Unveiling the Reaction Mechanism of the N(2D) + Pyridine Reaction: Ring-Contraction versus 7-Membered-Ring Formation Channels

Despite the relevance of the reactions of the prototypical nitrogen-containing six-membered aromatic molecule (N-heterocyclic) of pyridine (C6H5N) in environmental science, astrochemistry, planetary science, prebiotic chemistry, and materials science, few experimental/theoretical studies exist on the bimolecular reactions involving pyridine and neutral atomic/molecular radicals. We report a combined experimental and theoretical study on the elementary reaction of pyridine with excited nitrogen atoms, N(2D), aimed at providing information about the primary reaction products and their branching fractions (BFs). From previous crossed molecular beam (CMB) experiments with mass-spectrometric detection and present synergistic calculations of the reactive potential energy surface (PES) and product BFs we have unveiled the reaction mechanism. It is found that the reaction proceeds via N(2D) barrierless addition to pyridine that, via bridged intermediates followed by N atom "sliding" into the ring, leads to 7-membered-ring structures. They further evolve, mainly via ring-contraction mechanisms toward 5-membered-ring radical products and, to a smaller extent, via H-displacement mechanisms toward 7-membered-ring isomeric products and their isomers. Using the theoretical statistical estimates, an improved fit of the experimental data previously reported has been obtained, leading to the following results for the dominant product channels: C4H4N (pyrrolyl) + HCN (BF = 0.61 ± 0.20), C3H3N2 (1H-imidazolyl/1H-pyrazolyl) + C2H2 (BF = 0.11 ± 0.06), and C5H4N2 (7-membered-ring molecules or pyrrole carbonitriles) + H (BF = 0.28 ± 0.10). The ring-contraction product channels C4H4N (pyrrolyl) + HCN, C3H3N2 (1H-imidazolyl) + C2H2, C3H3N2 (1H-pyrazolyl) + C2H2, and C5H5 (cyclopentadienyl) + N2 have statistical BFs of 0.54, 0.09, 0.11, and 0.07, respectively. Among the H-displacement channels, the cyclic-CHCHCHCHNCN + H channel and cyclic-CHCHCHCHCN2 + H are theoretically predicted to have a comparable BF (0.07 and 0.06, respectively), while the other isomeric 7-membered-ring molecule + H channel has a BF of 0.03. Pyrrole-carbonitriles and 1H-ethynyl-1H-imidazole (+ H) isomeric channels have an overall BF of 0.03. Implications for the chemistry of Saturn's moon Titan and prebiotic chemistry, as well as for understanding the N-doping of graphene or carbon nanotubes, are noted.

Noncovalent Interactions Steer the Formation of Polycyclic Aromatic Hydrocarbons

Aromatic molecules play an important role in the chemistry of astronomical environments such as the cold interstellar medium (ISM) and (exo)planetary atmospheres. The observed abundances of (polycyclic) aromatic hydrocarbons such as benzonitrile and cyanonaphthalenes are, however, highly underestimated by astrochemical models. This demonstrates the need for more experimentally verified reaction pathways. The low-temperature ion-molecule reaction of benzonitrile•+ with acetylene is studied here using a multifaceted approach involving kinetics and spectroscopic probing of the reaction products. A fast radiative association reaction via an in situ experimentally observed prereactive complex shows the importance of noncovalent interactions in steering the pathway during cold ion-molecule reactions. Product structures of subsequent reactions are unambiguously identified using infrared action spectroscopy and reveal the formation of nitrogen-containing, linked bicyclic structures such as phenylpyridine•+ and benzo-N-pentalene+ structures. The results, contradicting earlier assumptions on the product structure, demonstrate the importance of spectroscopic probing of reaction products and emphasize the possible formation of linked bicyclic molecules and benzo-N-pentalene+ structures in astronomical environments.

Growth mechanism of aromatic prebiotic molecules: insights from different processes of ion-molecule reactions in benzonitrile-ammonia and benzonitrile-methylamine clusters

Benzonitrile (BN, C6H5CN) has been detected in the cold molecular cloud Taurus molecular cloud-1 (TMC-1) in 2018, which is suggested to be a precursor in the formation of more complex nitrogen-containing aromatic interstellar compounds. In this study, we utilized mass-selected infrared (IR) photodissociation spectroscopy and quantum chemical calculations to investigate the structures and gaseous ion-molecule reactions of benzonitrile-ammonia (BN-NH3) and benzonitrile-methylamine (BN-MA) clusters. The spectral observations indicate that the cyclic hydrogen bonding structure predominates in both neutral clusters. After VUV (118 nm) single-photon ionization, a new C-N covalent bond formed between BN and NH3 in the (BN-NH3)+ cluster. However, proton sharing constitutes the primary structure of the (BN-MA)+ cluster. The two nitrogen-containing interstellar molecules react with BN to yield distinct products due to difference in charge distribution and molecular polarity in the ionized clusters. The reactions of BN with other molecules contribute to our understanding of the growth mechanisms of complex interstellar molecules.

