The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H2CCO ice versus hydrogenation of mixed H2CCO/H2O ice at 10 K

Water ice plays an important role in reactions taking place on the surface of interstellar ice grains, ranging from catalytic effects that reduce reaction barrier heights to effects that stabilize the reaction products and intermediates formed, or that favor one reaction pathway over another, passing through water-involvement in the reaction to produce more complex molecules that cannot be formed without water or water-derived fragments H, O and OH. In this context, we have combined experimental and theoretical studies to investigate ketene (CH2CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH2CO and CH2CO/H2O ices. We show in the present study that with or without water, the ketene molecule reacts with H atoms to form four reaction products, namely CO, H2CO, CH4 and CH3CHO. Based on the amounts of CH2CO consumed during the hydrogenation processes, the CH2CO + 2H reaction appears to be more efficient in the presence of water. This underlines the catalytic role of water ice in reactions occurring on the surface of interstellar ice grains. However, if we refer to the yields of reaction products formed during the hydrogenation of CH2CO and CH2CO/H2O ices, we find that water molecules favor the reaction pathway to form CH3CHO and deactivate that leading to CH4 and H2CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H2O on the CH2CO + H reaction, whose potential energy barrier drops from 4.6 kcal mol-1 (without water) to 3.8 and 3.6 kcal mol-1 with one and two water molecules respectively.

Ionization, intrinsic basicity, and intrinsic acidity of unsaturated diols of astrochemical interest: 1,1- and 1,2-ethenediol: A theoretical survey

The structure, stability, and bonding characteristics of 1,1- and 1,2-ethenediol, their radical cations, and their protonated and deprotonated species were investigated using high-level ab initio G4 calculations. The electron density of all the neutral and charged systems investigated was analyzed using the QTAIM, ELF, and NBO approaches. The vertical ionization potential (IP) of the five stable tautomers of 1,2-ethenediol and the two stable tautomers of 1,1-ethenediol go from 11.81 to 12.27 eV, whereas the adiabatic ones go from 11.00 to 11.72 eV. The adiabatic ionization leads to a significant charge delocalization along the O-C-C-O skeleton. The most stable protonated form of (Z)-1,2-ethenediol can be reached by the protonation of both the anti-anti and the syn-anti conformers, whereas the most stable deprotonated form arises only from the syn-anti one. Both charged species are extra-stabilized by the formation of an O-H···O intramolecular hydrogen bond (IHB) which is not found in the neutral system. (Z)-1,2-ethenediol is predicted to be less stable, less basic, and more acidic than its cis-glycolaldehyde isomer. The most stable protonated species of (E)-1,2-ethenediol comes from its syn-syn conformer, although the anti-anti conformer is the most basic one. Contrarily, the three conformers yield a common deprotonated species, so their acidity follows exactly their relative stability. Again, the (E)-1,2-ethenediol is predicted to be less stable, less basic, and more acidic than its trans-glycolaldehyde isomer. Neither the neutral nor the protonated or the deprotonated forms of 1,1-ethenediol show the formation of any O-H···O IHB. The most stable protonated species is formed by the protonation of any of the two tautomers, but the most stable deprotonated form arises exclusively from the syn-anti neutral conformer. The conformers of 1,1-ethenediol are much less stable and significantly less basic than their isomer, acetic acid, and only slightly more acidic.

Free Ethynylarsinidene and Ethynylstibinidene: Heavier Analogues of Nitrenes and Phosphinidenes

Until now, there has been very little experimental evidence for the existence of free arsinidenes and stibinidenes, apart from the hydrides, AsH and SbH. Here, we report on photogeneration of triplet ethynylarsinidene, HCCAs, and triplet ethynylstibinidene, HCCSb, from ethynylarsine and ethynylstibine, respectively, in solid argon matrices. The products were identified using infrared spectroscopy and the associated UV absorption spectra are interpreted with the aid of theoretical predictions.

Methylenecyanamide (CH2═NCN) and (Z)- and (E)-Iminoacetonitriles (NC-CH═NH), Dimers of Hydrogen Cyanide

(Z)- and (E)-iminoacetonitriles (NCCH═NH), two hydrogen cyanide dimers, are described as key compounds in prebiotic chemistry. Among the many possible dimers of HCN with covalent bonds, even the second on the scale of thermodynamic stability, methylenecyanamide (CH₂═NCN), has been investigated little. We show that this compound can be isolated, is stable enough to give an adduct with a nucleophile or a diene, and can be easily generated under prebiotic conditions, highlighting a possible role in this medium. Comparison between isomers shows significant differences in formation and chemical reactivity.

