Article

Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, Nondetection of HC11N, and a Search for New Organic Molecules

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Abstract

The 100 m Robert C. Byrd Green Bank Telescope K-band (KFPA) receiver was used to perform a high-sensitivity search for rotational emission lines from complex organic molecules in the cold interstellar medium towards TMC-1 (cyanopolyyne peak), focussing on the identification of new carbon-chain-bearing species as well as molecules of possible prebiotic relevance. We report a detection of the carbon-chain oxide species HC$_7$O and derive a column density of $(7.8\pm0.9)\times10^{11}$~cm$^{-2}$. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C$_7$H$^-$, and/or reaction of C$_6$H$_2$$^+$ with CO (followed by dissociative electron recombination). Upper limits are given for the related HC$_6$O, C$_6$O and C$_7$O molecules. In addition, we obtained the first detections of emission from individual $^{13}$C isotopologues of HC$_7$N, and derive abundance ratios HC$_7$N/HCCC$^{13}$CCCCN = $110\pm16$ and HC$_7$N/HCCCC$^{13}$CCCN = $96\pm 11$, indicative of significant $^{13}$C depletion in this species relative to the local interstellar elemental $^{12}$C/$^{13}$C ratio of 60-70. The observed spectral region covered two transitions of HC$_{11}$N, but emission from this species was not detected, and the corresponding column density upper limit is $7.4\times10^{10}$ cm$^{-2}$ (at 95% confidence). This is significantly lower than the value of $2.8\times10^{11}$ cm$^{-2}$ previously claimed by Bell et al. (1997) and confirms the recent non-detection of HC$_{11}$N in TMC-1 by Loomis et al. (2016). Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline and pyrimidine.

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... Spectral line models were generated for each source using a custom Python routine (based on the code used by Cordiner et al. 2017). Spectroscopic parameters were taken from the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al. 2001), where available, and additional data for HDO were taken from the JPL Molecular Spectroscopy Database (Pickett et al. 1998). ...
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The Chemical Evolution of Phosphorus: An Interdisciplinary Approach to Astrobiology is an exploration of "the phosphorus enigma." The volume attempts to answer the questions: How did phosphorus atoms, which are produced inside the inner cores of a handful of huge stars, become concentrated in relatively high proportions in the organisms composing Earth's biosphere? And: How did these phosphate derivatives manage to be included in such a great variety of organic molecules playing essential biochemical roles in all known life forms? Due to the interdisciplinary nature of the topic, the volume is arranged in three main sections. The first section introduces the fundamental concepts and notions of physics, chemistry, and biology necessary for the proper understanding of the topics discussed within an astronomical framework. It covers the basic features of the astrophysics, cosmochemistry, and astrobiology disciplines and provides a brief summary of the main physical and chemical properties of phosphorus compounds of interest. The second section focuses on the role of phosphorus and its compounds within the context of chemical evolution in galaxies. Following an interdisciplinary approach, the author discusses the position of P among the main biogenic elements by considering its relevance in most essential biochemical functions as well as its peculiar chemistry under different physicochemical conditions. The phosphorus distribution in different cosmic sites-such as terrestrial planets, interplanetary dust particles, cometary dust, planetary atmospheres, and the interstellar medium (ISM)-is reviewed, showing that this element is both scarce and ubiquitous in the universe. Also addressed are some possible routes allowing for the incorporation of phosphorus compounds of prebiotic interest during the earlier stages of solar system formation. The third section features an overall perspective on the role of phosphorus and its compounds in current areas of research of solid state physics, materials engineering, nanotechnology or medicine, as well as some perspectives for future work in order to properly solve the enigma of phosphorus in chemical evolution. The book can be used as reader-friendly resource for undergraduate, graduate, or postgraduate students, senior scientists, and researchers coming from diverse related fields of physics, chemistry, and materials science, as well as senior scientists interested in primary sources of the main biogenic elements.