Microhydration of small protonated polyaromatic hydrocarbons: a first principles study

Using first principles methodology, we investigate the microsolvation of protonated benzene (BzH+), protonated coronene (CorH+) and protonated dodecabenzocoronene (DbcH+). Gas phase complexes of these small protonated polyaromatic hydrocarbons (H+PAHs) with mono-, di-, and tri-hydrated water molecules are considered. Their most stable forms are presented, where we discuss their structural, energetic aromaticity and IR and UV spectral features. In particular, we focus on the analysis of the bonding and various non-bonded interactions between these protonated aromatics and water clusters. The strength of non-bonded interactions is quantified and correlated with their electron density profiles. Furthermore, insights into the interfacial interactions and stability of these complexes were obtained through non-covalent index and symmetry-adapted perturbation theory (SAPT0) analyses. We also discuss the effects of the extension of the π aromatic cloud on the water solvation of these protonated aromatics. In particular, we extended our predictions for the S0 → S1 and S0 → T1 wavelength transitions of micro hydrated H+PAHs to deduce those of these species solvated in aqueous solution. The present findings should be useful for understanding, at the microscopic level, the effects of water interacting with H+PAHs, which are relevant for organic chemistry, astrochemistry, atmospheric chemistry, combustion and materials science.

Photoelectron spectrum of the pyridyl radical

We report the photoelectron spectrum of the pyridyl radical (C5H4N), a species of interest in astrochemistry and combustion. The radicals were produced via hydrogen abstraction in a fluorine discharge and ionized with synchrotron radiation. Mass-selected slow photoelectron spectra of the products were obtained from photoelectron-photoion coincidence spectra. A Franck-Condon simulation based on computed geometries and vibrational frequencies identified contributions of the o- and p-pyridyl radicals. For the o-isomer an adiabatic ionisation energy of 7.70 eV was obtained, in excellent agreement with a computed value of 7.72 eV. The spectrum of o-pyridyl is characterized by a long progression in an in-plane bending mode and the N-C stretch that contains the radical site.

Possible Roles of Transition Metal Cations in the Formation of Interstellar Benzene via Catalytic Acetylene Cyclotrimerization

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous interstellar molecules. However, the formation mechanisms of PAHs and even the simplest cyclic aromatic hydrocarbon, benzene, are not yet fully understood. Recently, we reported the statistical and dynamical properties in the reaction mechanism of Fe+-catalyzed acetylene cyclotrimerization, whereby three acetylene molecules are directly converted to benzene. In this study, we extended our previous work and explored the possible role of the complex of other 3d transition metal cations, TM+ (TM = Sc, Ti, Mn, Co, and Ni), as a catalyst in acetylene cyclotrimerization. Potential energy profiles for bare TM+-catalyst (TM = Sc and Ti), for TM+NC--catalyst (TM = Sc, Ti, Mn, Co, and Ni), and for TM+-(H2O)8-catalyst (TM = Sc and Ti) systems were obtained using quantum chemistry calculations, including the density functional theory levels. The calculation results show that the scandium and titanium cations act as efficient catalysts in acetylene cyclotrimerization and that reactants, which contain an isolated acetylene and (C2H2)2 bound to a bare (ligated) TM cation (TM = Sc and Ti), can be converted into a benzene-metal-cation product complex without an entrance barrier. We found that the number of electrons in the 3d orbitals of the transition metal cation significantly contributes to the catalytic efficiency in the acetylene cyclotrimerization process. On-the-fly Born-Oppenheimer molecular dynamics (BOMD) simulations of the Ti+-NC- and Ti+-(H2O)8 complexes were also performed to comprehensively understand the nuclear dynamics of the reactions. The computational results suggest that interstellar benzene can be produced via acetylene cyclotrimerization reactions catalyzed by transition metal cation complexes.

Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile

The threshold photoionization and dissociative ionization of benzonitrile (C6H5CN) were studied using double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source (SLS). The threshold photoelectron spectrum was recorded from 9.6 to 12.7 eV and Franck-Condon simulations of ionization into the ionic ground state, X̃+, as well as the B̃+ and C̃+ states were performed to assign the observed vibronic structures. The adiabatic ionization energies of the X̃+, B̃+ and C̃+ states are determined to be (9.72 ± 0.02), (11.85 ± 0.03) and, tentatively, (12.07 ± 0.04) eV, respectively. Threshold ionization mass spectra were recorded from 13.75 to 19.75 eV and the breakdown diagram was constructed by plotting the fractional abundances of the parent ion and ionic dissociation products as a function of photon energy. The seven lowest energy dissociative photoionization channels of benzonitrile were found to yield CN˙ + c-C6H5+, HCN + C6H4˙+, C2H4 + HC5N˙+, HC3N + C4H4˙+, H2C3N˙ + C4H3+, CH2CHCN + C4H2˙+ and H2C4N˙ + c-C3H3+. HCN loss from the benzonitrile cation is the dominant dissociation channel from the dissociation onset of up to 18.1 eV and CH2CHCN loss becomes dominant from 18.1 eV and up. We present extensive potential energy surface calculations on the C6H5CN˙+ surface to rationalize the detected products. The breakdown diagram and time-of-flight mass spectra are fitted using a Rice-Ramsperger-Kassel-Marcus statistical model. Anchoring the fit to the CBS-QB3 result (3.42 eV) for the barrier to HCN loss, we obtained experimental dissociation barriers for the products of 4.30 eV (CN loss), 5.53 eV (C2H4 loss), 4.33 eV (HC3N loss), 5.15 eV (H2C3N loss), 4.93 eV (CH2CHCN loss) and 4.41 eV (H2C4N loss). We compare our work to studies of the electron-induced dissociative ionization of benzonitrile and isoelectronic phenylacetylene (C8H6), as well as the VUV-induced dissociation of protonated benzonitrile (C6H5CNH+). Also, we discuss the potential role of barrierless association reactions found for some of the identified fragments as a source of benzonitrile(˙+) in interstellar chemistry and in Titan's atmosphere.