Formation of CO, CH4, H2CO and CH3CHO through the H2CCO + H surface reaction under interstellar conditions

The reaction of ketene (H₂CCO) with hydrogen atoms has been studied under interstellar conditions through two different experimental methods, occurring on the surface and in the bulk of H₂CCO ice. We show that ketene interaction with H-atoms at 10 K leads mainly to four reaction products, carbon monoxide (CO), methane (CH₄), formaldehyde (H₂CO) and acetaldehyde (CH₃CHO). A part of these results shows a chemical link between a simple organic molecule such as H₂CCO and a complex one such as CH₃CHO, through H-addition reactions taking place in dense molecular clouds. The H-addition processes are very often proposed by astrophysical models as mechanisms for the formation of complex organic molecules based on the abundance of species already detected in the interstellar medium. However, the present study shows that the hydrogenation of ketene under non-energetic conditions may also lead efficiently to fragmentation processes and the formation of small species such as CO, CH₄ and H₂CO, without supplying external energy such as UV photons or high energy particles. Such fragmentation pathways should be included in the astrophysical modeling of H₂CCO + H in the molecular clouds of the interstellar medium. To support these results, theoretical calculations have explicitly showed that, under our experimental conditions, H-atom interactions with the CC bond of ketene lead mainly to CH₃CHO, CH₄ and CO. By investigating the formation and reactivity of the reaction intermediate H₃C-CO radical, our calculations demonstrate that the H₃C-CO + H reaction evolves through two barrierless pathways to form either CH₃CHO or CH₄ and CO fragments.

Rotational Spectra of Unsaturated Carbon Chains Produced by Pyrolysis: The Case of Propadienone, Cyanovinylacetylene, and Allenylacetylene

Several interstellar molecules are highly reactive unsaturated carbon chains, which are unstable under terrestrial conditions. Laboratory studies in support of their detection in space thus face the issue of how to produce these species and how to correctly model their rotational energy levels. In this work, we introduce a general approach for producing and investigating unsaturated carbon chains by means of selected test cases. We report a comprehensive theoretical/experimental spectroscopic characterization of three species, namely, propadienone, cyanovinylacetylene, and allenylacetylene, all of them being produced by means of flash vacuum pyrolysis of a suitable precursor. For each species, quantum-chemical calculations have been carried out with the aim of obtaining accurate predictions of the missing spectroscopic information required to guide spectral analysis and assignment. Rotational spectra of the title molecules have been investigated up to 400 GHz by using a frequency-modulation millimeter-/submillimeter-wave spectrometer, thus significantly extending spectral predictions over a wide range of frequency and quantum numbers. A comparison between our results and those available in the literature points out the clear need of the reported laboratory measurements at higher frequencies for setting up accurate line catalogs for astronomical searches.

Spectroscopic and Computational Characterization of 2-Aza-1,3-butadiene, a Molecule of Astrochemical Significance

Being N-substituted unsaturated species, azabutadienes are molecules of potential relevance in astrochemistry, ranging from the interstellar medium to Titan’s atmosphere. 2-Azabutadiene and butadiene share a similar conjugated π system, thus allowing investigation of the effects of heteroatom substitution. More interestingly, 2-azabutadiene can be used to proxy the abundance of interstellar butadiene. To enable future astronomical searches, the rotational spectrum of 2-azabutadiene has been investigated up to 330 GHz. The experimental work has been supported and guided by accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the two possible forms, trans and gauche. The trans species, more stable by about 7 kJ/mol than gauche-2-azabutadiene, has been experimentally observed, and its rotational and centrifugal distortion constants have been obtained with remarkable accuracy, while theoretical estimates of the spectroscopic parameters are reported for gauche-2-azabutadiene.

Gas-phase identification of (Z)-1,2-ethenediol, a key prebiotic intermediate in the formose reaction

Prebiotic sugars are thought to be formed on primitive Earth by the formose reaction. However, their formation is not fully understood and it is plausible that key intermediates could have formed in extraterrestrial environments and subsequently delivered on early Earth by cometary bodies. 1,2-Ethenediol, the enol form of glycolaldehyde, represents a highly reactive intermediate of the formose reaction and is likely detectable in the interstellar medium. Here, we report the identification and first characterization of (Z)-1,2-ethenediol by means of rotational spectroscopy. The title compound has been produced in the gas phase by flash vacuum pyrolysis of bis-exo-5-norbornene-2,3-diol at 750 °C, through a retro-Diels-Alder reaction. The spectral analysis was guided by high-level quantum-chemical calculations, which predicted spectroscopic parameters in very good agreement with the experiment. Our study provides accurate spectral data to be used for searches of (Z)-1,2-ethenediol in the interstellar space.

