Electronically excited states of formic acid investigated by theoretical and experimental methods.

Absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements of gas-phase formic acid (HCOOH) in the photon energy range 4.7-10.8 eV (265-115 nm), together with quantum chemical calculations to provide vertical energies and oscillator strengths. The combination of experimental and theoretical methods has allowed a comprehensive assignment of the electronic transitions. The VUV spectrum reveals various vibronic features not previously reported in the literature, notably associated with (3pa'←10a'), (3p'a'←10a'), (3sa'←2a″) and (3pa'←2a″) Rydberg transitions. The assignment of vibrational features in the absorption bands reveal that the C=O stretching, v3'a', the H'-O-C' deformation, v5'a', the C-O stretching, v6'a', and the O=C-O' deformation, v7'a' modes are mainly active. The measured absolute photoabsorption cross sections have also been used to estimate the photolysis lifetime of HCOOH in the upper stratosphere (30-50 km), showing that solar photolysis is an important sink at altitudes above 30 km but not in the troposphere. Potential energy curves for the lowest-lying electronic excited states, as a function of the C=O coordinate, are obtained employing time dependent density functional theory (TD-DFT). These calculations have shown the relevance of internal conversion from Rydberg to valence character governing the nuclear dynamics, yielding clear evidence of the rather complex multidimensional nature of the potential energy surfaces involved.

The mystery of unidentified infrared emission bands.

A family of unidentified infrared emission (UIE) bands has been observed throughout the Universe. The current observed spectral properties of the UIE bands are summarized. These properties are discussed in the frameworks of different models of the chemical carriers of these bands. The UIE carriers represent a large reservoir of carbon in the Universe, and play a significant role in the physical and chemical processes in the interstellar medium and galactic environment. A correct identification of the carrier of the UIE bands is needed to use these bands as probes of galactic evolution.

Laboratory Detection of Cyanoacetic Acid: A Jet-Cooled Rotational Study.

Herein we present a laboratory rotational study of cyanoacetic acid (CH2(CN)C(O)OH), an organic acid as well as a -CN bearing molecule, that is a candidate molecular system to be detected in the interstellar medium (ISM). Our investigation aims to provide direct experimental frequencies of cyanoacetic acid to guide its eventual astronomical search in low-frequency surveys. Using different jet-cooled rotational spectroscopic techniques in the time domain, we have determined a precise set of the relevant rotational spectroscopic constants, including the 14N nuclear quadrupole coupling constants for the two distinct structures, cis- and gauche- cyanoacetic acid. We believe this work will potentially allow the detection of cyanoacetic acid in the interstellar medium, whose rotational features have remained unknown until now.

First detection of doubly deuterated methyl acetylene (CHD2CCH and CH2DCCD)☠.

We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD2CCH and CH2DCCD were identified in the dense core L483 through nine and eight, respectively, rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The astronomical frequencies observed here were combined with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 ± 0.5) × 1012 cm-2 for CHD2CCH and (2.2 ± 0.4) × 1012 cm-2 for CH2DCCD, which translate to abundance ratios CH3CCH/CHD2CCH = 34 ± 10 and CH3CCH/CH2DCCD = 42 ± 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, concretely around 2.4 times less abundant than CH3CCD. The abundances of the different deuterated isotopologues with respect to CH3CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C3H6D+ and C3H5D+, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as responsible for the formation and deuterium fractionation of CH3CCH in L483. The abundance ratios CH2DCCH/CH3CCD = 3.0 ± 0.9 and CHD2CCH/CH2DCCD = 1.25 ± 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD2CCH compared to CH2DCCD being well explained by the chemical model.

O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3.