Counterintuitive Catalytic Reactivity of the Aluminum Oxide "Passivation" Shell of Aluminum Nanoparticles Facilitating the Thermal Decomposition of exo-Tetrahydrodicyclopentadiene (JP-10)

High energy density aluminum nanoparticles (AlNPs) have been at the center of attention as additives to hydrocarbon jet fuels like exo-tetrahydrodicyclopentadiene (JP-10, C10H16) aiming at the superior performance of volume-limited air-breathing propulsion systems. However, a fundamental understanding of the ignition and combustion chemistry of JP-10 in the presence of AlNPs has been elusive. Exploiting an isomer-selective comprehensive identification of the decomposition products in a newly designed high-temperature chemical microreactor coupled to vacuum ultraviolet photoionization, we reveal an active low-temperature heterogeneous surface chemistry commencing at 650 K involving the alumina (Al2O3) shell. Contrary to textbook knowledge of an "inactive alumina surface", this unconventional reactivity, where oxygen is transferred from alumina to JP-10, leads to generating cyclic, oxygenated organics like phenol (C6H5OH) and 2,4-cyclopentadiene-1-one (C5H4O)─key tracers of an alumina-mediated interfacial chemistry. This counterintuitive reactivity transforms our knowledge of the (catalytic) processes of alumina-coated AlNPs on the molecular level.

Accurate Prediction of Adiabatic Ionization Energies for PAHs and Substituted Analogues

The accurate calculation of adiabatic ionization energies (AIEs) for polycyclic aromatic hydrocarbons (PAHs) and their substituted analogues is essential for understanding their electronic properties, reactivity, stability, and environmental/health implications. This study demonstrates that the M06-2X density functional theory method excels in predicting the AIEs of polycyclic aromatic hydrocarbons and related molecules, rivaling the (R)CCSD(T)-F12 method in terms of accuracy. These findings suggest that M06-2X, coupled with an appropriate basis set, represents a reliable and efficient method for studying polycyclic aromatic hydrocarbons and related molecules, aligning well with the experimental techniques. The set of molecules examined in this work encompasses numerous polycyclic aromatic hydrocarbons from m/z 67 up to m/z 1,176, containing heteroatoms that may be found in biofuels or nucleic acid bases, making the results highly relevant for photoionization experiments and mass spectrometry. For coronene-derivative molecular species with the C6n2H6n chemical formula, we give an expression to predict their AIEs (AIE (n) = 4.359 + 4.8743n-0.72057, in eV) upon extending the π-aromatic cloud until reaching graphene. In the long term, the application of this method is anticipated to contribute to a deeper understanding of the relationships between PAHs and graphene, guiding research in materials science and electronic applications and serving as a valuable tool for validating theoretical calculation methods.

Selective adsorption of polycyclic aromatic hydrocarbons by isostructural hydrogen-bonded organic frameworks

Two isostructural hydrogen-bonded organic frameworks (HOFs) with 1-D hexagonal-shaped pores were crystallised using the molecules biphenyl-3,3',5,5'-tetracarboxylic acid (BPTCA) and [1,1':4',1'']terphenyl- 3,3'',5,5''-tetracarboxylic acid (TPTCA). The desolvated HOFs, named BPTCA-2 and TPTCA-2, exhibited selective adsorption towards naphthalene and anthracene, respectively, during competitive adsorption experiments.

Fingerprinting fragments of fragile interstellar molecules: dissociation chemistry of pyridine and benzonitrile revealed by infrared spectroscopy and theory

The cationic fragmentation products in the dissociative ionization of pyridine and benzonitrile have been studied by infrared action spectroscopy in a cryogenic ion trap instrument at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. A comparison of the experimental vibrational fingerprints of the dominant cationic fragments with those from quantum chemical calculations revealed a diversity of molecular fragment structures. The loss of HCN/HNC is shown to be the major fragmentation channel for both pyridine and benzonitrile. Using the determined structures of the cationic fragments, potential energy surfaces have been calculated to elucidate the nature of the neutral fragment partner. In the fragmentation chemistry of pyridine, multiple non-cyclic structures are formed, whereas the fragmentation of benzonitrile dominantly leads to the formation of cyclic structures. Among the fragments are linear cyano-(di)acetylene˙+, methylene-cyclopropene˙+ and o- and m-benzyne˙+ structures, the latter possible building blocks in interstellar polycyclic aromatic hydrocarbon (PAH) formation chemistry. Molecular dynamics simulations using density functional based tight binding (MD/DFTB) were performed and used to benchmark and elucidate the different fragmentation pathways based on the experimentally determined structures. The implications of the difference in fragments observed for pyridine and benzonitrile are discussed in an astrochemical context.