Gas-phase spectroscopic identification of the chlorovinyl radical

Fourier transform microwave spectra for two isomers of the chlorine substituted vinyl radical have been observed in the 4-52 GHz frequency region. The observed radicals (2A´) have been generated using...

An Efficient Photochemical Route Towards Triplet Ethynylphosphinidene, HCCP

While the archetypal free phosphinidene, H-P, has been studied for over a century, reports on uncomplexed, univalent phosphorus compounds are very sparse. Here we demonstrate production of HCCP in solid argon through the UV-induced rearrangement and subsequent dehydrogenation of phosphapropyne, CH₃ CP. Migration of H atoms along the CCP backbone of CH₃ CP resulted in production of the previously unobserved species 1-phosphapropadiene, CH₂ =C=PH, followed by ethynylphosphine, HCCPH₂ .

Unimolecular decomposition of methyl ketene and its dimer in the gas phase: theory and experiment

We present a combined theoretical and experimental investigation on the single photoionization and dissociative photoionization of gas-phase methyl ketene (MKE) and its neutral dimer (MKE2). The performed experiments entail the recording of photoelectron photoion coincidence (PEPICO) spectra and slow photoelectron spectra (SPES) in the energy regime 8.7-15.5 eV using linearly polarized synchrotron radiation. We observe both dimerization and trimerization of the monomer which brings about significantly complex and abstruse dissociative ionization patterns. These require the implementation of theoretical calculations to explore the potential energy surfaces of the monomer and dimer's neutral and ionized geometries. To this end, explicitly correlated quantum chemical methodologies involving the coupled cluster with single, double and perturbative triple excitations (R)CCSD(T)-F12 method, are utilized. An improvement in the adiabatic ionization energy of MKE is presented (AIE = 8.937 ± 0.020 eV) as well as appearance energies for multiple fragments formed through dissociative ionization of either the MKE monomer or dimer. In this regard, the synergy of experiment and theory is crucial to interpreting the obtained results. We discuss the potential astrochemical implications of this work in the context of recent advances in the field of astrochemistry and speculate on the potential presence and eventual fate of interstellar MKE molecules.

VUV photoionization of the CH2NC radical: adiabatic ionization energy and cationic vibrational mode wavenumber determinations

The photoelectron spectroscopy of CH₂NC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2 eV in the vicinity of the first photoionizing transition X⁺¹A₁ ← X ²B₁.

Direct Experimental Observation of in situ Dehydrogenation of an Amine-Borane System Using Gas Electron Diffraction

In situ dehydrogenation of azetidine-BH₃, which is a candidate for hydrogen storage, was observed with the parent and dehydrogenated analogue subjected to rigorous structural and thermochemical investigations. The structural analyses utilized gas electron diffraction supported by high-level quantum calculations, while the pathway for the unimolecular hydrogen release reaction in the absence and presence of BH₃ as a bifunctional catalyst was predicted at the CBS-QB3 level. The catalyzed dehydrogenation pathway has a barrier lower than the predicted B-N bond dissociation energy, hence favoring the dehydrogenation process over the dissociation of the complex. The predicted enthalpy of dehydrogenation at the CCSD(T)/CBS level indicates that mild reaction conditions would be required for hydrogen release and that the compound is closer to thermoneutral than linear amine boranes. The entropy and free energy change for the dehydrogenation process show that the reaction is exergonic, energetically feasible, and will proceed spontaneously toward hydrogen release, all of which are important factors for hydrogen storage.

Alkaline and Alkaline-earth Cyanoacetylides: A Combined Theoretical and Rotational Spectroscopic Investigation

The metallic cyanoacetylides LiC3N, NaC3N, MgC3N and CaC3N have been investigated by combined spectroscopy measurements and theoretical calculations. The theoretical calculations predict for the four species that the linear isomer with formula MCCCN (M= Li, Na, Mg and Ca) is the most stable one. We used the laser ablation molecular beam Fourier transform microwave spectroscopy to synthesize these species by the reaction of metal vapors, produced by laser ablation, and the 3-bromo-2-propynenitrile (BrCCCN). The pure rotational spectra were observed by Fourier transform microwave spectroscopy in the 2-18 GHz frequency region only for LiCCCN and NaCCCN, while no spectral signatures for MgCCCN and CaCCCN could be detected. Finally, we have searched for LiCCCN and NaCCCN species towards the carbon-rich evolved star IRC + 10216 but only upper limits to their abundances have been obtained.

