Experimental Investigations into Astrophysically Relevant Ionic Reactions

The Cyanoketenyl Anion [NC3O]. Angew Chem Int Ed Engl 2024;63:e202403766. [PMID: 38470943 DOI: 10.1002/anie.202403766] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Cumulenes and heterocumulenes with three or more cumulative multiple bonds are usually reactive species that serve as valuable building blocks for more complex molecules but tend to isomerize or cyclize and therefore are difficult to isolate. Using a mild ligand exchange reaction at the carbon in α-metalated ylides, we have now succeeded in the synthesis and gram-scale isolation of the elusive cyanoketenyl anion [NC3O]-. Despite its assumed cumulene-like structure and the delocalization of the negative charge across the whole 5-atom molecule, it features a bent geometry with a nucleophilic central carbon atom. Computational studies reveal an ambiguous bonding situation in the anion, which can be illustrated only by a combination of different resonance structures. Nonetheless, the anion features remarkable stability, thus allowing the storage of its potassium-crown ether salt and its application as a highly functional synthetic building block. The cyanoketenyl anion readily reacts with a series of small molecules to form more complex organic compounds, including industrially valuable compounds such as cyanoacetate. This work demonstrated that reactive species can be generated by novel synthesis methods and open up atom-economic pathways to complex compounds from small abundant molecules.

Ultrafast photoinduced C-H bond formation from two small inorganic molecules

The formation of carbon-hydrogen (C-H) bonds via the reaction of small inorganic molecules is of great significance for understanding the fundamental transition from inorganic to organic matter, and thus the origin of life. Yet, the detailed mechanism of the C-H bond formation, particularly the time scale and molecular-level control of the dynamics, remain elusive. Here, we investigate the light-induced bimolecular reaction starting from a van der Waals molecular dimer composed of two small inorganic molecules, H2 and CO. Employing reaction microscopy driven by a tailored two-color light field, we identify the pathways leading to C-H photobonding thereby producing HCO+ ions, and achieve coherent control over the reaction dynamics. Using a femtosecond pump-probe scheme, we capture the ultrafast formation time, i.e., 198 ± 16 femtoseconds. The real-time visualization and coherent control of the dynamics contribute to a deeper understanding of the most fundamental bimolecular reactions responsible for C-H bond formation, thus contributing to elucidate the emergence of organic components in the universe.

Cold Ion-Molecule Reactions in the Extreme Environment of a Coulomb Crystal

Coulomb crystals provide a unique environment in which to study ion-neutral gas-phase reactions. In these cold, trapped ensembles, we are able to study the kinetics and dynamics of small molecular systems. These measurements have connections to chemistry in the Interstellar Medium (ISM) and planetary atmospheres. This Feature Article will describe recent work in our laboratory that uses Coulomb crystals to study translationally cold, ion-neutral reactions. We provide a description of how the various affordances of our experimental system allow for detailed studies of the reaction mechanisms and the corresponding products. In particular, we will describe quantum-state resolved reactions, isomer-dependent reactions, and reactions with a rarely studied, astrophysically relevant ion, CCl+.

Perspectives of Gas Phase Ion Chemistry: Spectroscopy and Modeling

The study of ions in the gas phase has a long history and has involved both chemists and physicists. The interplay of their competences with the use of very sophisticated commercial and/or homemade instrumentations and theoretical models has improved the knowledge of thermodynamics and kinetics of many chemical reactions, even if still many stages of these processes need to be fully understood. The new technologies and the novel free-electron laser facilities based on plasma acceleration open new opportunities to investigate the chemical reactions in some unrevealed fundamental aspects. The synchrotron light source can be put beside the FELs, and by mass spectrometric techniques and spectroscopies coupled with versatile ion sources it is possible to really change the state of the art of the ion chemistry in different areas such as atmospheric and astro chemistry, plasma chemistry, biophysics, and interstellar medium (ISM). In this manuscript we review the works performed by a joint combination of the experimental studies of ion–molecule reactions with synchrotron radiation and theoretical models adapted and developed to the experimental evidence. The review concludes with the perspectives of ion–molecule reactions by using FEL instrumentations as well as pump probe measurements and the initial attempt in the development of more realistic theoretical models for the prospective improvement of our predictive capability.

Mutual neutralisation of O+ with O-: investigation of the role of metastable ions in a combined experimental and theoretical study

The mutual neutralisation of O⁺ with O^- has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O⁺(²D^o), τ ≈ 3.6 hours), while no contributions from the second metastable state (O⁺(²P^o), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau-Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O⁺(²D^o) metastable state as well as channels involving the O(3s ⁵S^o) state.