Gas-phase formation of the resonantly stabilized 1-indenyl (C9H7•) radical in the interstellar medium

The 1-indenyl (C9H7•) radical, a prototype aromatic and resonantly stabilized free radical carrying a six- and a five-membered ring, has emerged as a fundamental molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures in deep space and combustion systems. However, the underlying formation mechanisms have remained elusive. Here, we reveal an unconventional low-temperature gas-phase formation of 1-indenyl via barrierless ring annulation involving reactions of atomic carbon [C(3P)] with styrene (C6H5C2H3) and propargyl (C3H3•) with phenyl (C6H5•). Macroscopic environments like molecular clouds act as natural low-temperature laboratories, where rapid molecular mass growth to 1-indenyl and subsequently complex PAHs involving vinyl side-chained aromatics and aryl radicals can occur. These reactions may account for the formation of PAHs and their derivatives in the interstellar medium and carbonaceous chondrites and could close the gap of timescales of their production and destruction in our carbonaceous universe.

Molecular growth of PANH via intermolecular Coulombic decay

Nitrogen-bearing polycyclic aromatic hydrocarbons (PANHs) are ubiquitous in space. They are considered precursors to advanced biomolecules identified in meteorites. However, their chemical evolution into biomolecules in photodestructive astrophysical mediums remains a paradox. Here, we show that light can efficiently initiate the molecular mass growth of PANHs. Ultraviolet-photoexcited quinoline monomers, the smallest PANH, were observed to associate and intermolecular Coulombic decay between the associating monomers formed the cations of quinoline-dimer. Molecular rearrangements in the dimer cation lead to a dominant formation of cations heavier than quinoline. The enrichment of these heavier cations over all the other cations reveals the efficiency of this route for the mass growth of PANHs in space. This mechanism also leads to a highly reactive unsaturated PANH-ring via CH loss, a hitherto unknown channel in any photon-driven process. The occurrence of this efficient pathway toward complex molecules points to a rich chemistry in dense interstellar clouds.

Divergent Construction of N-Doped Polycyclic Aromatic Hydrocarbons with Indole as the Nitrogen Source Building Block

An Ag/Au-catalyzed divergent cascade reaction of alkyne embedded diazoketones with indoles has been described. Preliminary mechanistic studies indicate that the reaction goes through a [4+2]-cycloaddition of an in situ formed isobenzopyrylium intermediate with indole, followed by a sequential retro-Michael addition/carbene N-H insertion process to give the benzo[i]phenanthridines products with gold catalysis; whereas a dearomatization/rearomatization sequence occurs favourably when the reaction is catalyzed by a silver catalyst, delivering benzo[b]carbazoles in generally high to excellent yields. Notably, this is a rare example of using indole as the dienophile for cycloaddition with the isobenzopyrylium species, providing a concise and practical approach for the selective construction of N-doped polycyclic aromatic hydrocarbons (PAHs) with structural diversity and broad functional-group compatibility.

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon

Electron bombardment of aniline (PhNH₂) in an Ar matrix mainly generated the aniline cation (PhNH₂⁺), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH₂⁺ and PhN, and resulted in the formation of PhNH₂, PhNH, and Ph. In separate experiments, irradiation of the PhNH₂/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH₂⁺ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

Threshold Photoelectron Spectroscopy of Quinoxaline, Quinazoline, and Cinnoline

The threshold photoelectron spectra of cinnoline, quinazoline, and quinoxaline, three small naphthalene-analogue polycyclic nitrogen-containing hydrocarbons of C₈H₆N₂ composition, were recorded. The spectra are assigned to understand their electronic structure and the role of isomerism. Furthermore, this work provides reference data for the selective identification of such species as gas-phase reaction products at low number densities. Imaging photoelectron photoion coincidence spectroscopy was used at the VUV beamline of the Swiss Light Source to record the spectra from the ionization onset to 12 eV. To assign and interpret the spectral features, we relied on (time-dependent) density functional theory and EOM-IP-CCSD calculations and computed vertical and adiabatic ionization energies as well as Franck-Condon factors to simulate ground- and excited-state spectra. Vibrational progressions belonging to four electronic states could be simulated in each of the samples, and we report a total of 12 adiabatic ionization energies, including the ones to the ground and excited cation states. Such a wealth of spectral information, as well as the reliable ab initio modeling, is promising with regards to analytical applications. While cinnoline can be easily distinguished by its lowest adiabatic ionization energy, quinazoline and quinoxaline show different vibrational fingerprints, which can be used to distinguish the three isomers even in complex reaction mixtures. Finally, we also relate the cation electronic states to the neutral molecular orbitals and note that Koopmans' approximation fails in these N₂-containing species very much like it does in N₂.

Unconventional excited-state dynamics in the concerted benzyl (C7H7) radical self-reaction to anthracene (C14H10)

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical – radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (C₇H₇) radical self-reaction to phenanthrene and anthracene (C₁₄H₁₀) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more “rapid” synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

The reaction of benzyl radical self-reaction to anthracene opens-up a previously overlooked avenue for a more efficient synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

Synthesis of CxNy-rich polycyclic oligomers from primeval monomers in aqueous media

A multichannel, non-thermolytic and efficient pathway is described toward the formation of functionalized carbon nitride-like oligomers, starting from readily available cyanamide and glyoxal (in ratios >2), in aqueous media under mild conditions. Such oligomers can be isolated as stable solids that result from structures involving cyanamide self-additions along with structures formally derived from the condensation of cyanamide, dicyandiamide or melamine with glyoxal, leading occasionally to oxygen-containing units. The oligomeric aggregates have masses up to 500 u, as inferred from mass spectra analyses, and their formation can be rationalized in terms of polyadditions of cyanamide (up to 10-mer) and glyoxal. The latter is not only a willing reaction partner, but also promotes facile condensation by enhancing the reactivity of nitrile fragments and inducing a significant lowering of the energy barriers. This mechanistic surmise is also supported by DFT calculations of the early condensation steps. As a result, melamine/triazine-type structures are obtained in aquatic environments under much milder conditions than those usually required by other synthetic procedures. Moreover, our results also help unveil the abiotic processes affording complex organic matter on celestial bodies and early earth.