Spectroscopic Studies on Hydrazine-Boranes, Key Compounds for Chemical Hydrogen Storage

Hydrazine-boranes (H₂NNH₂·BH₃ and H₃B·NH₂NH₂·BH₃) have been proposed for the storage of hydrogen, but these compounds have not created scope for extensive research works as ammonia- and methylamine-boranes have made these last decades. In the exciting research devoted to energy storage with environmentally friendly processes, hydrazine-borane, hydrazine-bisborane, and their simply substituted derivatives could provide a satisfactory response for hydrogen production and recyclability of the formed products. To date, knowledge of the physical and chemical properties of these compounds is still scarce. In this paper, the electronic structure of various hydrazine-boranes complexes is studied by ultraviolet-photoelectron spectroscopy (UV-PES), which is the experimental technique giving direct access to the energy of occupied molecular orbitals. Thus, UV-PE spectra were registered and first ionization energies were determined. Understanding of different types of interactions between nitrogen lone pairs and their variations by complexation has been our essential goal in these studies. In particular, clear stabilization of all molecular orbital energies is noted when complexation with borane takes place. Evolution of the σ_BN bond during the hydrogen release process upon thermal activation has also been studied experimentally by UV-PES and supported by quantum chemical calculations.

Photoionization and dissociative photoionization of propynal in the gas phase: theory and experiment

Propynal (HCCCHO) is a complex organic compound (COM) of astrochemical and astrobiological interest. We present a combined theoretical and experimental investigation on the single photon ionization of gas-phase propynal, in the 10 to 15.75 eV energy range. Fragmentation pathways of the resulting cation were investigated both theoretically and experimentally. The adiabatic ionization energy (AIE) has been measured to be AIEexp = 10.715 ± 0.005 eV using tunable VUV synchrotron radiation coupled with a double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer. In the energy range under study, three fragments formed by dissociative photoionization were identified experimentally: HC3O+, HCO+ and C2H2+, and their respective appearance energies (AE) were found to be AE = 11.26 ± 0.03, 13.4 ± 0.3 and 11.15 ± 0.03 eV, respectively. Using explicitly correlated coupled cluster calculations and after inclusion of the zero point vibrational energy, core-valence and scalar relativistic effects, the AIE is calculated to be AIEcalc = 10.717 eV, in excellent agreement with the experimental finding. The appearance energies of the fragments were calculated using a similar methodological approach. To further interpret the observed vibrational structure, anharmonic frequencies were calculated for the fundamental electronic state of the propynal cation. Moreover, MRCI calculations were carried out to understand the population of excited states of the cationic species. This combined experimental and theoretical study will help to understand the presence and chemical evolution of propynal in the external parts of interstellar clouds where it has been observed.

Isomerization of cyanopropyne in solid argon

Cyanopropyne, CH3-C[triple bond, length as m-dash]C-CN, is a simple molecule whose photochemistry is still unexplored. Here we investigate the UV photolysis of this astrophysically significant nitrile trapped in solid argon. The FTIR study was assisted with 15N-isotopic substitution data and with DFT-level computations including the analyses of ground- and excited-state potential energy surfaces. Cyanopropyne was found to decay mainly via a two-step isomerization process. Infrared absorption spectra evolved to show signals from allenyl cyanide, CH2[double bond, length as m-dash]C[double bond, length as m-dash]CH-CN, which then further convert into propargyl cyanide, H-C[triple bond, length as m-dash]C-CH2-CN. Some evidence for the presence of allenyl isocyanide, propargyl isocyanide, 3-cyanocyclopropene, and 1,2,3-butatrien-1-imine under particular experimental conditions was also observed. Although cyano/isocyano interconversion has been observed during photolysis of other closely related species in solid argon matrices, including H-C[triple bond, length as m-dash]C-CN, no evidence could be found for production of 1-isocyano-1-propyne, CH3-C[triple bond, length as m-dash]C-NC for these experiments.

Photochemistry of XCH2CN (X = -Cl, -SH) in Argon Matrices

We report infrared spectra and photochemical behavior of the potentially astrochemically significant species, mercaptoacetonitrile (HS-CH₂C≡N) and, for comparison purposes, chloroacetonitrile (Cl-CH₂C≡N), both suspended in an argon matrix at 6 K. Photolytic formation of the isocyano products HS-CH₂-NC and Cl-CH₂-NC were observed as well as CH₃NSC and CH₃SCN (in HS-CH₂CN photolysis). While no dissociation products were observed for Cl-CH₂-CN, photolysis of HS-CH₂-CN produced compounds necessitating the loss of the CN group to form CH₂═S, the SH group to form H₂C-CN and HC-CN, or both CN and SH to form CH₃ and CH₄. Observation of emission spectra upon annealing indicates the presence of free sulfur atom in matrices of photolyzed HS-CH₂-CN.