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“This book beautifully traces the stellar origin of the element phosphorous, its chemical properties, and the observations of phosphorous-based molecules and minerals in the interstellar medium and in the solar system. [The author] then connects the astronomical studies with the role of phosphorous played in living organisms, presenting the biochemistry of biomolecules that incorporate phosphorous, and the roles that these molecules play in the origin of life on Earth. This book presents a comprehensive summary of our current understanding of the astrochemical and astrobiological significance of phosphorous. It is invaluable for researchers and students who are interested in the question of origin of life and the search for extraterrestrial life. ” —From the Foreword by Sun Kwok, President, International Astronomical Union Commission on Astrobiology (2015–2018), University of British Columbia, Vancouver, Canada
Article
Star-forming regions show a rich and varied chemistry, including the presence of complex organic molecules—in both the cold gas distributed on large scales and the hot regions close to young stars where protoplanetary disks arise. Recent advances in observational techniques have opened new possibilities for studying this chemistry. In particular, the Atacama Large Millimeter/submillimeter Array has made it possible to study astrochemistry down to Solar System–size scales while also revealing molecules of increasing variety and complexity. In this review, we discuss recent observations of the chemistry of star-forming environments, with a particular focus on complex organic molecules, taking context from the laboratory experiments and chemical models that they have stimulated. The key takeaway points include the following: ▪ The physical evolution of individual sources plays a crucial role in their inferred chemical signatures and remains an important area for observations and models to elucidate. ▪ Comparisons of the abundances measured toward different star-forming environments (high-mass versus low-mass, Galactic Center versus Galactic disk) reveal a remarkable similarity, which is an indication that the underlying chemistry is relatively independent of variations in their physical conditions. ▪ Studies of molecular isotopologues in star-forming regions provide a link with measurements in our own Solar System, and thus may shed light on the chemical similarities and differences expected in other planetary systems. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 58 is August 18, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Observations of IRC +10216 with the Yebes 40 m telescope between 31 and 50 GHz have revealed more than 150 unidentified lines. Some of them can be grouped into a new series of 26 doublets, harmonically related with integer quantum numbers ranging from J up = 54 to 80. The separation of the doublets increases systematically with J , that is to say, as expected for a linear species in one of its bending modes. The rotational parameters resulting from the fit to these data are B = 290.8844 ± 0.0004 MHz, D = 0.88 ± 0.04 Hz, and q = 0.1463 ± 0.0001 MHz. The rotational constant is very close to that of the ground state of HC 9 N. Our ab initio calculations show an excellent agreement between these parameters and those predicted for the lowest energy vibrationally excited state, ν 19 = 1, of HC 9 N. This is the first detection, and complete characterization in space, of vibrationally excited HC 9 N. An energy of 41.5 cm ⁻¹ is estimated for the ν 19 state. In addition, 17 doublets of HC 7 N in the ν 15 = 1 state, for which laboratory spectroscopy is available, were detected for the first time in IRC +10216. Several doublets of HC 5 N in its ν 11 = 1 state were also observed. The column density ratio between the ground and the lowest excited vibrational states are ≈127, 9.5, and 1.5 for HC 5 N, HC 7 N, and HC 9 N, respectively. We find that these lowest-lying vibrational states are most probably populated via infrared pumping to vibrationally excited states lying at ≈600 cm ⁻¹ . The lowest vibrationally excited states thus need to be taken into account to precisely determine absolute abundances and abundance ratios for long carbon chains. The abundance ratios N(HC 5 N)/N(HC 7 N) and N(HC 7 N)/N(HC 9 N) are 2.4 and 7.7, respectively.
Article
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.
Article
Using the Yebes 40m and IRAM 30m radio telescopes, we detected a series of harmonically related lines with a rotational constant B0=4460.590±0.001 MHz and a distortion constant D0=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.
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Using APEX-1 and APEX-2 observations, we have detected and studied the rotational lines of the HC3N molecule (cyanoacetylene) in the powerful outflow/hot molecular core G331.512−0.103. We identified 31 rotational lines at J levels between 24 and 39; 17 of them in the ground vibrational state v = 0 (9 lines corresponding to the main C isotopologue and 8 lines corresponding to the 13C isotopologues), and 14 in the lowest vibrationally excited state v7 = 1. Using local thermodynamic equilibrium (LTE)-based population diagrams for the beam-diluted v = 0 transitions, we determined Texc = 85 ± 4 K and N(HC3N) = (6.9 ± 0.8) × 1014 cm−2, while for the beam-diluted v7 = 1 transitions we obtained Texc = 89 ± 10 K and N(HC3N) = (2 ± 1) × 1015 cm−2. Non-LTE calculations using H2 collision rates indicate that the HC3N emission is in good agreement with LTE-based results. From the non-LTE method, we estimated Tkin ≃90 K, n(H2) ≃ 2 × 107 cm−3 for a central core of 6 arcsec in size. A vibrational temperature in the range from 130 to 145 K was also determined, values which are very likely lower limits. Our results suggest that rotational transitions are thermalized, while infrared radiative pumping processes are probably more efficient than collisions in exciting the molecule to the vibrationally excited state v7 = 1. Abundance ratios derived under LTE conditions for the 13C isotopologues suggest that the main formation pathway of HC3N is C2H2 + CN → HC3N + H.
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A specific interstellar aromatic molecule Aromatic molecules such as polycyclic aromatic hydrocarbons (PAHs) are known to exist in the interstellar medium owing to their characteristic infrared emission features. However, the infrared emission only indicates the general class of molecule, and identifying which specific molecular species are present is difficult. McGuire et al. used radio astronomy to detect rotational transitions of benzonitrile emitted from a well-known nearby cloud of interstellar gas (see the Perspective by Joblin and Cernicharo). This molecule may be a precursor to more complex PAHs. The identification of benzonitrile sheds light on the composition of aromatic material within the interstellar medium—material that will eventually be incorporated into new stars and planets. Science , this issue p. 202 ; see also p. 156
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We report the detection of the carbon-chain radical HC$_5$O for the first time in the interstellar medium toward the dark cloud TMC-1 using the 100 m Green Bank Telescope. We observe four hyperfine components of this radical in the $J = 17/2 \rightarrow 15/2$ rotational transition that originates from the $^2\Pi_{1/2}$ fine structure level of its ground state, and calculate an abundance of $n/n_{H_2}$ = $1.7\times 10^{-10}$, assuming an excitation temperature of $T_{ex} = 7$~K. No indication of HC$_3$O, HC$_4$O, HC$_6$O, is found in these or archival observations of the source, while we report tentative evidence for HC$_7$O. We compare calculated upper limits, and the abundance of HC$_5$O to predictions based on (1) the abundance trend of the analogous HC$_n$N family in TMC-1 and (2) a gas-grain chemical model. We find that the gas-grain chemical model well reproduces the observed abundance of HC$_5$O, as well as the upper limits of HC$_3$O, HC$_6$O, and HC$_7$O, but HC$_4$O is over produced. The prospects for astronomical detection of both shorter and longer HC$_n$O chains are discussed.