A chemical dynamics study of the reaction of the methylidyne radical (CH, X2Π) with dimethylacetylene (CH3CCCH3, X1A1g)

The gas-phase reaction of the methylidyne (CH; X²Π) radical with dimethylacetylene (CH₃CCCH₃; X¹A_1g) was studied at a collision energy of 20.6 kJ mol^-1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C₅H₇ reaction intermediate(s) ultimately dissociating to C₅H₆ isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon-carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon-hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively 'screen' the carbon-carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C₂H₆) forming propylene (CH₃C₂H₃). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH₃CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne - ethane system.

High-level ab initio quartic force fields and spectroscopic characterization of C2N. Phys Chem Chem Phys 2021;23:26227-26240. [PMID: 34787132 DOI: 10.1039/d1cp03505c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

While it is now well established that large carbon chain species and radiative electron attachment (REA) are key ingredients triggering interstellar anion chemistry, the role played by smaller molecular anions, for which REA appears to be an unlikely formation pathway, is as yet elusive. Advancing this research undoubtedly requires the knowledge (and modeling) of their astronomical abundances which, for the case of C₂N^-, is largely hindered by a lack of accurate spectroscopic signatures. In this work, we provide such data for both ground -CCN^-(³Σ^-) and low-lying c-CNC^-(¹A₁) isomers and their singly-substituted isotopologues by means of state-of-the-art rovibrational quantum chemical techniques. Their quartic force fields are herein calibrated using a high-level composite energy scheme that accounts for extrapolations to both one-particle and (approximate) -particle basis set limits, in addition to relativistic effects, with the final forms being subsequently subject to nuclear motion calculations. Besides standard spectroscopic attributes, the full set of computed properties includes fine and hyperfine interaction constants and can be readily introduced as guesses in conventional experimental data reduction analyses through effective Hamiltonians. On the basis of benchmark calculations performed anew for a minimal test set of prototypical triatomics and limited (low-resolution) experimental data for -CCN^-(³Σ^-), the target accuracies are determined to be better than 0.1% of experiment for rotational constants and 0.3% for vibrational fundamentals. Apart from laboratory investigations, the results here presented are expected to also prompt future astronomical surveys on C₂N^-. To this end and using the theoretically-predicted spectroscopic constants, the rotational spectra of both -CCN^-(³Σ^-) and c-CNC^-(¹A₁) are derived and their likely detectability in the interstellar medium is further explored in connection with working frequency ranges of powerful astronomical facilities. Our best theoretical estimate places c-CNC^-(¹A₁) at about 15.3 kcal mol^-1 above the ground-state -CCN^-(³Σ^-) species.

Rotational Spectra of Three Cyanobutadiene Isomers (C5H5N) of Relevance to Astrochemistry and Other Harsh Reaction Environments

Three cyanobutadiene isomers have been synthesized and their rotational spectra analyzed in the 130-375 GHz frequency range. These species, which are close analogues of known interstellar molecules and are isomers of the heterocyclic aromatic molecule pyridine (C₅H₅N), offer the opportunity of revealing important insights concerning the chemistry in astronomical environments. The s-trans conformers of E-1-cyano-1,3-butadiene and Z-1-cyano-1,3-butadiene are observed, while both the anti-clinal and syn-periplanar conformers of 4-cyano-1,2-butadiene are evident in the rotational spectra. Over 1000 transitions for s-trans-Z-1-cyano-1,3-butadiene and for syn-periplanar-4-cyano-1,2-butadiene are fit to an octic, distorted-rotor Hamiltonian with low uncertainty (<50 kHz). Although neither s-trans-E-1-cyano-1,3-butadiene nor anti-clinal-4-cyano-1,2-butadiene can be fully treated with a distorted-rotor Hamiltonian in this frequency range, we provide herein minimally perturbed, single-state least-squares fits of over 1000 transitions for each species, yielding sets of spectroscopic constants that are expected to enable accurate prediction of high-intensity transitions at frequencies up to 370 GHz for both isomers. The assigned transitions and spectroscopic constants for these cyanobutadienes have already enabled the identification of two isomers in harsh reaction environments and should be sufficient to enable their identification in astronomical environments by radio astronomy.

Cyclopentadienyl radical formation from the reaction of excited nitrogen atoms with benzene: a theoretical study

Ab initio CCSD(T)/CBS//ωB97X-D/6-311+G(d,p) calculations of the C6H6N potential energy surface were performed to investigate the reaction mechanism underlying the reaction of atomic nitrogen (2D) with benzene. Thereafter, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of reaction rate constants and product branching ratios were performed under single-collision conditions. The results revealed that the N(2D) + C6H6 reaction in the case of statistical behavior is expected to produce hydrogen cyanide plus a cyclopentadienyl radical (91.5-88.9%), acetylene plus a pyrrole radical (5.8-7.5%), 1-cyano-2,4-cyclopentadiene + H (2.3-3.0%) and 1-ethynyl-pyrrole + H (0.4-0.6%), with the most favorable pathways being the initial adduct i1 leading to the formation of a seven-membered cyclic intermediate i12 through an exothermic ring expansion process and a multistep route i12 → i15 → i16 → C5H5 + HCN featuring an intramolecular ring-shrinking process involving a C-C bond fusion elimination channel to yield the bicyclic intermediate i15, followed by hydrogen cyanide elimination, thus forming a cyclopentadienyl radical. The calculated product branching ratios were consistent with the available experimental data; however, some quantitative deviations from the experimental results and the possible reasons are also discussed. The possible effects of the title reaction on the upper atmosphere of Titan, with critical implications for the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons, were compared with the mass growth processes of their polycyclic aromatic hydrocarbon counterparts produced through ring expansion.

