NCCN in TMC-1 and IRC+10216

We employ quantum chemical calculations using the CBS-RAD ('Complete Basis Set – Radicals') technique on the C₂N₂H potential energy surface to show that the reaction of HNC with CN is a viable and plausible route to NCCN in cold astrophysical environments. We use detailed chemical kinetic models to predict the abundance of NCCN in TMC-1 and IRC+10216. Radio-astronomical detection of NCCN is precluded by its lack of a dipole moment. We discuss other prospects for its observation in interstellar and circumstellar environments, by space-borne infrared spectroscopy, indirectly by detection of the NCCNH⁺ ion, or inferentially by detection of its higher-energy, polar isomer CNCN.     

Calculations on the rates, mechanisms, and interstellar importance of the reactions between C and NH2 and between N and CH2

The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH₂→HNC+H, and N+CH₂→HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.     

Detection of a new interstellar molecular ion, H2COH+ (protonated formaldehyde)

A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+.     

Detection of a new interstellar molecule, H2CN

We have detected a new interstellar molecule, H2CN (methylene amidogen), in the cold, dark molecular cloud TMC-l. The column density of H2CN is estimated to be approximately 1.5 x 10(11) cm-2 by assuming an excitation temperature of 5 K. This column density corresponds to a fractional abundance relative to H2 of approximately 1.5 x 10(-11). This value is more than three orders of magnitude less than the abundance of the related molecule HCN in TMC-1. We also report a tentative detection of H2CN in Sgr B2(N). The formation mechanism of H2CN is discussed. Our detection of the H2CN molecule may suggest the existence of a new series of carbon-chain molecules, CH2CnN (n = 0, 1, 2,...).     

Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: the case of IRC +10216

The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type C _n H, C _n H₂, C _n N, HC _n N. We present the detection of new species (CH₂CHCN, CH₂CN, H₂CS, CH₃CCH and C₃O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar envelope around an AGB star. These five molecules are most likely formed in the outer expanding envelope rather than in the stellar photosphere. A pure gas phase chemical model of the circumstellar envelope is reasonably successful in explaining the derived abundances, and additionally allows to elucidate the chemical formation routes and to predict the spatial distribution of the detected species.     

The rate of the reaction between C2H and C2H2 at interstellar temperatures

The reaction between the radical C2H and the stable hydrocarbon C2H2 is one of the simplest neutral-neutral hydrocarbon reactions in chemical models of dense interstellar clouds and carbon-rich circumstellar shells. Although known to be rapid at temperatures > or = 300 K, the reaction has yet to be studied at lower temperatures. We present here ab initio calculations of the potential surface for this reaction and dynamical calculations to determine its rate at low temperature. Despite a small potential barrier in the exit channel, the calculated rate is large, showing that this reaction and, most probably, more complex analogs contribute to the formation of complex organic molecules in low-temperature sources.     

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.     

The rate of the reaction between CN and C2H2 at interstellar temperatures

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.     

Theoretical investigation of the interstellar CH3NC/CH3CN ratio

Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds. According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors. We have calculated the ratio of the formation rates of the protonated precursor ions-- CH3NCH+ and CH3CNH+ --synthesized via the radiative association reaction between CH3+ and HCN, which is thought to the dominant formation process of the two isomeric ions. Our calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH+/CH3CNH+ formation rate ratio between 0.1 and 0.4. If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.     

A quantum chemical study on the formation of ethanimine (CH<Subscript>3</Subscript>CHNH) in the interstellar ice

Ethanimine (CH₃CHNH) is an important prebiotic molecule since it is a precursor of amino acid \(\alpha \)-alanine in Strecker synthesis. Two isomers (E and Z) of ethanimine were detected in the molecular cloud Sagittarius B2 north during GBT-PRIMOS survey. A possible radical-molecule reaction pathway has been proposed for the formation of ethanimine in the interstellar medium (ISM) from some previously detected interstellar molecules like methylene (both triplet CH₂ (³B₁) and singlet CH₂ (¹A₁)) and methyenimine (CH₂NH). The mechanism has been studied in the gas phase and in water ice with the help of density functional theory at B2PLYPD/6-311++G (2d, p) level of theory. It is observed that E-ethanimine forms efficiently in gas phase but ice reactions are favorable only in the hot core of molecular clouds. Same is true for the formation of Z-ethanimine which forms only at the surface of water cluster as the height of entrance barrier for formation of Z-ethanimine is similar to that of E-ethanimine. Isomerization from E to Z form is also studied and found to be forbidden due to large entrance barrier. Out of the two reaction system CH₂ (³B₁) + CH₂NH and CH₂ (¹A₁) + CH₂NH, later is more favorable then the former one due to the small entrance barrier. Still, much of the detected abundance of ethanimine comes from the reaction of CH₂ (³B₁) with CH₂NH as since CH₂ (¹A₁) has very low abundance compared to the CH₂ (³B₁) in ISM. The proposed pathway seems to be a promising candidate for the ethanimine formation in ISM.     