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Bell et al. (1997) reported the first detection of the cyanopolyyne HC$_{11}$N toward the cold dark cloud TMC-1; no subsequent detections have been reported toward any source. Additional observations of cyanopolyynes and other carbon-chain molecules toward TMC-1 have shown a log-linear trend between molecule size and column density, and in an effort to further explore the underlying chemical processes driving this trend, we have analyzed GBT observations of HC$_9$N and HC$_{11}$N toward TMC-1. Although we find an HC$_9$N column density consistent with previous values, HC$_{11}$N is not detected and we derive an upper limit column density significantly below that reported in Bell et al. Using a state-of-the-art chemical model, we have investigated possible explanations of non-linearity in the column density trend. Despite updating the chemical model to better account for ion-dipole interactions, we are not able to explain the non-detection of HC$_{11}$N, and we interpret this as evidence of previously unknown carbon-chain chemistry. We propose that cyclization reactions may be responsible for the depleted HC$_{11}$N abundance, and that products of these cyclization reactions should be investigated as candidate interstellar molecules.
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Recent detections of complex organic molecules in dark clouds have rekindled interest in the astrochemical modeling of these environments. Because of its relative closeness and rich molecular complexity, TMC-1 has been extensively observed to study the chemical processes taking place in dark clouds. We use local thermodynamical equilibrium radiative transfer modeling coupled with a Bayesian statistical method which takes into account outliers to analyze the data from the Nobeyama spectral survey of TMC-1 between 8 and 50 GHz. We compute the abundance relative to molecular hydrogen of 57 molecules, including 19 isotopologues in TMC-1 along with their associated uncertainty. The new results are in general agreement with previous abundance determination from Ohishi & Kaifu and the values reported in the review from Agúndez & Wakelam. However, in some cases, large opacity and low signal to noise effects allow only upper or lower limits to be derived, respectively. © 2016. The American Astronomical Society. All rights reserved..
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It was 1969 when the first organic molecule in space, H2CO, was discovered. Since then many organic molecules were discovered by using the NRAO 11 m (upgraded later to 12 m), Nobeyama 45 m, IRAM 30 m, and other highly sensitive radio telescopes as a result of close collaboration between radio astronomers and microwave spectroscopists. It is noteworthy that many famous organic molecules such as CH3OH, C2H5OH, (CH3)2O and CH3NH2 were detected by 1975. Organic molecules were found in so-called hot cores where molecules were thought to form on cold dust surfaces and then to evaporate by the UV photons emitted from the central star. These days organic molecules are known to exist not only in hot cores but in hot corinos (a warm, compact molecular clump found in the inner envelope of a class 0 protostar) and even protoplanetary disks. As was described above, major organic molecules were known since 1970s. It was very natural that astronomers considered a relationship between organic molecules in space and the origin of life. Several astronomers challenged to detect glycine and other prebiotic molecules without success. ALMA is expected to detect such important materials to further consider the gexogenous deliveryh hypothesis. In this paper I summarize the history in searching for complex organic molecules together with difficulties in observing very weak signals from larger species. The awfully long list of references at the end of this article may be the most useful part for readers who want to feel the exciting discovery stories.
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The importance of comets for the origin of life on Earth has been advocated for many decades. Amino acids are key ingredients in chemistry, leading to life as we know it. Many primitive meteorites contain amino acids, and it is generally believed that these are formed by aqueous alterations. In the collector aerogel and foil samples of the Stardust mission after the flyby at comet Wild 2, the simplest form of amino acids, glycine, has been found together with precursor molecules methylamine and ethylamine. Because of contamination issues of the samples, a cometary origin was deduced from the (13)C isotopic signature. We report the presence of volatile glycine accompanied by methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko measured by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) mass spectrometer, confirming the Stardust results. Together with the detection of phosphorus and a multitude of organic molecules, this result demonstrates that comets could have played a crucial role in the emergence of life on Earth.
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We observed the J = 9- 8 and 16-15 rotational transitions of the normal species and five 13C isotopologues of HC5N to study its formation mechanisms toward the cyanopolyyne peak in Taurus Molecular Cloud-1, with the 45-m radio telescope of the Nobeyama Radio Observatory. We detected the five 13C isotopologues with high signal-to-noise ratios between 12 and 20, as well as the normal species. The abundance ratios of the five 13C isotopologues of HC5N are found to be 1.00:0.97:1.03:1.05:1.16 (±0.19) (1σ) for [H13CCCCCN]:[HC13CCCCN]:[HCC13CCCN]:[HCCC13CCN]:[HCCCC13CN]. We do not find any significant differences among the five isotopologues. The averaged [HC5N]/[13C isotopologues] abundance ratio is determined to be 94 ±6 (1σ), which is slightly higher than the local interstellar elemental 12C/13C ratio of 60-70. Possible formation pathways are discussed on the basis of these results. © 2016. The American Astronomical Society. All rights reserved..