An Aromatic Universe-A Physical Chemistry Perspective

This Perspective presents recent advances in our knowledge of the fundamental elementary mechanisms involved in the low- and high-temperature molecular mass growth processes to polycyclic aromatic hydrocarbons in combustion systems and in extraterrestrial environments (hydrocarbon-rich atmospheres of planets and their moons, cold molecular clouds, circumstellar envelopes). Molecular beam studies combined with electronic structure calculations extracted five key elementary mechanisms: Hydrogen Abstraction-Acetylene Addition, Hydrogen Abstraction-Vinylacetylene Addition, Phenyl Addition-DehydroCyclization, Radical-Radical Reactions, and Methylidyne Addition-Cyclization-Aromatization. These studies, summarized here, provide compelling evidence that key classes of aromatic molecules can be synthesized in extreme environments covering low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35-150 K) to high-temperature environments like circumstellar envelopes of carbon-rich Asymptotic Giant Branch Stars stars and combustion systems at temperatures above 1400 K thus shedding light on the aromatic universe we live in.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

Nitrogen-containing ions and molecules in the gas phase have been detected in non-Earth environments such as dark molecular clouds and more recently in the atmosphere of Saturn's moon Titan. These molecules may serve as precursors to larger heterocyclic structures that provide the foundation of complex biological molecules. On Titan, molecules of m/z 66 have been detected by the Cassini mission, and species of the empirical formula C₄H₄N may contribute to this signature. We have characterized seven isomers of C₄H₄N in anionic, neutral radical, and cationic states using density functional theory. Structures were optimized using the range-separated hybrid ωB97X-V with the cc-pVTZ and aug-cc-pVTZ basis sets. Anionic and radical C₄H₄N favor cyclic structures with aromatic and quasi-aromatic electron arrangements, respectively. Interestingly, ionization from the radical surface to the cation induces significant changes in structural stability, and the global minimum for positively charged isomers is CH₂CCHCNH⁺, a pseudo-linear species reminiscent of cyanoallene. Select formation pathways to these structures from Titan's existing or postulated gas-phase species, reactions that are also relevant for other astrophysical environments, are discussed. By characterizing C₄H₄N isomers, we have identified energetically stable anionic, radical, and cationic structures that may be present in Titan's atmosphere and dark molecular clouds.

Barrierless Reactions of Three Benzonitrile Radical Cations with Ethylene

Reactions of three protonated benzonitrile radical cations with ethylene are investigated. Product branching ratios and reaction kinetics, measured using ion-trap mass spectrometry, are reported and mechanisms are developed with support from quantum chemical calculations. Reactions proceed via pre-reactive van der Waals complexes with no energy barrier (above the reactant energy) and form radical addition and addition–elimination product ions. Rate coefficients are 4-dehydrobenzonitrilium: 1.72±0.01×10−11 cm3 molecule−1 s−1, 3-dehydrobenzonitrilium: 1.85±0.01×10−11 cm3 molecule−1 s−1, and 2-dehydrobenzonitrilium: 5.96±0.06×10−11 cm3 molecule−1 s−1 (with±50% absolute uncertainty). A ring-closure mechanism involving the protonated nitrile substituent is proposed for the 2-dehydrobenzonitrilium case and suggests favourable formation of the protonated indenimine cation.

Decomposition of Picolyl Radicals at High Temperature: A Mass Selective Threshold Photoelectron Spectroscopy Study

The reaction products of the picolyl radicals at high temperature were characterized by mass-selective threshold photoelectron spectroscopy in the gas phase. Aminomethylpyridines were pyrolyzed to initially produce picolyl radicals (m/z=92). At higher temperatures further thermal reaction products are generated in the pyrolysis reactor. All compounds were identified by mass-selected threshold photoelectron spectroscopy and several hitherto unexplored reactive molecules were characterized. The mechanism for several dissociation pathways was outlined in computations. The spectrum of m/z=91, resulting from hydrogen loss of picolyl, shows four isomers, two ethynyl pyrroles with adiabatic ionization energies (IEad ) of 7.99 eV (2-ethynyl-1H-pyrrole) and 8.12 eV (3-ethynyl-1H-pyrrole), and two cyclopentadiene carbonitriles with IE's of 9.14 eV (cyclopenta-1,3-diene-1-carbonitrile) and 9.25 eV (cyclopenta-1,4-diene-1-carbonitrile). A second consecutive hydrogen loss forms the cyanocyclopentadienyl radical with IE's of 9.07 eV (T0 ) and 9.21 eV (S1 ). This compound dissociates further to acetylene and the cyanopropynyl radical (IE=9.35 eV). Furthermore, the cyclopentadienyl radical, penta-1,3-diyne, cyclopentadiene and propargyl were identified in the spectra. Computations indicate that dissociation of picolyl proceeds initially via a resonance-stabilized seven-membered ring.