Breaking inversion symmetry by protonation: experimental and theoretical NEXAFS study of the diazynium ion, N2H. Phys Chem Chem Phys 2021;23:17166-17176. [PMID: 34346432 DOI: 10.1039/d1cp02002a]

As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N₂H⁺. By ab initio calculations we show that the spectrum consists of a superposition of two nitrogen 1s absorption spectra, each including a π* band, and a nitrogen 1s to H⁺ charge transfer band followed by a weak irregular progression of high energy excitations. Calculations also show that, as an effect of symmetry breaking by protonation, the π* transitions are separated by 0.23 eV, only slightly exceeding the difference in the corresponding dark (symmetry forbidden) and bright (symmetry allowed) core excitations of neutral N₂. By DFT and calculations and vibrational analysis, the complex π* excitation band of N₂H⁺ is understood as due to the superposition of the significantly different vibrational progressions of excitations from terminal and central nitrogen atoms, both leading to bent final state geometries. We also show computationally that the electronic structure of the charge transfer excitation smoothly depends on the nitrogen-proton distance and that there is a clear extension of the spectra going from infinity to close nitrogen-proton distance where fine structures show some, although not fully detailed, similarities. An interesting feature of partial localization of the nitrogen core orbitals, with a strong, non-monotonous, variation with nitrogen-proton distance could be highlighted. Specific effects could be unraveled when comparing molecular cation NEXAFS spectra, as represented by recently recorded spectra of N₂⁺ and CO⁺, and spectra of protonated molecules as represented here by the N₂H⁺ ion. Both types containing rich physical effects not represented in NEXAFS of neutral molecules because of the positive charge, whereas protonation also breaks the symmetry. The effect of the protonation on dinitrogen can be separated in charge, which extends the high-energy part of the spectrum, and symmetry-breaking, which is most clearly seen in the low-energy π* transition.

Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH₂⁺ + CH₄⁺, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH₃-CH₃]²⁺ to [CH₂-CH₄]²⁺ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH₃⁺ + CH₃⁺, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH₃-CH₃]²⁺ isomerization to intermediate [H₂C(H₂)CH₂]²⁺ of the diborane-like double-bridged structure.

Infrared spectroscopy and anharmonic theory of H3 +Ar2,3 complexes: The role of symmetry in solvation

The vibrational spectra of H₃ ⁺Ar_2,3 and D₃ ⁺Ar_2,3 are investigated in the 2000 cm^-1 to 4500 cm^-1 region through a combination of mass-selected infrared laser photodissociation spectroscopy and computational work including the effects of anharmonicity. In the reduced symmetry of the di-argon complex, vibrational activity is detected in the regions of both the symmetric and antisymmetric hydrogen stretching modes of H₃ ⁺. The tri-argon complex restores the D_3h symmetry of the H₃ ⁺ ion, with a concomitant reduction in the vibrational activity that is limited to the region of the antisymmetric stretch. Throughout these spectra, additional bands are detected beyond those predicted with harmonic vibrational theory. Anharmonic theory is able to reproduce some of the additional bands, with varying degrees of success.

Probing the ionization potentials of the formaldehyde dimer

In this work, we present a computational investigation on the ionization potentials (IPs) of the formaldehyde dimer, (H₂CO)₂. Twelve lowest lying IPs (corresponding to the entire valence orbitals) for both C_2h and C_s symmetry conformers have been computed at the coupled cluster level of theory using large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double (EOMIP-CCSD) excitations method with the aug-cc-pVXZ and aug-cc-pCVXZ (X = T, Q, and 5) basis sets combined with the Feller-Peterson-Dixon approach was employed, as well as CCSD with perturbative triples [CCSD(T)] with the aug-cc-pVTZ basis sets. In general, excellent agreement was observed from the comparison between the results obtained through the use of these approaches. In addition, the IPs for the formaldehyde monomer were also obtained using such methodologies and the results compared with existing experimental data; excellent agreement was also observed in this case. To the best of our knowledge, this work represents the first of its kind to determine the IPs for all these systems using a high level theory approach and is presented to motivate experimental investigations, e.g., studies involving photoionization, particularly for the formaldehyde dimer. The equilibrium binding energy of the C_2h dimer is calculated in this work at the CCSD(T)/aug-cc-pVTZ level of theory to be -4.71 kcal/mol. At this same level of theory, the equilibrium isomerization energy between C_2h and C_s conformers is 0.76 kcal/mol (C_s conformer being more stable).