We report the detection of the oxygen-bearing complex organic molecules propenal (C2H3CHO), vinyl alcohol (C2H3OH), methyl formate (HCOOCH3), and dimethyl ether (CH3OCH3) toward the cyanopolyyne peak of the starless core TMC-1. These molecules are detected through several emission lines in a deep Q-band line survey of TMC-1 carried out with the Yebes 40m telescope. These observations reveal that the cyanopolyyne peak of TMC-1, which is the prototype of cold dark cloud rich in carbon chains, contains also O-bearing complex organic molecules like HCOOCH3 and CH3OCH3, which have been previously seen in a handful of cold interstellar clouds. In addition, this is the first secure detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH are detected in a cold environment, adding new pieces in the puzzle of complex organic molecules in cold sources. We derive column densities of (2.2 ± 0.3) × 1011 cm™2, (2.5 ± 0.5) × 1012 cm-2, (1.1 ± 0.2) × 1012 cm-2, and (2.5 ± 0.7) × 1012 cm-2 for C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3, respectively. Interestingly, C2H3OH has an abundance similar to that of its well known isomer acetaldehyde (CH3CHO), with C2H3OH/CH3CHO ~ 1 at the cyanopolyyne peak. We discuss potential formation routes to these molecules and recognize that further experimental, theoretical, and astronomical studies are needed to elucidate the true mechanism of formation of these O-bearing complex organic molecules in cold interstellar sources.

Cumulene carbenes in TMC-1: Astronomical discovery of l-H2C5 ☠.

We report the first detection in space of the cumulene carbon chain l-H2C5. A total of eleven rotational transitions, with Jup = 7-10 and Ka = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8±0.5)×1010 cm-2. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that l-H2C5 is ~4.0 times less abundant than the larger cumulene carbene l-H2C6, while it is ~300 and ~500 times less abundant than the shorter chains l-H2C3 and l-H2C4. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C n H2 isomers with n > 3.

Quantum study of reaction O(3 P) + H2 (v, j) → OH + H: OH formation in strongly UV-irradiated gas.

The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the dependence of the reaction rate coefficient on the rotational and vibrational state of H2 and evaluate the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons, where H2 can be efficiently pumped to excited vibrational states. We use a recently calculated potential energy surface and carry out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O(3 P) + H2 (v, j) → OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2, although even for high vibrational states of H2 (v = 4-5) for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to A V = 1.3-2.3, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, of about 60%. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.

Discovery of allenyl acetylene, H2CCCHCCH, in TMC-1. A study of the isomers of C5H4.

We present the discovery in TMC-1 of allenyl acetylene, H2CCCHCCH, through the observation of nineteen lines with a signal-to-noise ratio ~4-15. For this species, we derived a rotational temperature of 7±1K and a column density of 1.2±0.2×1013 cm-2. The other well known isomer of this molecule, methyl diacetylene (CH3C4H), has also been observed and we derived a similar rotational temperature, Tr=7.0±0.3 K, and a column density for its two states (A and E) of 6.5±0.3×1012 cm-2. Hence, allenyl acetylene and methyl diacetylene have a similar abundance. Remarkably, their abundances are close to that of vinyl acetylene (CH2CHCCH). We also searched for the other isomer of C5H4, HCCCH2CCH (1.4-Pentadiyne), but only a3σ upper limit of 2.5×1012 cm-2 to the column density can be established. These results have been compared to state-of-the-art chemical models for TMC-1, indicating the important role of these hydrocarbons in its chemistry. The rotational parameters of allenyl acetylene have been improved by fitting the existing laboratory data together with the frequencies of the transitions observed in TMC-1.

Discovery of the propargyl radical (CH2CCH) in TMC-1: one of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds.

We present the first identification in interstellar space of the propargyl radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. The six strongest hyperfine components of the 20,2-10,1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios in the range 4.6-12.3 σ. We derive a column density of 8.7 × 1013 cm-2 for CH2CCH, which translates to a fractional abundance relative to H2 of 8.7 × 10-9. This radical has a similar abundance to methyl acetylene, with an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH2CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C2H4 reaction and by the dissociative recombination of C3Hn + ions with n = 4-6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.

TMC-1, the starless core sulfur factory: Discovery of NCS, HCCS, H2CCS, H2CCCS, and C4S and detection of C5S.