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The presence of numerous complex organic molecules (COMs; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. These COMs may be part of the material from which planetesimals and, ultimately, planets formed. Comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. Molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few COMs, such as ethylene glycol and glycine. We report the detection of 21 molecules in comet C/2014 Q2 (Lovejoy), including the first identification of ethyl alcohol (ethanol, C2H5OH) and the simplest monosaccharide sugar glycolaldehyde (CH2OHCHO) in a comet. The abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar- type protostars. Overall, the high abundance of COMs in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk.
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The rotational spectral lines of c-C$_3$H$_2$ and two kinds of the $^{13}$C isotopic species, c-$^{13}$CCCH$_2$ ($C_{2v}$ symmetry) and c-CC$^{13}$CH$_2$ ($C_s$ symmetry) have been observed in the 1-3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C$_3$H$_2$, c-$^{13}$CCCH$_2$ , and c-CC$^{13}$CH$_2$, respectively, with the Nobeyama 45 m telescope, and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where 7, 2, and 2 transitions, respectively, are observed with the both telescopes. With these data, we have evaluated the column densities of the normal and $^{13}$C isotopic species. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] ratio is determined to be $310\pm80$, while the [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratio is determined to be $61\pm11$. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] and [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratios expected from the elemental $^{12}$C/$^{13}$C ratio are 60-70 and 30-35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C$_3$H$_2$. Hence, this observation further confirms the dilution of the $^{13}$C species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of $^{13}$C$^+$ in the gas-phase C$^+$ due to the isotope exchange reaction: $\mathrm{^{13}C^++CO\rightarrow{}^{13}CO+C^+}$. Moreover, the abundances of the two $^{13}$C isotopic species are different from each other. The ratio of c-$\mathrm{^{13}CCCH_2}$ species relative to c-$\mathrm{CC^{13}CH_2}$ is determined to be $0.20\pm0.05$. If $^{13}$C were randomly substituted for the three carbon atoms, the [c-$\mathrm{^{13}CCCH_2}$]/[c-$\mathrm{CC^{13}CH_2}$] ratio would be 0.5. Hence, the observed ratio indicates that c-$\mathrm{CC^{13}CH_2}$ exists more favorably. Possible origins of the different abundances are discussed.
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This paper is the result of the International Cometary Workshop, held in Toulouse, France in April 2014, where the participants came together to assess our knowledge of comets prior to the ESA Rosetta Mission. In this paper, we look at the composition of the gas and dust from the comae of comets. With the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. We also discuss what has been learned from mass spectrometers during flybys. A few caveats for our interpretation are discussed. With dust, much of our information comes from flybys. They include {\it in situ} analyses as well as samples returned to Earth for laboratory measurements. Remote sensing IR observations and polarimetry are also discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and Philae lander will bring to bear to improve our understanding of comet 67P/Churyumov-Gerasimenko as "ground-truth" for our previous comprehensive studies. Finally, we summarize some of the initial Rosetta Mission findings.
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Observations of comets and asteroids show that the solar nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth's surface. Unlike asteroids, comets preserve a nearly pristine record of the solar nebula composition. The presence of cyanides in comets, including 0.01 per cent of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like compositions of simple and complex volatiles are found in protostars, and can readily be explained by a combination of gas-phase chemistry (to form, for example, HCN) and an active ice-phase chemistry on grain surfaces that advances complexity. Simple volatiles, including water and HCN, have been detected previously in solar nebula analogues, indicating that they survive disk formation or are re-formed in situ. It has hitherto been unclear whether the same holds for more complex organic molecules outside the solar nebula, given that recent observations show a marked change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks. Here we report the detection of the complex cyanides CH3CN and HC3N (and HCN) in the protoplanetary disk around the young star MWC 480. We find that the abundance ratios of these nitrogen-bearing organics in the gas phase are similar to those in comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of our solar nebula was not unique.
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Nucleobases, together with deoxyribose/ribose and phosphoric acid, are the building blocks of DNA and RNA for all known life. The presence of nucleobase-like compounds in carbonaceous chondrites delivered to the Earth raises the question of an extraterrestrial origin for the molecules that triggered life on our planet. Whether these molecules are formed in interstellar/protostellar environments, in small parent bodies in the solar system, or both, is currently unclear. Recent experiments show that the UV irradiation of pyrimidine (C4H4N2) in H2O-rich ice mixtures that contain NH3, CH3OH, or CH4 leads to the formation of the pyrimidine-based nucleobases uracil, cytosine, and thymine. In this work, we discuss the low-temperature UV irradiation of pyrimidine in realistic astrophysical ice mixtures containing H2O, CH3OH, and NH3, with or without CH4, to search for the production of nucleobases and other prebiotic compounds. These experiments show the presence of uracil, urea, glycerol, hexamethylenetetramine, small amino acids, and small carboxylic acids in all samples. Cytosine was only found in one sample produced from ices irradiated with a higher UV dose, while thymine was not found in any sample, even after irradiation with a higher UV dose. Results are discussed to evaluate the role of the photochemistry of pyrimidine in the inventory of organic molecules detected in meteorites and their astrophysical/astrobiological implications.