The reaction of C5N- with acetylene as a possible intermediate step to produce large anions in Titan's ionosphere

A theoretical and experimental investigation of the reaction C₅N^- + C₂H₂ has been carried out. This reaction is of astrophysical interest since the growth mechanism of large anions that have been detected in Titan's upper atmosphere by the Cassini plasma spectrometer are still largely unknown. The experimental studies have been performed using a tandem quadrupole mass spectrometer which allows identification of the different reaction channels and assessment of their reaction thresholds. Results of these investigations were compared with the predictions of ab initio calculations, which identified possible pathways leading to the observed products and their thermodynamical properties. These computations yielded that the majority of these products are only accessible via energy barriers situated more than 1 eV above the reactant energies. In many cases, the thresholds predicted by the ab initio calculations are in good agreement with the experimentally observed ones. For example, the chain elongation reaction leading to C₇N^-, although being slightly exoergic, possesses an energy barrier of 1.91 eV. Therefore, the title reaction can be regarded to be somewhat unlikely to be responsible for the formation of large anions in cold environments such as interstellar medium or planetary ionospheres.

Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH₃⁺ + CH₃CCCH₃ (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C₅H₇⁺, stemming from H₂ loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Laboratory Studies of Methane and Its Relationship to Prebiotic Chemistry

To examine how prebiotic chemical evolution took place on Earth prior to the emergence of life, laboratory experiments have been conducted since the 1950s. Methane has been one of the key molecules in these investigations. In earlier studies, strongly reducing gas mixtures containing methane and ammonia were used to simulate possible reactions in the primitive atmosphere of Earth, producing amino acids and other organic compounds. Since Earth's early atmosphere is now considered to be less reducing, the contribution of extraterrestrial organics to chemical evolution has taken on an important role. Such organic molecules may have come from molecular clouds and regions of star formation that created protoplanetary disks, planets, asteroids, and comets. The interstellar origin of organics has been examined both experimentally and theoretically, including laboratory investigations that simulate interstellar molecular reactions. Endogenous and exogenous organics could also have been supplied to the primitive ocean, making submarine hydrothermal systems plausible sites of the generation of life. Experiments that simulate such hydrothermal systems where methane played an important role have consequently been conducted. Processes that occur in other Solar System bodies offer clues to the prebiotic chemistry of Earth. Titan and other icy bodies, where methane plays significant roles, are especially good targets. In the case of Titan, methane is both in the atmosphere and in liquidospheres that are composed of methane and other hydrocarbons, and these have been studied in simulation experiments. Here, we review the wide range of experimental work in which these various terrestrial and extraterrestrial environments have been modeled, and we examine the possible role of methane in chemical evolution. Key Words: Methane-Interstellar environments-Submarine hydrothermal systems-Titan-Origin of life. Astrobiology 17, 786-812.

Isotopic and quantum-rovibrational-state effects for the ion-molecule reaction in the collision energy range of 0.03-10.00 eV

We report detailed quantum-rovibrational-state-selected integral cross sections for the formation of H₃O⁺via H-transfer (σ_HT) and H₂DO⁺via D-transfer (σ_DT) from the reaction in the center-of-mass collision energy (E_cm) range of 0.03-10.00 eV, where (vvv) = (000), (100), and (020) and . The E_cm inhibition and rotational enhancement observed for these reactions at E_cm < 0.5 eV are generally consistent with those reported previously for H₂O⁺ + H₂(D₂) reactions. However, in contrast to the vibrational inhibition observed for the latter reactions at low E_cm < 0.5 eV, both the σ_HT and σ_DT for the H₂O⁺ + HD reaction are found to be enhanced by (100) vibrational excitation, which is not predicted by the current state-of-the-art theoretical dynamics calculations. Furthermore, the (100) vibrational enhancement for the H₂O⁺ + HD reaction is observed in the full E_cm range of 0.03-10.00 eV. The fact that vibrational enhancement is only observed for the reaction of H₂O⁺ + HD, and not for H₂O⁺ + H₂(D₂) reactions suggests that the asymmetry of HD may play a role in the reaction dynamics. In addition to the strong isotopic effect favoring the σ_HT channel of the H₂O⁺ + HD reaction at low E_cm < 0.5 eV, competition between the σ_HT and σ_DT of the H₂O⁺ + HD reaction is also observed at E_cm = 0.3-10.0 eV. The present state-selected study of the H₂O⁺ + HD reaction, along with the previous studies of the H₂O⁺ + H₂(D₂) reactions, clearly shows that the chemical reactivity of H₂O⁺ toward H₂ (HD, D₂) depends not only on E_cm, but also on the rotational and vibrational states of H₂O⁺(X²B₁). The detailed σ_HT and σ_DT values obtained here with single rovibrational-state selections of the reactant H₂O⁺ are expected to be valuable benchmarks for state-of-the-art theoretical calculations on the chemical dynamics of the title reaction.