Synthesis of functionalized nitrogen-containing polycyclic aromatic hydrocarbons and other prebiotic compounds in impacting glycine solutions

Proteinogenic amino acids can be produced on or delivered to a planet via impacting abiotic sources and consequently were likely present before the emergence of life on Earth. However, the role that these materials played in prebiotic scenarios remains an open question, in part because little is known about the survivability and reactivity of astrophysical organic compounds upon impact with a planetary surface. To this end, we use a force-matched semi-empirical quantum simulation method to study impacts of aqueous proteinogenic amino acids at conditions reaching 48 GPa and 3000 K. Here, we probe a relatively unstudied mechanism for prebiotic synthesis where sudden heating and pressurization causes condensation of complex carbon-rich structures from mixtures of glycine, the simplest protein-forming amino acid. These carbon-containing clusters are stable on short timescales and undergo a fundamental structural transition upon expansion and cooling from predominantly sp3-bonded tetrahedral-like moieties to those that are more sp2-bonded and planar. The recovered sp2-bonded structures include large nitrogen containing polycyclic aromatic hydrocarbons (NPAHs) with a number of different functional groups and embedded bonded regions akin to oligo-peptides. A number of small organic molecules with prebiotic relevance are also predicted to form. This work presents an alternate route to gas-phase synthesis for the formation of NPAHs of high complexity and highlights the significance of both the thermodynamic path and local chemical self-assembly in forming prebiotic species during shock synthesis. Our results help determine the role of comets and other celestial bodies in both the delivery and synthesis of potentially significant life building compounds on early Earth.

What Will Photo-Processing of Large, Ionized Amino-Substituted Polycyclic Aromatic Hydrocarbons Produce in the Interstellar Medium?

Collision-energy resolved tandem mass spectrometry was used to probe the trends in unimolecular fragmentation in a series of ionized amino-substituted polycyclic aromatic hydrocarbons ranging from naphthalene to pyrene. As the ring system expands, the dominant dissociation process changes from HNC loss (aniline) to H loss for 1-aminopyrene. Imaging photoelectron photoion coincidence spectroscopy of 1-aminopyrene yielded threshold photon-energy resolved breakdown curves, the Rice-Ramsperger-Kassel-Marcus modeling of which gave a 0 K activation energy, E₀, for H loss of 3.8 ± 0.4 eV. Calculations at the CCSD/6-31G(d)//B3LYP/6-31G(d) level of theory were used to explore the possible reaction mechanisms for H, HNC, and C,N,H₂ losses, and details of the reaction pathways are presented. The H atom loss was found to be due both to direct N-H bond cleavage and isomerization to form an azepine derivative.

On the formation of phosphorous polycyclic aromatics hydrocarbons (PAPHs) in astrophysical environments

The formation of phosphorous-containing polycyclic aromatic hydrocarbons (PAPHs) in astrophysical contexts is proposed and analyzed by means of computational methods [B3LYP-D3BJ/ma-def2-TZVPP, MP2-F12, CCSD-F12b and CCSD(T)-F12b levels of theory]. A "bottom-up" approach based on a radical-neutral reaction scheme between acetylene (C₂H₂) and the CP radical was used investigating: (a) the synthesis of the first PAPH (C₅H₅P) "phosphinine"; (b) PAPH growth by addition of C₂H₂ to the C₅H₄P radical; (c) PAPH synthesis by addition reactions of one CP radical and nC₂H₂ to a neutral PAH. Results show: (I) the formation of the phosphinine radical has a strong thermodynamic tendency (-133.3 kcal mol^-1) and kinetic barriers ≤5.4 kcal mol^-1; (II) PAPH growth by nC₂H₂ addition on the radical phosphinine easily and exothermically produces radicals (1a- or 1-phospha-naphtalenes with kinetic barriers ≤7.1 kcal mol^-1 and reaction free energies ≤-102.5 kcal mol^-1); (III) the addition of a single CP + nC₂H₂ to a neutral benzene generates a complex chemistry where the main product is 2-phospha-naphtalene; (IV) because of the CP radical character, its barrierless addition to a PAH produces a resonant stabilized PAPH, becoming excellent candidates for addition reactions with neutral or radical hydrocarbons and PAHs; (V) the same energy trend between all four levels of theory continues a well-calibrated computational protocol to analyze complex organic reactions with astrochemical interest using electronic structure theory.

On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

Pure methane (CH4) ices processed by energetic electrons under ultra-high vacuum conditions to simulate secondary electrons formed via galactic cosmic rays (GCRs) penetrating interstellar ice mantles have been shown to produce an array of complex hydrocarbons with the general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7). By monitoring the in situ chemical evolution of the ice combined with temperature programmed desorption (TPD) studies and tunable single photon ionization coupled to a reflectron time-of-flight mass spectrometer, specific isomers of C3H4, C3H6, C4H4, and C4H6 were probed. These experiments confirmed the synthesis of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), 1-butyne (HCCC2H5), 2-butyne (CH3CCCH3), 1,2-butadiene (H2CCCH(CH3)), and 1,3-butadiene (CH2CHCHCH2) with yields of 2.17 ± 0.95 × 10-4, 3.7 ± 1.5 × 10-3, 1.23 ± 0.77 × 10-4, 1.28 ± 0.65 × 10-4, 4.01 ± 1.98 × 10-5, 1.97 ± 0.98 × 10-4, 1.90 ± 0.84 × 10-5, and 1.41 ± 0.72 × 10-4 molecules eV-1, respectively. Mechanistic studies exploring the formation routes of methylacetylene, propene, and vinylacetylene were also conducted, and revealed the additional formation of the 1,2,3-butatriene isomer. Several of the above isomers, methylacetylene, propene, vinylacetylene, and 1,3-butadiene, have repeatedly been shown to be important precursors in the formation of polycyclic aromatic hydrocarbons (PAHs), but until now their interstellar synthesis has remained elusive.

