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.

Primary α-Phosphino- and α-Arseno-Nitriles, Analogues of α-Aminonitriles.

More than 170 years after the synthesis of α-aminonitriles by the reaction of Strecker, α-phosphinonitriles, and α-arsenonitriles have been prepared and characterized by NMR and IR spectroscopy. For the simplest derivatives, the reaction was carried out by reaction of cyanomethyltributylstannane with phosphorus or arsenic trichloride, followed by a chemoselective reduction of the dichlorophosphine and dichloroarsine formed to the corresponding primary phosphine and arsine. The phosphinoacetonitrile can be stored at -80 °C for months in its pure state, but arsenoacetonitrile decomposes at this temperature. Chiral compounds can be synthesized from C-substituted cyano-1-alkyltributylstannanes. In 1H NMR spectroscopy, these chiral phosphines and arsines display diastereotopic protons for the PH2 and AsH2 groups, a property never observed for the NH2 group of α-aminonitriles. This work paves the way for further studies on the chemistry of these compounds, including a comparative chemistry of these phosphorus and arsenic derivatives with the well-known α-aminonitriles.

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.

On the Spectroscopy of Phosphaalkynes: Millimeter- and Submillimeter-Wave Study of C2H5CP.

Ethyl phosphaethyne, C2H5CP, has been characterized spectroscopically in the gas phase for the first time, employing millimeter- and submillimeter-wave spectroscopy in the frequency regime from 75 to 760 GHz. Spectroscopic detection and analysis was guided by high-level quantum-chemical calculations of molecular structures and force fields performed at the coupled-cluster singles and doubles level extended by a perturbative correction for the contribution from triple excitations, CCSD(T), in combination with large basis sets. Besides the parent isotopologue, the three singly substituted 13C species were observed in natural abundance up to frequencies as high as 500 GHz. Despite the comparably low astronomical abundance of phosphorus, phosphaalkynes, R-CP, such as C2H5CP are promising candidates for future radio astronomical detection.

Tracing Prebiotic Molecules: Rotational Spectroscopy of Deuterated Glycolaldehyde and (Z)-1,2-Ethenediol.

Glycolaldehyde, an important prebiotic molecule, along with its monodeuterated species and its higher energy tautomer, (Z)-1,2-ethenediol, has been detected in the interstellar medium. Although the elemental D/H ratio in the universe is only ∼1.6 × 10-5, the deuterium relative abundance in interstellar molecules might be by far larger than this. As such, it provides a remarkable and almost unique diagnostic tool. In particular, it might help elucidate the reaction mechanisms that lead to the formation of the so-called complex organic molecules. It is therefore crucial to extend the census of the interstellar deuterated molecules. To this aim, in this work, we present for the first time a spectroscopic investigation of the rotational spectra of the CHDOD-CHO bideuterated variant of glycolaldehyde and of mono- and bideuterated species of (Z)-1,2-ethenediol (CHOD═CHOD, CHOD═CHOH, and CHOH═CHOD rotamers). For each species, more than a hundred transitions have been assigned. Their analysis led to the accurate determination of all rotational constants as well as quartic and sextic centrifugal distortion terms, thus providing spectroscopic line catalogs suitable for supporting astronomical searches. In addition, the rotational constants of the bideuterated glycolaldehyde isotopologue studied in this work allowed us to improve the semiexperimental equilibrium structure determination for this molecule.

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.

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 electric discharges of diluted dichloro derivatives of ethylene as molecular precursors. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study to two isotopologues (35Cl and 37Cl), and precise molecular constants are determined for both radicals.

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.

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.

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.

Unusual Quartet-Doublet Phosphorescence from the Phosphaethynyl Radical, CP.