We report the detection of the sulfur-bearing species NCS, HCCS, H2CCS, H2CCCS, and C4S for the first time in space. These molecules were found towards TMC-1 through the observation of several lines for each species. We also report the detection of C5S for the first time in a cold cloud through the observation of five lines in the 31-50 GHz range. The derived column densities are N(NCS) = (7.8±0.6)×1011 cm-2, N(HCCS) = (6.8±0.6)×1011 cm-2, N(H2CCS) = (7.8±0.8)×1011 cm-2, N(H2CCCS) = (3.7±0.4)×1011 cm-2, N(C4S) = (3.8±0.4)×1010 cm-2, and N(C5S) = (5.0±1.0)×1010 cm-2. The observed abundance ratio between C3S and C4S is 340, that is to say a factor of approximately one hundred larger than the corresponding value for CCS and C3S. The observational results are compared with a state-of-the-art chemical model, which is only partially successful in reproducing the observed abundances. These detections underline the need to improve chemical networks dealing with S-bearing species.

Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1.

We present the discovery in TMC-1 of vinyl acetylene, CH2CHCCH, and the detection, for the first time in a cold dark cloud, of HCCN, HC4N, and CH3CH2CN. A tentative detection of CH3CH2CCH is also reported. The column density of vinyl acetylene is (1.2±0.2)×1013 cm-2, which makes it one of the most abundant closed-shell hydrocarbons detected in TMC-1. Its abundance is only three times lower than that of propylene, CH3CHCH2. The column densities derived for HCCN and HC4N are (4.4±0.4)×1011 cm-2 and (3.7±0.4)×1011 cm-2, respectively. Hence, the HCCN/HC4N abundance ratio is 1.2±0.3. For ethyl cyanide we derive a column density of (1.1 ±0.3)×1011 cm-2. These results are compared with a state-of-the-art chemical model of TMC-1, which is able to account for the observed abundances of these molecules through gas-phase chemical routes.

Discovery of the acetyl cation, CH3CO+, in space and in the laboratory.

Using the Yebes 40m and IRAM 30m radiotelescopes, we detected two series of harmonically related lines in space that can be fitted to a symmetric rotor. The lines have been seen towards the cold dense cores TMC-1, L483, L1527, and L1544. High level of theory ab initio calculations indicate that the best possible candidate is the acetyl cation, CH3CO+, which is the most stable product resulting from the protonation of ketene. We have produced this species in the laboratory and observed its rotational transitions Ju = 10 up to Ju = 27. Hence, we report the discovery of CH3CO+ in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CH3CO+ with high accuracy up to 500 GHz. We derive an abundance ratio N(H2CCO)/N(CH3CO+)~44. The high abundance of the protonated form of H2CCO is due to the high proton affinity of the neutral species. The other isomer, H2CCOH+, is found to be 178.9 kJ mol-1 above CH3CO+. The observed intensity ratio between the K=0 and K=1 lines, ~2.2, strongly suggests that the A and E symmetry states have suffered interconversion processes due to collisions with H and/or H2, or during their formation through the reaction of H 3 + with H2CCO.

SiO, 29SiO, and 30SiO emission from 67 oxygen-rich stars. A survey of 61 maser lines from 7 to 1 mm.

Circumstellar environments of oxygen-rich stars are among the strongest SiO maser emitters. Physical processes such as collisions, infrared pumping and overlaps favors the inversion of level population and produce maser emission at different vibrational states. Despite numerous observational and theoretical efforts, we still do not have an unified picture including all the physical processes involved in the SiO maser emission. The aim of this work is to provide homogeneous data in a large sample of oxygen-rich stars. We present a survey of 67 oxygen-rich stars from 7 to 1 mm, in their rotational transitions from J = 1 → 0 to J = 5 → 4, for vibrational numbers v from 0 to 6 in the three main SiO isotopologues. We have used one of the 34 m NASA antennas at Robledo and the IRAM 30 m radio telescope. The first tentative detection of a v = 6 line is reported, as well as the detection of new maser lines. The highest vibrational levels seem confined to small volumes, presumably close to the stars. The J = 1 → 0, v = 2 line flux is greater than the corresponding v = 1 in almost half of the sample, which may confirm a predicted dependence on the pulsation cycle. This database is potentially useful in models which should consider most of the physical agents, time dependency, and mass-loss rates. As by-product, we report detections of 27 thermal rotational lines from other molecules, including isotopologues of SiS, H2S, SO, SO2, and NaCl.