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

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.

Mid-infrared concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy of a continuous supersonic expansion discharge source

Concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is implemented for the first time on a continuous gas-flow pinhole supersonic expansion discharge source for the study of cooled molecular ions. The instrument utilizes a continuous-wave optical parametric oscillator easily tunable from 2.5 to 3.9 μm and demonstrates a noise equivalent absorption of ∼1 × 10(-9) cm(-1). The effectiveness of concentration-modulated NICE-OHMS is tested through the acquisition of transitions in the ν1 fundamental band of HN2 (+) centered near 3234 cm(-1), with a signal-to-noise of ∼40 obtained for the strongest transitions. The technique is used to characterize the cooling abilities of the supersonic expansion discharge source itself, and a Boltzmann analysis determines a rotational temperature of ∼29 K for low rotational states of HN2 (+). Further improvements are discussed that will enable concentration-modulated NICE-OHMS to reach its full potential for the detection of molecular ions formed in supersonic expansion discharges.

Relevance and Significance of Extraterrestrial Abiological Hydrocarbon Chemistry

Astrophysical observations show similarity of observed abiological "organics"-i.e., hydrocarbons, their derivatives, and ions (carbocations and carbanions)-with studied terrestrial chemistry. Their formation pathways, their related extraterrestrial hydrocarbon chemistry originating from carbon and other elements after the Big Bang, their parent hydrocarbon and derivative (methane and methanol, respectively), and transportation of derived building blocks of life by meteorites or comets to planet Earth are discussed in this Perspective. Their subsequent evolution on Earth under favorable "Goldilocks" conditions led to more complex molecules and biological systems, and eventually to humans. The relevance and significance of extraterrestrial hydrocarbon chemistry to the limits of science in relation to the physical aspects of evolution on our planet Earth are also discussed.

Dissociative recombination and mutual neutralization of heavier molecular ions: C10H8(+), WF5(+), and C(n)F(m)(+)

Dissociative recombination (DR) rate coefficients for the naphthalene cation, C10H8(+), and WF5(+), and mutual neutralization (MN) rate coefficients for these species and five CnFm(+) ions, were determined at 300 K using variable electron and neutral density attachment mass spectrometry (VENDAMS). DR proceeds at 9 ± 3 × 10(-7) cm(3) s(-1) for C10H8(+) and at 6.1 ± 1.4 × 10(-7) cm(3) s(-1) for WF5(+). Consistent with previous results, MN for the polyatomic cations with the halide anions Cl(-), Br(-), and I(-) exhibits an approximate μ(-1/2) reduced mass dependence of the reactant partners, demonstrating that ion collision velocities influence the rate coefficients. This work is an extension of VENDAMS to systems, where low reactant concentrations are necessary to avoid significant reaction of product ions with the neutral precursor, i.e., conditions not suitable for traditional flowing afterglow measurements, as well as to ions of masses > ∼ 100 Da, which are not amenable to the study of DR in magnetic storage rings. Our results expand the sparse literature on DR and MN of heavier ions.

Astrochemistry of dust, ice and gas: introduction and overview

A brief introduction and overview of the astrochemistry of dust, ice and gas and their interplay is presented. The importance of basic chemical physics studies of critical reactions is illustrated through a number of recent examples. Such studies have also triggered new insight into chemistry, illustrating how astronomy and chemistry can enhance each other. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas–grain chemistry rather than pure gas-phase chemistry, and a critical discussion of the state of such models is given. Recent developments in studies of diffuse clouds and PDRs, cold dense clouds, hot cores, protoplanetary disks and exoplanetary atmospheres are summarized, both for simple and more complex molecules, with links to papers presented in this volume. In spite of many lingering uncertainties, the future of astrochemistry is bright: new observational facilities promise major advances in our understanding of the journey of gas, ice and dust from clouds to planets.