We report the first detection of phosphorescence from the phosphaethynyl radical. This rare instance of quartet-doublet emission, studied here in solid argon, is presumably promoted by efficient intersystem crossing from the originally photoexcited doublet (B2 Σ+ ) to the adjacent second quartet state, 14 Δ. Vibronic progressions were traced for the a4 Σ+ -X2 Σ+ and a4 Σ+ -A2 Пi systems from their origins up to (v'=0)→(v''=5) and (v'=0)→(v''=2) bands, respectively. The measured phosphorescence lifetime is 108 ± 3 ms.

From Molecular to Cluster Properties: Rotational Spectroscopy of 2-Aminopyridine and of Its Biomimetic Cluster with Water.

We report the observation and analysis of the rotational spectrum of a 1:1 cluster between 2-aminopyridine and water (AMW) carried out with supersonic expansion Fourier transform microwave spectroscopy at 4.7-16.5 GHz. Measurements of the 2-aminopyridine monomer (AMP) were also extended up to 333 GHz for the room-temperature rotational spectrum and to resolved hyperfine splitting resulting from the presence of two 14N quadrupolar nuclei. Supersonic expansion measurements for both AMP and AMW were also carried out for two synthesized isotopic species with single deuteration on the phenyl ring. Nuclear quadrupole hyperfine structure has also been resolved for AMW and the derived splitting constants were used as an aid in structural analysis. The structure of the AMW cluster was determined from the three sets of available rotational constants and the hydrogen bonding configuration is compared with those for clusters with water of similarly sized single-ring molecules. Experimental results aided by quantum chemistry computations allow the conclusion that the water molecule is unusually strongly bound by two hydrogen bonds, OH...N and O...HN, to the NCNH atomic chain of AMP with the potential to replace hydrogen bonds to the identical structural segment in cytosine and adenine in CT and AT nucleic acid base pairs.

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 .

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. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CH3NC + F → CH2NC + HF reaction, a VUV synchrotron radiation excitation, and a double imaging electron/ion coincidence spectrometer which allows the recording of mass-selected threshold photoelectron spectra. Assignment of the observed vibrational structure of the CH2NC+ cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CH2NC radical is measured as 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation: [small nu, Greek, tilde]1+(CH2 symmetric stretch) = 2999(80) cm-1, [small nu, Greek, tilde]2+(NC stretch) = 1925(40) cm-1, [small nu, Greek, tilde]4+(H2C-N stretch) = 1193(40) cm-1, [small nu, Greek, tilde]6+(CNC out-of-plane bend) = 237(50) cm-1, and [small nu, Greek, tilde]8+(CH2 rock) = 1185(60) cm-1.

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.

Quasi-symmetry effects in the threshold photoelectron spectrum of methyl isocyanate.

The vacuum-ultraviolet threshold photoelectron spectrum of methyl isocyanate CH3NCO has been recorded from 10.4 eV to 12 eV using synchrotron radiation and a coincidence technique allowing for a mass-discrimination of the photoelectron signal. A significant improvement is achieved over previous investigations as this experimental setup leads to a much more resolved spectrum. Ten sharp peaks and a broad feature spanning 1.2 eV were recorded. This spectrum consists of X̃+ 2A″←X̃ 1A' and Ã+ 2A'←X̃ 1A' ionizing transitions. For the former, the adiabatic ionization energy was determined experimentally to be 10.596(6) eV; for the latter, its value was estimated to be 10.759(50) eV. Seven sharp peaks could be assigned to vibrational modes of the cation X̃+ 2A″ and neutral X̃ 1A' ground electronic states involving only the NCO group atoms. Theoretical modeling of the threshold photoelectron spectrum has proven difficult as methyl isocyanate is a non-rigid molecule displaying large amplitude internal rotation of the methyl group and ∠CNC bending mode, leading to the quasi-symmetry. With the help of ab initio calculations, a theoretical model in which these two large amplitude motions are included in addition to the five small amplitude vibrational modes involving NCO group atoms is proposed. Comparison with the experimental spectrum shows that the broad feature and the strongest peak line positions are well accounted for; their intensities are also fairly well reproduced after adjusting a few parameters.

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.

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.

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.