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Abstract The informational subunits of RNA or DNA consist of substituted N-heterocyclic compounds that fall into two groups: those based on purine (C5H4N4) (adenine and guanine) and those based on pyrimidine (C4H4N2) (uracil, cytosine, and thymine). Although not yet detected in the interstellar medium, N-heterocycles, including the nucleobase uracil, have been reported in carbonaceous chondrites. Recent laboratory experiments and ab initio calculations have shown that the irradiation of pyrimidine in ices containing H2O, NH3, or both leads to the abiotic production of substituted pyrimidines, including the nucleobases uracil and cytosine. In this work, we studied the methylation and oxidation of pyrimidine in CH3OH:pyrimidine, H2O:CH3OH:pyrimidine, CH4:pyrimidine, and H2O:CH4:pyrimidine ices irradiated with UV photons under astrophysically relevant conditions. The nucleobase thymine was detected in the residues from some of the mixtures. Our results suggest that the abundance of abiotic thymine produced by ice photolysis and delivered to the early Earth may have been significantly lower than that of uracil. Insofar as the delivery of extraterrestrial molecules was important for early biological chemistry on early Earth, these results suggest that there was more uracil than thymine available for emergent life, a scenario consistent with the RNA world hypothesis. Key Words: Pyrimidine-Nucleobases-Interstellar ices-Cometary ices-Molecular processes-Prebiotic chemistry. Astrobiology 13, 948-962.
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Nitrogen-14 quadrupole hyperfine structure has been resolved in the microwave spectrum of propanedinitrile, CH 2 (CN) 2 . Measurements between 5 and 30 GHz by Fourier transform and Stark modulation spectroscopy have been analysed. The general matrix elements for nuclear quadrupole interaction in molecules containing two equivalent coupling nuclei have been derived as simple formulae. The quadrupole coupling constants determined for the inertial axis system of CH 2 (CN) 2 are χ aa = - 2.368(28) MHz, χ bb = 0.318(20) MHz, and χ cc = 2.050(20) MHz. These values are used to establish that the electric field gradient tensor at nitrogen is cylindrically symmetric about the C≡N bond with a value of χ CN = - 4.09(4) MHz, close to values found for other organic cyanides.
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We completed the first spectral line survey toward a cold, dark cloud TMC-1 (cyanopolyyne peak) in a frequency range between 8.8 and 50.0 GHz by the 45-m radio telescope of Nobeyama Radio Observatory. We detected 414 lines of 38 molecular species including 11 new molecules, such as $\mathrm{C_6H}$, CCO, $\mathrm{C_3O}$, CCS, $\mathrm{C_3S}$, HNCCC, HCCNC, $\mathrm{HC_3NH^{+}}$, HCCCHO, $\mathrm{CH_2CN}$, and cyclic $\mathrm{C_3H}$ ($c\hbox{-}\mathrm{C_3H}$), which were previously reported; only one significant line, U49536, remains unidentified. 177 out of 414 lines are reported for the first time. Some half of the detected molecules are linear carbon-chain species and their derivatives. We also discovered three new rarer isotopomers: CC${{34\atop} \mathrm{S}}, \mathrm{C_3}$${{34\atop} \mathrm{S}}$, and HDCS. Spectral charts are presented together with the observed line parameters of all detected lines. Improved molecular constants of several carbon-chain molecules obtained during the course of identification are also presented.
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Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B and TMC-1C. The C6H-/C6H column density ratio is greater than about 1.0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H-/C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud. We also report the first detection of C4H- in TMC-1 (at 4.8-sigma confidence), and derive an anion-to-neutral ratio C4H-/C4H = (1.2 +- 0.4) x 10^-5 (= 0.0012 +- 0.0004%). Such a low value compared with C6H- highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H- could be produced as a result of reactions of oxygen atoms with C5H- and C6H-.
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The 13C substitutions of the molecule HC7N were observed in TMC-1 using the J = 12-11, J = 13-12 rotational transitions in the frequency range 12.4-13.6 GHz. We present the first detection of the 13C isotopic species of HC7N in the interstellar medium, based on the average of a number of weak rotational transitions. This paper describes the calibration and data-averaging process that is also used in a search for large cyanopolyyne molecules in TMC-1 using the 100 m Robert C. Byrd Green Bank Telescope (GBT). The capabilities of the GBT 11-15 GHz observing system are described, along with a discussion of numerical methods for averaging observations of a number of weak spectral lines to detect new interstellar molecules.
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New measurements of 12C/13C ratios in Galactic molecular clouds have been conducted using the N = 1 → 0 transition of the CN radical. This species is unique in that it has extensive hyperfine structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular clouds, and they include sources that lie in the range of 0.09-16.41 kpc in distance from the Galactic center (DGC). The ratios derived from CN indicate a gradient with Galactic distance of 12C/13C = 6.01DGC + 12.28. This gradient agrees rather closely with those derived from measurements of CO and H2CO. The least-squares fit to all data points for the three molecules is 12C/13C = 6.21DGC + 18.71. CO, CN, and H2CO are synthesized from quite varied reactions, and any 13C fractionation must follow different pathways for these three species. The relatively good agreement between the 12C/13C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the 12C/13C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system (12C/13C = 89) and local interstellar medium values (12C/13C ≈ 68) of this ratio may be a result of 13C enrichment since the formation of the solar system, as predicted by recent models.