Origin band of the first photoionizing transition of hydrogen isocyanide

The photoelectron spectrum of the X1Σ+ → X+2Σ+ ionizing transition of hydrogen isocyanide (HNC) is measured for the first time at a fixed photon energy (13 eV). The assignment of the spectrum is supported by wave-packet calculations simulating the photoionization transition spectrum and using ab initio calculations of the potential energy surfaces for the three lowest electronic states of the cation. The photoelectron spectrum allows the retrieval of the fundamental of the CN stretching mode of the cationic ground state ([small nu, Greek, tilde]3 = 2260 ± 80 cm-1) and the adiabatic ionization energy of hydrogen isocyanide: IE(HNC) = 12.011 ± 0.010 eV, which is far below that of HCN (IE(HCN) = 13.607 eV). In light of this latter result, the thermodynamics of the HCN+/HNC+ isomers is discussed and a short summary of the values available in the literature is given.

Synthesis of N-unsubstituted cycloalkylimines containing a 4 to 8-membered ring

Primary cycloalkylimines with a 4 to 8-membered ring have been synthesized by dehydrocyanation of the corresponding α-aminonitriles.

Reduction of C[double bond, length as m-dash]O functional groups through H addition reactions: a comparative study between H2CO + H, CH3CH2CHO + H and CH3OCHO + H under interstellar conditions

H-addition reactions on the icy interstellar grains may play an important role in the formation of complex organic molecules. In the present work we propose a comparative study of H2CO + H, CH3CH2CHO + H and CH3OCHO + H solid state reactions at 10 K under interstellar conditions in order to characterize the main reaction pathways involved in the hydrogenation of a CHO functional group. We show that the most probable mechanism for the formation of alcohols under non-energetic conditions through the saturation of the CHO group corresponds to the attachment of the H atom to the CH group with noticeable variations of the energy barriers for each studied reaction. These energy barriers have been calculated to be 8.3, 14.6 and 32.7 kJ mol-1 for H2CO + H, CH3CH2CHO + H and CH3OCHO + H, respectively. The coupling of the experimental and theoretical analysis proves that while the simplest aldehyde, formaldehyde, is easily reduced to methanol, methylformate and propanal behave differently under H-bombardments but they cannot be a source of alcohol formation through H-addition reactions. Consequently, for the formation of alcohols larger than CH3OH, other chemical pathways should be taken into account, probably energetic processing such as the photolysis of interstellar ice analogues containing C-, H- and O-bearing compounds or the coupling of the H-addition reaction and photon-irradiation on species with a CHO functional group.

Synthesis and Electronic Phosphorescence of Dicyanooctatetrayne (NC10N) in Cryogenic Matrixes

The rodlike 1,8-dicyano-octa-1,3,5,7-tetrayne (NC₁₀N) molecule was synthesized with UV-assisted coupling of rare-gas matrix-isolated cyanobutadiyne (HC₅N) molecules. Detection of NC₁₀N molecule was possible due to its strong orange-red (origin at 618 nm) electronic luminescence. Excitation spectra of this emission (ã ³Σ_u⁺-X̃ ¹Σ_g⁺ phosphorescence) gave access to studying the fully allowed H̃ ¹Σ_u⁺-X̃ ¹Σ_g⁺ UV system of NC₁₀N. The identification of observed spectral features was assisted with quantum chemical computations. Certain regularities shaping the electronic spectroscopy of NC_{2 n}N molecules have been discussed.

Isoselenocyanates versus Isothiocyanates and Isocyanates

Alkyl and aryl isoselenocyanates are well known intermediates in the synthesis of various organoselenium compounds, but the knowledge of the physicochemical properties of simple unsaturated derivatives is still fragmentary. Vinyl-, 2-propenyl-, and cyclopropyl isoselenocyanates have been prepared by reaction of selenium in powder with the corresponding isocyanides. The isoselenocyanates of this series, with a variable distance between the N═C═Se group and the unsaturated or pseudounsaturated group, have been studied by UV-photoelectron spectroscopy and quantum-chemical calculations. For each of these three isoselenocyanates, the exploration of conformers and geometrical optimization always converge toward only one local minimum. The vinyl and cyclopropyl derivatives are characterized by similar order of magnitude of interactions between the NCSe group and the substituent, while for allylic compound two noninteracting moieties should be considered. The same conclusions were obtained for vinylic and cyclopropylic sulfur and oxygen derivatives. Thus the type and extent of interactions between the N═C═X (X = O, S, Se) group and an unsaturated (vinyl, allyl, or cyclopropyl) moiety are now clarified.