Yebes 40 m radio telescope and the broad band NANOCOSMOS receivers at 7 mm and 3 mm for line surveys.

CONTEXT: Yebes 40m radio telescope is the main and largest observing instrument at Yebes Observatory and it is devoted to Very Long Baseline Interferometry (VLBI) and single dish observations since 2010. It has been covering frequency bands between 2 GHz and 90 GHz in discontinuous and narrow windows in most of the cases, to match the current needs of the European VLBI Network (EVN) and the Global Millimeter VLBI Array (GMVA). AIMS: Nanocosmos project, a European Union funded synergy grant, opened the possibility to increase the instantaneous frequency coverage to observe many molecular transitions with single tunnings in single dish mode. This reduces the observing time and maximises the output from the telescope. METHODS: We present the technical specifications of the recently installed 31.5 - 50GHz (Q band) and 72 - 90.5 GHz (W band) receivers along with the main characteristics of the telescope at these frequency ranges. We have observed IRC+10216, CRL 2688 and CRL 618, which harbour a rich molecular chemistry, to demonstrate the capabilities of the new instrumentation for spectral observations in single dish mode. RESULTS: The results show the high sensitivity of the telescope in the Q band. The spectrum of IRC+10126 offers a signal to noise ratio never seen before for this source in this band. On the other hand, the spectrum normalised by the continuum flux towards CRL 618 in the W band demonstrates that the 40 m radio telescope produces comparable results to those from the IRAM 30 m radio telescope, although with a smaller sensitivity. The new receivers fulfil one of the main goals of Nanocosmos and open the possibility to study the spectrum of different astrophysical media with unprecedented sensitivity.

Interstellar nitrile anions: Detection of C3N- and C5N- in TMC-1.

We report on the first detection of C3N- and C5N- towards the cold dark core TMC-1 in the Taurus region, using the Yebes 40 m telescope. The observed C3N/C3N- and C5N/C5N- abundance ratios are ~140 and ~2, respectively; that is similar to those found in the circumstellar envelope of the carbon-rich star IRC +10216. Although the formation mechanisms for the neutrals are different in interstellar (ion-neutral reactions) and circumstellar clouds (photodissociation and radical-neutral reactions), the similarity of the C3N/C3N- and C5N/C5N- abundance ratios strongly suggests a common chemical path for the formation of these anions in interstellar and circumstellar clouds. We discuss the role of radiative electronic attachment, reactions between N atoms and carbon chain anions C n -, and that of H- reactions with HC3N and HC5N as possible routes to form C n N-. The detection of C5N- in TMC-1 gives strong support for assigning to this anion the lines found in IRC +10216, as it excludes the possibility of a metal-bearing species, or a vibrationally excited state. New sets of rotational parameters have been derived from the observed frequencies in TMC-1 and IRC +10216 for C5N- and the neutral radical C5N.

Discovery of HC4NC in TMC-1: A study of the isomers of HC3N, HC5N, and HC7N.

We present a study of the isocyano isomers of the cyanopolyynes HC3N, HC5N, and HC7N in TMC-1 and IRC+10216 carried out with the Yebes 40m radio telescope. This study has enabled us to report the detection, for the first time in space, of HCCCCNC in TMC-1 and to give upper limits for HC6NC in the same source. In addition, the deuterated isotopologues of HCCNC and HNCCC were detected, along with all 13C substitutions of HCCNC, also for the first time in space. The abundance ratios of HC3N and HC5N, with their isomers, are very different in TMC-1 and IRC+10216, namely, N(HC5N)/N(HC4NC) is ~300 and ≥2100, respectively. We discuss the chemistry of the metastable isomers of cyanopolyynes in terms of the most likely formation pathways and by comparing observational abundance ratios between different sources.

Discovery of HC3O+ in space: The chemistry of O-bearing species in TMC-1.