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The molecular ions HC and HC have recently been identified both in the laboratory and in astronomical environments. In addition, carbon chain anions have been proposed as potential carriers of the diffuse interstellar bands. Using a selected ion flow tube apparatus, we have determined the rate constants for the reactions of C and HC with H, N, and O atoms in order to characterize their interstellar chemistry. Reactions between the carbon chain anions and H and O occur readily, with rate constants ranging from mid to high 10-10 cm3 s -1. H atom reactions proceed primarily by associative detachment, and O atom reactions proceed both by associative detachment and by extrusion of CO. Reactions between the carbon chains and N atoms are slower, but the reaction products that are observed could provide a negative ion pathway for the synthesis of observed interstellar species.
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The negative molecular ion C8H- has been detected in the Galactic molecular source TMC-1. Four rotational transitions have been observed in the centimeter-wave band with the NRAO 100 m Green Bank Telescope (GBT) at precisely the frequencies calculated from the recent laboratory spectroscopy of this large carbon chain anion. C8H- is about 5% as abundant as C8H, or somewhat more than C6H- relative to C6H (1.6%). Improved values of the column densities of C6H- and C6H, and an upper limit for the abundance of the smaller carbon chain C4H- of 0.014% with respect to C4H, have also been determined.
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Aims. We report new laboratory spectroscopic data on the rotational spectrum of the pyrimidine nucleic acid base uracil (C4H4N2O2). The dataset has been extended both into the microwave region and towards mm-wavelengths with the aim of providing accurate tran- sition rest frequencies for astrophysical searches. Methods. The microwave measurements have been performed with a molecular beam Fourier transform microwave spectrometer in the frequency range from 9-19 GHz and the mm-wave band transitions have been recorded with a Stark-modulated free jet spectrome- ter up to 100 GHz. The global dataset has been analysed with a standard S -reduced Hamiltonian and precise spectroscopic parameters up to quartic order have been obtained. The hyperfine structure due to the two 14 N nuclei has been resolved in the microwave mea- surements and the nuclear quadrupole coupling constants could be derived to high accuracy. Results. Based on the new laboratory data and analysis, highly precise rotational transition rest frequencies are available for astro- physical important lines of uracil up to 300 GHz.
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Two consecutive rotational transitions of the long cyanopolyyne HC11N, J = 39 --> 38 and 38 --> 37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers and coworkers, and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km s-1, in good agreement with the shorter cyanopolyynes HC7N and HC9N observed in this very sharp line source [5.82(5) and 5.84(5) km s-1, respectively]. The column density of HC11N is calculated to be 2.8 x 1011 cm-2. The abundance of the cyanopolyynes decreases smoothly with length to HC11N, the decrement from one to the next being about six for the longer carbon chains.
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In order to test gas-phase reaction schemes for the production of small oxides of carbon in cold, dense interstellar clouds, we have searched for the radical CCO and for propadienone (H2C3O) in Taurus Molecular Cloud 1, a nearby cloud which exhibits a rich organic chemistry. The radical CCO has been detected with a fractional abundance some two orders of magnitude less than that of CCS, about one order of magnitude less than that of H2CCO, and slightly less than that of C3O. An upper limit has been obtained on the abundance of propadienone which is slightly less than that of its isomer propynal (HC2CHO).
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We have studied the abundances of the (13)C isotopic species of C3S and C4H in the cold molecular cloud, Taurus Molecular Cloud-1 (Cyanopolyyne Peak), by radioastronomical observations of their rotational emission lines. The CCCS/(13)CCCS and CCCS/C(13)CCS ratios are determined to be >206 and 48(15), respectively. The C(13)CCS line is identified with the aid of laboratory microwave spectroscopy, and the range of the CCCS/CC(13)CS ratio is found to be from 30 to 206. The abundances of at least two (13)C isotopic species of C3S are thus found to be different. Similarly, it is found that the abundances of the four (13)C isotopic species of C4H are not equivalent. The CCCCH/(13)CCCCH, CCCCH/C(13)CCCH, CCCCH/CC(13)CCH, and CCCCH/CCC(13)CH ratios are evaluated to be 141(44), 97(27), 82(15), and 118(23), respectively. Here the errors denote three times the standard deviation. These results will constrain the formation pathways of C3S and C4H, if the non-equivalence is caused during the formation processes of these molecules. The exchange reactions after the formation of these two molecules may also contribute to the non-equivalence. In addition, we have confirmed that the $^{12}$C/(13)C ratio of some species are significantly higher than the interstellar elemental (12)C/(13)C ratio of 60-70. The observations of the (13)C isotopic species provide us with rich information on chemical processes in cold interstellar clouds.
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Rotational spectra of CnO with n=2, 4, 6, and 8 have been observed by using a Fabry–Perot type Fourier-transform microwave spectrometer cooperated with a pulsed discharge nozzle. The molecules have been generated by an electric discharge of carbon suboxide diluted in Ar, and adiabatically cooled to ≊2 K in a subsequent supersonic expansion. All the observed spectra for these species are characterized as linear molecules in the 3Σ− electronic ground state. Since all the three spin sublevels have been detected even in the free-jet condition, the spin–spin coupling constants have been determined precisely as well as other spectroscopic constants. The coupling constants show rapid increase as n becomes larger, indicating smaller energy gaps between the excited 1Σ+ state and the 3Σ− ground state for the longer species. Along with the recent observation of singlet CnO (n=5, 7, and 9) [Ogata, Ohshima, and Endo, J. Am. Chem. Soc. (submitted)], the present study has established the existence of a complete set of the linear carbon-chain series CnO up to n=9 in the gas phase. The effective C=C bond lengths evaluated from the rotational constants decrease gradually to a converging value of ≊1.28 A˚ as n becomes larger. No apparent quasilinearity has been observed in the centrifugal-distortion constants of all the members, in contrast to the relevant series of the pure carbon clusters, Cn, some of which (n=3 and 7) have shown substantial nonrigidity for the bending vibration.
