.15 cm-1. This agreement means that transition energies computed with our refined PES should be useful in the assignment of new high-resolution NH3 spectra and in correcting mistakes in previous assignments. Ideas for further improvements to our refined PES and for extension to other isotopolog are discussed. [ABSTRACT FROM AUTHOR]

The abundance of hydrogen sulfide, H2S, is underpredicted in standard astrophysical models and, thus, so are the effects of its rich chemistry. Its cation, H2S+, may have a similarly understated effect on the interstellar medium (ISM) but, unlike its neutral parent, its presence in the ISM is unconfirmed. Advancing the search for H2S+ requires the accurate characterization of its spectroscopic properties, both in the infrared (IR) and microwave regimes. Past spectroscopy experiments were unable to resolve its rotational structure to high accuracy and could not observe one of its fundamental vibrations. In this research, the H2S+ species has been studied at the highest available levels of electronic structure theory. Molecular structures, vibrational frequencies, rotational constants, and other rovibrational spectroscopic constants are predicted for H2S+ and eleven of its isotopologues. Previous ab initio studies fail to predict a separation of at least 5 cm−1 between the fundamental S−H stretching vibrations; a value equivalent to the uncertainty of previous photoelectron spectroscopy experiments. The characteristically different rotational properties of H2S and H2S+ are established concluding that most, if not all, large-dish telescopes and interferometers can easily discern the cation from the neutral. The best possible candidates for interstellar detection of H2S+ in the IR are determined to be the fundamental S−H stretches, due to their intensity and unique region in the IR. James Webb Space Telescope is best equipped for this search, whereas the Echelon-Cross-Echelle Spectrograph instrument on Stratospheric Observatory for Infrared Astronomy is better suited to find the bending fundamental of H2S+ and all fundamentals of its deuterated isotopologues. [ABSTRACT FROM AUTHOR]

Through established, highly accurate ab initio quartic force fields, a complete set of fundamental vibrational frequencies, rotational constants, and rovibrational coupling and centrifugal distortion constants have been determined for both the cyclic 1 1A′ and bent 2 1A′ DCCN, H13CCN, HC13CN, and HCC15N isotopologues of HCCN. Spectroscopic constants are computed for all isotopologues using second-order vibrational perturbation theory (VPT2), and the fundamental vibrational frequencies are computed with VPT2 and vibrational configuration interaction (VCI) theory. Agreement between VPT2 and VCI results is quite good, with the fundamental vibrational frequencies of the bent isomer isotopologues in accord to within a 0.1-3.2 cm–1 range. Similar accuracies are present for the cyclic isomer isotopologues. The data generated here serve as a reference for astronomical observations of these closed-shell, highly dipolar molecules using new, high-resolution telescopes and as reference for laboratory studies where isotopic labeling may lead to elucidation of the formation mechanism for the known interstellar molecule: X3A′ HCCN. [ABSTRACT FROM AUTHOR]