Internal Rotation of OH Group in 4-Hydroxy-2-butynenitrile Studied by Millimeter-Wave Spectroscopy

Cyanoacetylene, HCC-CN is a ubiquitous molecule in the Universe. However, its interstellar chemistry is not well understood and its understanding requires laboratory data including rotational spectroscopy of possible products coming from a reaction with another compounds. In this study we present the first spectroscopic characterization of gauche conformation of 4-hydroxy-2-butynenitrile (HOCH₂CCCN), a formal adduct of cyanoacetylene on formaldehyde, in the frequency range up to 500 GHz. The analysis of the rotational spectrum was complicated by internal rotation of the OH group that connects two equivalent gauche configurations. The spectral assignment was aided by high-level quantum chemical calculations that were particularly useful in the interpretation of torsional-rotational part of the problem. The applied reduced-axis-system (RAS) formalism allowed fitting within experimental accuracy the lines with K _a < 18. We also present the method of search for initial global solution of torsional-rotational problem within RAS formalism. Accurate spectroscopic parameters obtained in this study provide a reliable basis for the detection of 4-hydroxy-2-butynenitrile in the interstellar medium.

Laboratory spectroscopy of methoxymethanol in the millimeter-wave range

Methoxymethanol, CH₃OCH₂OH is a very interesting candidate for detection in the interstellar medium since it can be formed in the recombination reaction between two radicals considered as intermediates in methanol formation: CH₃O (already detected in the ISM) and CH₂OH.

Low Temperature Synthesis and Phosphorescence of Methylcyanotriacetylene

This paper reports on UV-stimulated synthesis of methylcyanotriacetylene carried out in cryogenic rare gas matrixes via coupling of smaller precursors: propyne and cyanodiacetylene. The detection was possible due to the strong visible ã ³A' → X̃ ¹A₁ phosphorescence of CH₃C₇N, discovered in the course of this work. The ensuing measurements of electronic spectroscopy revealed the formally forbidden B̃ ¹E-X̃ ¹A₁ system, as well as the allowed one Ẽ ¹A₁-X̃ ¹A₁, with origins at approximately 3.32 and 5.4 eV, respectively. It was also possible to revisit the spectroscopic characterization of cyanotriacetylene, HC₇N, formed in parallel to the title photoproduct. Spectral assignments were assisted with a density functional theory study.

Laboratory study of methyl isocyanate ices under astrophysical conditions

Methyl isocyanate has been recently detected in comet 67P/ Churyumov-Gerasimenko (67P/CG) and in the interstellar medium. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CH3NCO have been obtained at temperatures of relevance for astronomical environments. The spectra are dominated by a strong, characteristic multiplet feature at 2350-2250 cm-1, which can be attributed to the antisymmetric stretching of the NCO group. A phase transition from amorphous to crystalline methyl isocyanate is observed at ~ 90 K. The band strengths for the absorptions of CH3NCO in ice at 20 K have been measured. Deuterated methyl isocyanate is used to help with the spectral assignment. No X-ray structure has been reported for crystalline CH3NCO. Here we advance a tentative theoretical structure, based on Density Functional Theory (DFT) calculations, derived taking as a starting point the crystal of isocyanic acid. A harmonic theoretical spectrum is calculated then for the proposed structure, and compared with the experimental data. A mixed ice of H2O and CH3NCO was formed by simultaneous deposition of water and methyl isocyanate at 20 K. The absence of new spectral features indicates that methyl isocyanate and water do not react appreciably at 20 K, but form a stable mixture. The high CH3NCO/H2O ratio reported for comet 67P/CG, and the characteristic structure of the 2350-2250 cm-1 band, make of it a very good candidate for future astronomical searches.

Cryogenic Photochemical Synthesis and Electronic Spectroscopy of Cyanotetracetylene

HC₉N is a molecule of astrochemical interest. In this study, it was produced in cryogenic Ar and Kr matrices from UV-photolyzed diacetylene/cyanodiacetylene mixtures. Its strong phosphorescence was discovered and served for the identification of the compound. Vibrationally resolved phosphorescence excitation spectra gave insight into excited singlet electronic states. Two electronic systems were observed around 26 000-34 000 cm^-1 and 35 000-50 000 cm^-1. Energies of the second excited singlet and the lowest triplet state were derived from analysis of these systems. Vibrational and electronic spectroscopic features were assigned with the assistance of density functional theory calculations. Some trends concerning the electronic spectroscopy of HC_2n+1N family molecules are presented.