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Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J″⩽128 and ν⩽234 GHz. Complementary measurements were also made on low-J, centimeter-wave spectra observed in supersonic expansion and with fully resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are μa=0.14355(19), μb=2.0146(17), μtot=2.0197(17) D for quinoline, and μa=2.3602(21), μb=0.9051(14), μtot=2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported.
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Cometary nuclei contain the least modified material from the formative epoch of our planetary system, and their compositions reflect a range of processes experienced by material prior to its incorporation in the cometary nucleus. Dynamical models suggest that icy bodies in the main cometary reservoirs (Kuiper Belt, Oort Cloud) formed in a range of environments in the protoplanetary disk, and (for the Oort Cloud) even in disks surrounding neighboring stars of the Sun's birth cluster. Photometric and spectroscopic surveys of more than 100 comets have enabled taxonomic groupings based on free radical species and on crystallinity of rocky grains. Since 1985, new surveys have provided emerging taxonomies based on the abundance ratios of primary volatiles. More than 20 primary chemical species are now detected in bright comets. Measurements of nuclear spin ratios (in water, ammonia, and methane) and of isotopic ratios (D/H in water and HCN; 14N/15N in CN and HCN) have provided critical insights on factors affec...
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We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.
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Rotational spectra of carbon-chain molecules C5O, C7O, and C9O have been observed for the first time by using a Fabry-Perot type Fourier transform microwave (FTMW) spectrometer cooperated with a pulsed discharge nozzle (PDN). The molecules have been generated by the discharge of C3O2 diluted in Ar. The spectra consist of series of R-branch transitions typical of linear molecules in singlet electronic states. For C5O, transitions for all the possible singly substituted C-13 and O-18 isotope species have also been detected in natural abundance. These measurements with an assumption of linearity have enabled a complete substitution structure to be derived: r(s)(C-(1)-O) = 1.1562(11) Angstrom, r(s)(C-(2)-C-(1)) 1.2552(30) Angstrom, r(s)(C-(3)-C-(2)) = 1.2881(38) Angstrom, r(s)(C-(4)-C-(3)) = 1.2947(21) Angstrom, and r(s)(C-(5)-C-(4)) = 1.2736(10) Angstrom. The CC bond lengths in C5O were found to be much more uniform than those in an isoelectronic molecule HC5N. The canonical structure which reproduces observed internuclear distances in C5O has been suggested. By assuming the same partial structure as that in the C5O molecule, the averaged bond lengths for the rest of C-C bonds in C7O and C9O have been determined to be 1.270 and 1.274 Angstrom, respectively. Along with the recent results of tripler CnO (n = 2, 4, 6, and 8) [Ohshima, Endo, and Ogata, J. Chem. Phys. 1995, 102, 1493-1500], the CC bonds in CnO are found to become more uniform cumulene-type double bonds with increasing n and to converge to a constant length.
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The cyanopolyyne HC11N, the largest claimed interstellar molecule, has been detected in the laboratory. Its nearest microwave rotational transitions lie 0.13% lower in frequency than the lines identified in IRC +10216 and TMC-1, so it is concluded that the astronomical lines are something else entirely. The 20 laboratory lines measured with our Fourier transform molecular-beam spectrometer allow the entire microwave spectrum of HC11N to be determined to much better than 1 km s-1 in equivalent radial velocity. A search for HC11N at the correct frequencies in published and unpublished spectra of these sources has turned up no convincing candidate lines, but detection is probably possible with one of the existing large radio telescopes.
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We report observations of seven different transitions of HC9N between 9.8 and 23.3 GHz in the cold, dark cloud TMC-1. Because of its lower rotational constant, HC9N is expected to yield a rotational temperature closer to the kinetic temperature of the gas than the shorter cyanopolyynes. Assuming that HC9N in TMC-1 is optically thin, we obtain a rotational temperature Trot = 7.5-8.7 K and a corresponding column density of NL(HC9N) = (5.4-2.3) × 1012 cm-2, depending on the source size L assumed. The rate of radiative cooling is less efficient in the longer cyanopolyyne chains and is approximately proportional to (n + 1)-6, where n is the number of carbon atoms in the chain. In steady state this is balanced by the collisional rate of excitation. The rotational temperatures of the cyanopolyynes are found to increase with the number of heavy atoms over the range HC5N-HC9N, in reasonable agreement with calculations. Concomitantly, we find evidence that the longer cyanopolyynes are located in regions that become progressively smaller with chain length. We also report newly measured values for the rotational and centrifugal distortion constants of HC9N that improve the accuracy of the calculated millimeter wave transitions.
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We have conducted extensive astronomical searches for the N-bearing ring molecules pyridine, quinoline and isoquinoline towards the circumstellar envelopes of carbon-rich stars, and for interstellar pyrimidine in hot molecular cores. Here we report the derived upper limits on the column densities of these molecules, and summarize the current status of these observations.