Shock chemistry in the molecular clouds associated with SNR IC 443

Several interstellar molecules have been detected toward the highly perturbed B and G clouds associated with the supernova remnant IC 443 via their 3 mm transitions, including N2H+, SiO, SO, CN, HNC, and H13CO+. The (J, K) = (1, 1) and (2, 2) inversion lines of metastable ammonia have also been observed, as well as the J = 3-2 transition of HCO+ at 1.2 mm. Analysis of the (1, 1) and (2, 2) inversion lines of NH3 indicates minimum gas kinetic temperatures of TK = 70 K toward cloud B, and TK = 33 K in cloud G. Modeling of the J = 1-0 and J = 3-2 transitions of HCO+ implies densities greater than 10(5) cm-3 toward both positions. These data clearly show that hot and dense material is present in IC 443, and they suggest the presence of shocks in both regions. A careful analysis of the HCO+ lines indicates that the HCO+ abundance is at most enhanced by factors of a few over that found in cold, quiescent gas. This conclusion contradicts past claims of HCO+ abundance enhancements of several orders of magnitude in the perturbed regions. The N2H+ abundance was also found to be similar to that in cold gas, suggesting that there is no increase in ionization in the clouds. The abundances of SO and CS, as well as CN and NH3, do not appear to differ significantly from those found in cold dark clouds, although chemistry models predict sulfur-containing species to undergo high-temperature enhancements. SiO, however, is found to have an abundance in the perturbed gas 100 times larger than the upper limits observed in the dark cloud TMC 1, a result in agreement with high temperature chemistry models. In addition, the HNC/HCN ratio in both IC 443 B and G was found to be approximately 0.1--far from the ratio of 1 predicted by low-temperature ion-molecule chemistry, but similar to the values observed in clouds where elevated temperatures are present.     

Detections of 13C-substituted C3H2 in astronomical sources

We report observations of the 2(12)-1(01) rotational transition of the 13C isotopic species of cyclopropenylidene (C3H2) toward TMC-1, Sgr B2, and IRC +10216 using the laboratory rest frequencies which have recently become available. Our detections allow estimates to be made of the fractional abundance of the unsubstituted similar species in these sources. The fractional abundance relative to H2, f(C3H2), is 1-2 x 10(-8) in TMC-1, and this is similar to the abundance of HCN, one of the more abundant organic molecules in the interstellar medium. In IRC +10216 f(C3H2) is one order of magnitude greater than in TMC-1. The 12C species in Sgr B2 shows a self-absorbed profile and the relative abundance of C3H2 estimated to be about an order of magnitude less than in TMC-1.     

Sulphur-bearing carbon chains in IRC+10216

We have updated the chemical model of IRC+10216 developed by Millar, Herbst & Bettens to include recent routes to the formation of sulphuretted hydrocarbons. The routes are based on a quantum chemical study of the S+C₂H system. In addition, we have altered the parent species for sulphur to reflect new observational results. We find that the model calculations give excellent agreement with the observed column densities, and discuss the significance of these reactions to the formation of species as yet unobserved and to dark interstellar clouds.     

Detection of interstellar hydrogen sulfide in cold, dark clouds

We have detected interstellar hydrogen sulfide (H2S) toward the cold, dark clouds L134N and TMC 1. We derive total column densities of approximately 2.6 x 10(13) cm-2 and approximately 7.0 x 10(12) cm-2 at the SO peak of L134N and at the NH3 peak of TMC 1, respectively. Since the expected gas phase reactions leading to the formation of H2S are thought to be endothermic, grain surface reactions may play a major role in the synthesis of this species in cold, dark clouds. If the carbon abundance is high and grain surface reactions are the dominant formation route, H2CS would be expected to form instead of H2S, and the abundances of H2CS have been observed to be high where those of H2S are low in L134N and TMC 1.     

Hydrogen isotopic substitution of solid methylamine through atomic surface reactions at low temperatures: A potential contribution to the D/H ratio of methylamine in molecular clouds

We experimentally studied hydrogen (H)–deuterium (D) substitution reactions of solid methylamine (CH₃NH₂) under astrophysically relevant conditions. We also calculated the potential energy surface for the H–D substitution reactions of methylamine isotopologues using quantum chemical methods. Despite the relatively large energy barrier of more than 18 kJ mol^?1, CH₃NH₂ reacted with D atoms to yield deuterated methylamines at 10 K, suggesting that the H–D substitution reaction proceeds through quantum tunneling. Deuterated methylamines reacted with H atoms as well. On the basis of present results, we propose that methylamine has potential for D enrichment through atomic surface reactions on interstellar grains at very low temperatures in molecular clouds. D enrichment would occur in particular in the methyl group of methylamine.     