Transfer of Asymmetry between Proteinogenic Amino Acids under Harsh Conditions

The heating above 400 °C of serine, cysteine, selenocysteine and threonine leads to a complete decomposition of the amino acids and to the formation in low yields of alanine for the three formers and of 2-aminobutyric acid for the latter. At higher temperature, this amino acid is observed only when sublimable α-alkyl-α-amino acids are present, and with an enantiomeric excess dependent on several parameters. Enantiopure or enantioenriched Ser, Cys, Sel or Thr is not able to transmit its enantiomeric excess to the amino acid formed during its decomposition. The presence during the sublimation-decomposition of enantioenriched valine or isoleucine leads to the enantioenrichment of all sublimable amino acids independently of the presence of many decomposition products coming from the unstable derivative. All these studies give information on a potentially prebiotic key-reaction of abiotic transformations between α-amino acids and their evolution to homochirality.

Fourier-transform microwave spectroscopy of a halogen substituted Criegee intermediate ClCHOO

Pure rotational spectra of the chloro-substituted Criegee intermediate (ClCHOO) were observed by Fourier-transform microwave spectroscopy. Two conformers (syn and anti) of the isolated molecule were identified from the rotational spectra of the parent and ³⁷Cl and ¹³C isotopologues detected in natural abundance. Rotational constants, centrifugal distortion constants, and all components of the nuclear quadrupole coupling tensor were determined for both conformers. Structural features of the molecule have been rationalized with supporting ab initio calculations and the natural bond orbital analysis, which suggest that the conformational preferences are driven by hyperconjugative effects.

One-step synthesis of conjugated enynenitriles from bromocyanoacetylene

Bromocyanoacetylene is able to provide conjugated enynenitriles stereoselectively in one step from alkynes, secondary amines and co-catalysts.

Low-Temperature Reactivity of C2n+1N(-) Anions with Polar Molecules

Following the recent discovery of molecular anions in the interstellar medium, we report on the kinetics of proton transfer reactions between cyanopolyynide anions C2n+1N(-) (n = 0, 1, 2) and formic acid HCOOH. The results, obtained from room temperature down to 36 K by means of uniform supersonic flows, show a surprisingly weak temperature dependence of the CN(-) reaction rate, in contrast with longer chain anions. The CN(-) + HCOOH reaction is further studied theoretically via a reduced dimensional quantum model that highlights a tendency of the reaction probability to decrease with temperature, in agreement with experimental data but at the opposite of conventional long-range capture theories. In return, comparing HCOOH to HC3N as target molecules suggests that dipole-dipole interactions must play an active role in overcoming this limiting effect at low temperatures. This work provides new fundamental insights on prototypical reactions between polar anions and polar molecules along with critical data for astrochemical modeling.

Is the Reaction of C3N(-) with C2H2 a Possible Process for Chain Elongation in Titan's Ionosphere?

The reaction of C3N(-) with acetylene was studied using three different experimental setups, a triple quadrupole mass spectrometer (Trento), a tandem quadrupole mass spectrometer (Prague), and the "CERISES" guided ion beam apparatus at Orsay. The process is of astrophysical interest because it can function as a chain elongation mechanism to produce larger anions that have been detected in Titan's ionosphere by the Cassini Plasma Spectrometer. Three major products of primary processes, C2H(-), CN(-), and C5N(-), have been identified, whereby the production of the cyanide anion is probably partly due to collisional induced dissociation. The formations of all these products show considerable reaction thresholds and also display comparatively small cross sections. Also, no strong signals of anionic products for collision energies lower than 1 eV have been observed. Ab initio calculations have been performed to identify possible pathways leading to the observed products of the title reaction and to elucidate the thermodynamics of these processes. Although the productions of CN(-) and C5N(-) are exoergic, all reaction pathways have considerable barriers. Overall, the results of these computations are in agreement with the observed reaction thresholds. Due to the existence of considerable reaction energy barriers and the small observed cross sections, the title reaction is not very likely to play a major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

Microwave and Quantum Chemical Study of Intramolecular Hydrogen Bonding in 2-Propynylhydrazine (HC≡CCH2NHNH2)