The species responsible for the broad extended red emission (ERE). discovered in 1975 and now known to be widespread throughout the Galaxy, still is unidentified. Spanning the range from ∼540 to 900 nm, the ERE is a photoluminescent process associated with a wide variety of different interstellar environments. Over the years, a number of plausible candidates have been suggested, but subsequent observations ruled them out. The objects that present the ERE also emit the infrared features attributed to free polycyclic aromatic hydrocarbon (PAH) molecules, suggesting that closely related materials are plausible ERE carriers. Here, we show that the peculiar spectra and unique properties of closed-shell cationic PAH dimers satisfy the existing observational constraints and suggest that emission from mixtures of charged PAH clusters accounts for much of the ERE. This work provides a view into the structures, stabilities, abundances, and ionization balance of PAH-related species in the emission zones, which, in turn, reflects physical conditions in the emission zones and sheds fundamental light on the nanoscale processes involved in carbon-particle nucleation and growth and carbonaceous dust evolution in the interstellar medium. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH3 molecule. The singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations and the internally contracted averaged coupled-pair functional electronic structure methods have been used in conjunction with very large correlation-consistent basis sets, including diffuse functions. Extrapolation to the one-particle basis set limit was performed and core correlation and scalar relativistic contributions were included directly, while the diagonal Born–Oppenheimer correction was added. Our best purely ab initio PES, denoted “mixed,” is constructed from two PESs which differ in whether the ic-ACPF higher-order correlation correction was added or not. Rovibrational transition energies computed from the mixed PES agree well with experiment and the best previous theoretical studies, but most importantly the quality does not deteriorate even up to 10 300 cm-1 above the zero-point energy (ZPE). The mixed PES was improved further by empirical refinement using the most reliable J=0–2 rovibrational transitions in the HITRAN 2004 database. Agreement between high-resolution experiment and rovibrational transition energies computed from our refined PES for J=0–6 is excellent. Indeed, the root mean square (rms) error for 13 HITRAN 2004 bands for J=0–2 is 0.023 cm-1 and that for each band is always ≤=0.06 cm-1. For J=3–5 the rms error is always ≤=0.15 cm-1. This agreement means that transition energies computed with our refined PES should be useful in the assignment of new high-resolution NH3 spectra and in correcting mistakes in previous assignments. Ideas for further improvements to our refined PES and for extension to other isotopolog are discussed. [ABSTRACT FROM AUTHOR]

• The next most likely isomer of H 2 CS present in the ISM is trans-HCSH. • Even so, the cis-HCSH isomer may be the most likely to be observed. • H 2 SC is more observable but is likely too high in energy to be observed.. The missing sulfur of the interstellar medium may be hiding in molecules for which there are no spectral data yet available for any astrophysical observation, molecules like isomers of H 2 CS a known interstellar compound. Every known interstellar molecule is a pointer to the presence and detectability of related molecules in the ISM. This quantum chemical study provides the necessary spectral data for the observation of cis -, trans -, and 3 A HCSH as well as for 1 A 1 and 3 A ″ H 2 SC. Benchmarks of the CCSD(T)-F12/cc-pVTZ-F12 quartic force field for the known thioformaldehyde provide anharmonic vibrational frequencies of with an average of 1.9 cm−1 of gas-phase experiment demonstrating reliability for the computed fundamental frequencies. The 1 A 1 H 2 SC form has bright spectral features in the infrared and millimeter-wave regimes, but is the highest-energy species of this set. The trans -HCSH isomer is the lowest-energy (43.69 kcal/mol or 1.90 eV) isomer next to thioformaldehyde, but the slightly higher cis -HCSH isomer has greater intensities for its brightest fundamental frequencies and a larger dipole moment making it potentially more likely to be observed. These data determined here may also assist in laboratory characterization of these molecules and how their chemistry will likely progress. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) play a significant role in the chemistry of the interstellar medium (ISM) as well as in hydrocarbon combustion. These molecules can have high levels of diversity with the inclusion of heteroatoms and the addition or removal of hydrogens to form charged or radical species. There is an abundance of data on the cationic forms of these molecules, but there have been many fewer studies on the anionic species. The present study focuses on the anionic forms of deprotonated PAHs. It has been shown in previous work that PAHs containing nitrogen heteroatoms (PANHs) have the ability to form valence excited states giving anions electronic absorption features. This work analyzes how the isoelectronic pure PAHs behave under similar structural constructions. Singly deprotonated forms of benzene, naphthalene, anthracene, and tetracene classes are examined. None of the neutral-radicals possess dipole moments large enough to support dipole-bound excited states in their corresponding closed-shell anions. Even though the PANH anion derivatives support valence excited states for three-ringed structures, it is not until four-ringed structures of the pure PAH anion derivatives that valence excited states are exhibited. However, anisotropically extended PAHs larger than tetracene will likely exhibit valence excited states. The relative energies for the anion isomers are very small for all of the systems in this study. [ABSTRACT FROM AUTHOR]