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Cyanobutadiyne has been produced by gas phase copyrolysis of pyridine and phosphorus trichloride in a flow reactor. The yield of the reaction is sufficiently good to allow the detection of rotational transitions of the 13C and 15N containing species in natural abundance. Normal pyridine and its fully deuterated variant have been used as precursors, making it possible to study the ground-state rotational spectra of 14 isotopomers in the millimeter wave region. Very accurate values of the rotational and quartic centrifugal distortion constants have been obtained for all the isotopic species investigated, and in addition the sextic distortion constant has been precisely determined for the most abundant variants H12C514N and D12C514N, for which the measurements have been extended up to 460 GHz. A mixed experimental–theoretical equilibrium structure has been evaluated for cyanobutadiyne combining experimental ground-state rotational constants with theoretically computed zero-point contributions. The re geometry is compared with operationally defined purely experimental structures, namely r0, rs, and rm(1) molecular structures.
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Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of die spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.
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Long-chain hydrocarbon anions Cn H– (n = 4, 6, 8) have recently been found to be abundant in a variety of interstellar clouds. In order to explain their large abundances in the denser (prestellar/protostellar) environments, new chemical models are constructed that include gas-grain interactions. Models including accretion of gas-phase species onto dust grains and cosmic-ray-induced desorption of atoms are able to reproduce the observed anion-to-neutral ratios, as well as the absolute abundances of anionic and neutral carbon chains, with a reasonable degree of accuracy. Due to their destructive effects, the depletion of oxygen atoms onto dust results in substantially greater polyyne and anion abundances in high-density gas (with cm–3). The large abundances of carbon-chain-bearing species observed in the envelopes of protostars such as L1527 can thus be explained without the need for warm carbon-chain chemistry. The C6H– anion-to-neutral ratio is found to be most sensitive to the atomic O and H abundances and the electron density. Therefore, as a core evolves, falling atomic abundances and rising electron densities are found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray desorption of atoms in high-density models delays freeze-out, which results in a more temporally stable anion-to-neutral ratio, in better agreement with observations. Our models include reactions between oxygen atoms and carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O, and HC7O, the abundances of which depend on the assumed branching ratios for associative electron detachment.
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The unambiguous assignment of atomic or molecular lines in the interstellar or circumstellar medium (ISM or CSM) depends critically on the availability of laboratory data with appropriate accuracy. Above approximately 500 GHz the amount of transitions measured in the laboratory decreases fast. However, with upcoming missions such as the Stratospheric Observatory For Infrared Astronomy (SOFIA), the Herschel Space Observatory, or the Atacama Large Millimeter Array (ALMA) the need for accurate rest frequencies up to at least 2 THz increases considerably. The catalog section of the Cologne Database for Molecular Spectroscopy (CDMS) has been created to provide the astronomical community with line frequencies of atoms and molecules of astronomical interest. The CDMS is available on the internet free of charge via http:// www. ph1. uni-koeln. de/ vorhersagen/ or via the short-cut http:// www. cdms. de/ .
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Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.
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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 C(3)H(3)N(+), but the molecular structure(s) of this fragment was not determined. Microscopic reversibility means that pyrimidine could form under interstellar conditions should the required C(3)H(3)N(+) reactant be detected in the ISM. Hence C(3)H(3)N(+) 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 (C(3)H(3)N(+)) 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 H(2)CCCNH(+) is energetically much lower than either H(2)CC(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 C(3)H(3)N(+) 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.
Article
A comprehensive reinvestigation of the rotational spectrum of pyrimidine was carried out by using several different spectrometers. All singly substituted 13C- and 15N-isotopic species of pyrimidine have been measured in natural abundance with millimeter-wave free jet and waveguide Fourier transform microwave techniques, and complete rs and r0 heavy atom geometries have been determined. The ground state rotational spectrum in the centimeter-wave region was measured at sub-Doppler resolution of the cavity Fourier transform spectrometer and all elements in the inertial and principal nuclear quadrupole-coupling tensors of the nitrogen nuclei in pyrimidine have been determined. The room-temperature spectrum was measured up to 337 GHz and J = 66 with BWO-based spectrometers and sextic level centrifugal distortion constants in the rotational Hamiltonian have been determined for the ground state and three lowest vibrational fundamentals of pyrimidine. Copyright 1999 Academic Press.
Article
The effective structures (r(0)) of the three linear cyanopolyynes HC(7)N, HC(9)N, and HC(11)N have been determined to high accuracy by isotopic substitution, following detection in a supersonic molecular beam with a Fourier transform microwave spectrometer of all of the singly substituted rare isotopic species. For each chain, the lengths of the individual bonds have been determined to an accuracy of 0.001 Å or 0.1% toward the end of the chain and to 0.01 Å or 1.0% toward the center. The experimental structures are in excellent agreement with recent high-level theoretical calculations, or, in the case of HC(11)N, with extrapolation from HC(9)N. The three polyynes studied here represent the largest reactive carbon chain molecules for which accurate structures have been derived empirically. For HC(7)N and HC(9)N, it has been possible to resolve at high-resolution nitrogen hyperfine structure in the lower rotational transitions and determine eQq for all of the singly substituted isotopic species of HC(7)N and for normal HC(9)N. Copyright 2000 Academic Press.