Using the Yebes 40m and IRAM 30m radio telescopes, we detected a series of harmonically related lines with a rotational constant B 0=4460.590±0.001 MHz and a distortion constant D 0=0.511 ±0.005 kHz towards the cold dense core TMC-1. High-level-of-theory ab initio calculations indicate that the best possible candidate is protonated tricarbon monoxide, HC3O+. We have succeeded in producing this species in the laboratory and observed its J u -J l = 2-1 and 3-2 rotational transitions. Hence, we report the discovery of HC3O+ in space based on our observations, theoretical calculations, and laboratory experiments. We derive an abundance ratio N(C3O)/N(HC3O+)~7. The high abundance of the protonated form of C3O is due to the high proton affinity of the neutral species. The chemistry of O-bearing species is modelled, and predictions are compared to the derived abundances from our data for the most prominent O-bearing species in TMC-1.

Tentative detection of HC5NH+ in TMC-1.

Using the Yebes 40m radio telescope, we report the detection of a series of seven lines harmonically related with a rotational constant B 0=1295.81581 ± 0.00026 MHz and a distortion constant D 0 = 27.3 ± 0.5 Hz towards the cold dense cloud TMC-1. Ab initio calculations indicate that the best possible candidates are the cations HC5NH+ and NC4NH+. From a comparison between calculated and observed rotational constants and other arguments based on proton affinities and dipole moments, we conclude that the best candidate for a carrier of the observed lines is the protonated cyanodiacetylene cation, HC5NH+. The HC5N/HC5NH+ ratio derived in TMC-1 is 240, which is very similar to the HC3N/HC3NH+ ratio. Results are discussed in the framework of a chemical model for protonated molecules in cold dense clouds.

The millimeter-wave spectrum and astronomical search for ethyl methyl sulfide.

CONTEXT: Sulfur-containing molecules constitute only 8% of the molecules observed in the interstellar medium (ISM), in spite of the fact that sulfur has been shown to be an abundant element in the ISM. In order to understand the chemical behavior of the ISM and specific cases like the missing sulfur reservoir, a detailed chemical molecular composition in the ISM must be mapped out. AIMS: Our goal is to investigate the rotational spectrum of ethyl methyl sulfide, CH3CH2SCH3, which ms to be a potential candidate for observation in the ISM since the simpler analogs, CH3SH and CH3CH2SH, have already been detected. Rotational spectrum of ethyl methyl sulfide has been observed before, but its experimental rotational parameters are not precise enough to allow its detection in the ISM. METHODS: The rotational spectrum of ethyl methyl sulfide in the frequency range 72-116.5 GHz was measured using a broadband millimeter-wave spectrometer based on radio astronomy receivers with fast Fourier transform backends. The spectral searches and identification of the vibrational excited states of ethyl methyl sulfide was supported by high-level ab initio calculations on the harmonic and anharmonic force fields. RESULTS: The rotational spectra for the trans and gauche conformers of ethyl methyl sulfide was analyzed, and a total of 172 and 259 rotational transitions were observed for each one, respectively. The observation of A - E internal rotation splittings allowed the experimental determination of the V 3 hindered internal rotation barrier height for both trans and gauche species. In addition, the vibrational excited states, resulting from the lowest frequency vibrational mode ν 30 were identified for both conformers. The new experimental rotational parameters were employed to search for ethyl methyl sulfide in the warm and cold molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm and 2 mm.

Detectable interstellar anions: Examining the key factors.

Electron attachment energy (eAE) and the dipole moment of the neutral molecules are related to the radiative electron attachment (REA) process, which is an important formation route of interstellar anions under various astrophysical environments. Electron attachment energies (eAEs) have been calculated for over 80 carbon chain neutral molecules; Cn, CnX (X = N, O, S, H, Si, P), HCnN using ab initio quantum chemical method. Dipole moments of these neutral species and those of their corresponding anions are also theoretically estimated. The anions are found to be significantly more stable than their corresponding neutral analogues in most of the carbon chains considered. The high eAEs and the large dipole moment of most of the neutral species suggest a high detectability of their corresponding anions in the interstellar medium if they are abundant. Members of the C2nO-, C2nS-, C2n1Si, HC2nN-, CnP-, and C2n chains with increasing n are good candidates for future astronomical detection including the higher members of the C2nH- and C2n-1N- groups whose lower members have been observed.