Measurements in N2-CH4(C2H2) discharges of reaction rates and thermochemical constants for Titan atmosphere study

The kinetic reactions in N2-xCH4(C2H2) gas discharges with x less than 1% have been studied by emission spectroscopy in the afterglow of D.C. discharges and by mass spectroscopy from radiolysis ionization using alpha particles. The pressure range is from several Torr to 100 Torr. At the end of N2 D.C. discharges at room temperature, for a residence time of about 10(-2) s, the dominant active species are the N atoms with density of 10(14)-10(15) cm-3 for N2 density of about 10(17) cm-3 (3 Torr), the N2(X,V) vibrational molecules with for example [N2(X,V = 10)] approximately 10(14) cm-3 and the electronic metastable molecules N2(A 3 sigma u +) with a density of 10(12) cm-3. In such conditions, the following kinetic reactions have been studied: N2(A) + N2(A) --> N2(C,B,V') + N2(X), N2(A) + N2(X,V>5) --> N2(X) + N2(B,V') in pure N2 post-discharges and N2(A) + CH4 --> products, C + N + M2 --> CN(B,V') + M2, N2(X,V>4) + CN --> N2(X) + CN(B,A,V'), in N2-1% CH4 post-discharges. The clustering reactions of N2-(1-5%)CH4(C2H2) gas mixtures after radiolysis ionization have been studied for the H2CN+ nN2 ions and the equilibrium constants have been determined in the temperature range T = 140-300 K.     

Shock waves and the chemical structure of interstellar clouds

In this paper we study the effect of shock waves on the chemical structure of the interstellar clouds. A model of molecular cloud has been assumed. The chemistry is investigated in a time dependent model. Our chemical network contains 56 species in 251 reactions to including molecules of the elements H, O, C, N, S, and Si.The results indicate that the calculated fractional abundance of the molecules NS, H₂O, CN, NH, CO, and SO agrees well with the observations. The molecules OH, H₂S, CS, H₂CS, HS, NO, SiO, CH, CH₂, CH₃, HCO, C₂, and HCN reach high post-shock abundances.     

The detection of acetaldehyde in cold dust clouds

Observations of the 1(01) --> 0(00) rotational transitions of A and E state acetaldehyde are reported. The transitions were detected, for the first time in interstellar space, in the cold dust clouds TMC-1 and L134N, and in Sgr B2. This is also the first time acetaldehyde has been found in a dust cloud and is the most complex oxygen-bearing molecule yet known in this environment. We find a column density of 6 x 10(12) cm-2 in TMC-1, comparable to many other species detected there, and an approximately equal column density in L134N. In the direction of Sgr B2, the CH3CHO profile appears to consist of broad emission features from the hot molecular cloud core, together with absorption features resulting from intervening colder material. We also report the possible detection of HC9N toward IRC +10 degrees 216 through its J = 33 --> 32 transition. Implications for cold dust cloud chemistry and excitation are discussed.     

Formation of cyanoallene （buta-2, 3-dienenitrile） in theinterstellar medium： a quantum chemical and spectroscopicstudy

The interstellar medium, filling the vast space between stars, is a rich reser-voir of molecular material ranging from simple diatomic molecules to more com-plex, astrobiologically important molecules such as vinylcyanide, methylcyanodiac-cetylene, cyanoaUene, etc. Interstellar molecular cyanoallene is one of the most stableisomers of methylcynoacetylene. An attempt has been made to explore the possibilityof forming cyanoallene in interstellar space by radical-radical and radical-moleculeinteraction schemes in the gaseous phase. The formation of cyanoallene starting fromsome simple, neutral interstellar molecules and radicals has been studied using densityfunctional theory. The reaction energies and structures of the reactants and productsshow that the formation of cyanoallene is possible in the gaseous phase. Both of theconsidered reaction paths are totally exothermic and barrierless, thus giving rise to ahigh probability of occurrence. Rate constants for each step in the formation processof cyanoallene in both the reaction paths are estimated. A full vibrational analysishas been attempted for cyanoallene in the harmonic and anharmonic approximations.Anharmonic spectroscopic parameters such as rotational constants, rotation-vibrationcoupling constants and centrifugal distortion constants have been calculated.     

Reactions of nitriles in ices relevant to Titan, comets, and the interstellar medium: formation of cyanate ion, ketenimines, and isonitriles

Motivated by detections of nitriles in Titan's atmosphere, cometary comae, and the interstellar medium, we report laboratory investigations of the low-temperature chemistry of acetonitrile, propionitrile, acrylonitrile, cyanoacetylene, and cyanogen (CH₃CN, CH₃CH₂CN, CH₂CHCN, HCCCN, and NCCN, respectively). A few experiments were also done on isobutyronitrile and trimethylacetonitrile ((CH₃)₂CHCN and (CH₃)₃CCN, respectively). Trends were sought, and found, in the photo- and radiation chemical products of these molecules at 12-25 K. In the absence of water, all of these molecules isomerized to isonitriles, and CH₃CN, CH₃CH₂CN, and (CH₃)₂CHCN also formed ketenimines. In the presence of H₂O, no isonitriles were detected but rather the cyanate ion (OCN⁻) was seen in all cases. Although isonitriles, ketenimines, and OCN⁻ were the main focus of our work, we also describe cases of hydrogen loss, to make smaller nitriles, and hydrogen addition (reduction), to make larger nitriles. HCN formation also was seen in most experiments. The results are presented in terms of nitrile ice chemistry on Titan, in cometary ice, and in the interstellar medium. Possible connections to prebiotic chemistry are briefly discussed.