The microwave spectrum of 2-propynylhydrazine (HC≡CCH2NHNH2) was investigated in the 23-124 GHz spectral interval. The spectra of two conformers denoted I and II were assigned. I is the lower-energy form, and relative intensity measurements yielded an internal energy difference of 3.0(4) kJ/mol between I and II. The spectra of the ground and five vibrationally excited states were assigned for I, whereas only the spectrum of the ground vibrational state was assigned for II. Both I and II are each stabilized simultaneously by two intramolecular hydrogen bonds. The first of these hydrogen bonds is formed between the hydrogen atom of the -NH- part of the hydrazino group, and the second internal hydrogen bond is formed between one of the hydrogen atoms of the -NH2 part. The π-electrons of the triple bond is thus shared by these two hydrogen atoms. The shortest contact between a hydrogen atom of the hydrazino group and the π-electrons of the ethynyl group is found in lower-energy conformer I. The conformational properties of 2-propynylhydrazine were explored by MP2/cc-pVTZ and CCSD/cc-pVQZ calculations. The CCSD method predicts that seven rotameric forms exist for this compound. Five of these rotamers are stabilized by internal hydrogen bonding. The simultaneous sharing of the π-electrons of the triple bond by two hydrogen atoms occurs only in Conformers I and II, which are predicted to be the two forms with the lowest energies, with I 2.52 kJ/mol lower in energy than II. The effective rotational constants of the ground vibrational states of I and II were predicted by a combination of MP2 and CCSD calculations, whereas centrifugal distortion constants and vibration-rotation constants were calculated by the MP2 method. The theoretical spectroscopic constants are compared with the experimental counterparts. It is concluded that more refined calculations are necessary to obtain complete agreement.

Rotational Spectrum, Conformational Composition, Intramolecular Hydrogen Bonding, and Quantum Chemical Calculations of Mercaptoacetonitrile (HSCH2C≡N), a Compound of Potential Astrochemical Interest

The microwave spectra of mercaptoacetonitrile (HSCH2C≡N) and one deuterated species (DSCH2C≡N) were investigated in the 7.5-124 GHz spectral interval. The spectra of two conformers denoted SC and AP were assigned. The H-S-C-C chain of atoms is synclinal in SC and anti-periplanar in AP. The ground state of SC is split into two substates separated by a comparatively small energy difference resulting in closely spaced transitions with equal intensities. Several transitions of the parent species of SC deviate from Watson's Hamiltonian. Only slight improvements were obtained using a Hamiltonian that takes coupling between the two substates into account. Deviations from Watson's Hamiltonian were also observed for the parent species of AP. However, the spectrum of the deuterated species, which was investigated only for the SC conformer, fits satisfactorily to Watson's Hamiltonian. Relative intensity measurements found SC to be lower in energy than AP by 3.8(3) kJ/mol. The strength of the intramolecular hydrogen bond between the thiol and cyano groups was estimated to be ∼2.1 kJ/mol. The microwave work was augmented by quantum chemical calculations at CCSD and MP2 levels using basis sets of minimum triple-ζ quality. Mercaptoacetonitrile has astrochemical interest, and the spectra presented herein should be useful for a potential identification of this compound in the interstellar medium. Three different ways of generating mercaptoacetonitrile from compounds already found in the interstellar medium were explored by quantum chemical calculations.

Microwave and Quantum Chemical Study of Intramolecular Hydrogen Bonding in 2-Propenylhydrazine (H2C═CHCH2NHNH2)

The microwave spectrum of 2-propenylhydrazine (H2C═CHCH2NHNH2) was studied in the 12-61 and 72-123 GHz spectral regions. A variety of intramolecular hydrogen bonds between one or more of the hydrogen atoms of the hydrazino group and the π-electrons are possible for this compound. Assignments of the spectra of four conformers, all of which are stabilized with intramolecular hydrogen bonds are reported. One hydrogen bond exists in two of these conformers, whereas the π-electrons are shared by two hydrogen atoms in the two other rotamers. Vibrationally excited-state spectra were assigned for three of the four conformers. The internal hydrogen bonds are weak, probably in the 3-6 kJ/mol range. A total of about 4400 transitions were assigned for these four forms. The microwave work was guided by quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. These calculations indicated that as many as 18 conformers may exist for 2-propenylhydrazine and 11 of these have either one or two intramolecular hydrogen bonds. The four conformers detected in this work are among the rotamers with the lowest CCSD electronic energies. The CCSD method predicts rotational constants that are very close to the experimental rotational constants. The B3LYP calculations yielded quartic centrifugal distortion constants that deviated considerably from their experimental counterparts in most cases. The calculation of vibration-rotation constants and sextic centrifugal distortion constants by the B3LYP method were generally found to be in poor agreement with the corresponding experimental constants.

Metallic cyanoacetylides of copper, silver and gold: generation and structural characterization

Copper, silver and gold cyanoacetylides have been synthesized and characterized in the laboratory using a combination of a laser ablation technique and Fourier transform microwave spectroscopy. A study of the chemical bonding helps to understand the preference for metal–C bonding over metal–N bonding.