Isoquinoline gas-phase absorption spectrum in the vacuum ultraviolet between 3.7 and 10.7 eV. New valence and Rydberg electronic states

VUV photons from a synchrotron source were used to record the gas-phase absorption spectrum of isoquinoline over the range 3.5 to 10.7 eV. The rich spectrum exhibits both broad and sharp features, of varying intensities, that are analyzed into eight valence and eight Rydberg transitions. Previous data on the valence transitions of isoquinoline were essentially limited to solution spectra up to 5.4 eV. Our study increases their number considerably. The features in the 3.96 eV region are discussed in terms of vibronic coupling between the nπ* 1¹A′′ and ππ* 2¹A′ valence electronic states. The intensities of some spectral features are augmented by collective π-electron modes considered to be of plasmon-type. Assignments of the valence transitions were facilitated by our DFT calculations and by earlier Pariser–Parr–Pople MO calculations. The calculation results are compared and their relative value is discussed. The DFT calculations reproduce very well a number of experimentally determined properties of the ground state of isoquinoline, in particular its bond distances and angles, rotational constants, vibrational frequencies and dipole moment. No Rydberg series of isoquinoline have previously been observed. Three of the newly observed Rydberg series converge to the D₀ electronic ground state of the ion, while two converge to the D₁ and three to the D₃ excited electronic states of the cation. Astrophysical applications of the VUV absorption spectrum of isoquinoline, in particular the measured absorption cross-sections, are briefly discussed. A comparison between the absorption spectra of isoquinoline and quinoline highlights their similarities and differences, related to their respective molecular orbitals.

VUV photons from a synchrotron source were used to record the gas-phase absorption spectrum of isoquinoline over the range 3.5 to 10.7 eV.

A review on the production of nitrogen-containing compounds from microalgal biomass via pyrolysis

Nitrogen-containing compounds (NCCs) which may be produced from nitrogen-rich biomass such as microalgae, may find important biochemical and biomedical applications. This review summarizes the recent knowledge about the formation mechanism of NCCs during pyrolysis of microalgae. The key technical and biological aspects of microalgae and pyrolysis process parameters, which influence the formation of NCCs, have been analyzed. The mechanism of formation of NCCs such as indole, pyridine, amides, and nitriles during primary and secondary pyrolysis reactions are elaborated. It has been emphasized that the pyrolysis conditions and the use of catalysts had significant impacts on the yields and compositions of NCCs. The available information shows that the transformation of nitrogen and nitrogen functionalities during pyrolysis are strongly associated with the formation process of NCCs. The challenges in the development of pyrolysis technologies for the production of NCCs from microalgae are identified with future research needs identified.

Pressurized entrained-flow pyrolysis of microalgae: Enhanced production of hydrogen and nitrogen-containing compounds

Pressurized entrained-flow pyrolysis of Chlorella vulgaris microalgae was investigated. The impact of pressure on the yield and composition of pyrolysis products were studied. The results showed that the concentration of H₂ in bio-gas increased sharply with increasing pyrolysis pressure, while those of CO, CO₂, CH₄, and C₂H₆ were dramatically decreased. The concentration of H₂ reached 88.01 vol% in bio-gas at 900 °C and 4 MPa. Higher pressures promoted the hydrogen transfer to bio-gas. The bio-oils derived from pressurized pyrolysis were rich in nitrogen-containing compounds and PAHs. The highest concentration of nitrogen-containing compounds in bio-oil was achieved at 800 °C and 1 MPa. Increasing pyrolysis pressure promoted the formation of nitrogen-containing compounds such as indole, quinoline, isoquinoline and phenanthridine. Higher pyrolysis pressures led to increased sphericity, enhanced swelling, and higher carbon order of bio-chars. Pressurized pyrolysis of biomass has a great potential for poly-generation of H₂, nitrogen containing compounds and bio-char.

Highly efficient gas-phase reactivity of protonated pyridine radicals with propene

Small nitrogen containing heteroaromatics are fundamental building blocks for many biological molecules, including the DNA nucleotides. Pyridine, as a prototypical N-heteroaromatic, has been implicated in the chemical evolution of many extraterrestrial environments, including the atmosphere of Titan. This paper reports on the gas-phase ion-molecule reactions of the three dehydro-N-pyridinium radical cation isomers with propene. Photodissociation ion-trap mass spectrometry experiments are used to measure product branching ratios and reaction kinetics. Reaction efficiencies for 2-dehydro-N-pyridinium, 3-dehydro-N-pyridinium and 4-dehydro-N-pyridinium with propene are 70%, 47% and 41%, respectively. The m/z 106 channel is the major product channel across all cases and assigned 2-, 3-, and 4-vinylpyridinium for each reaction. The m/z 93 channel is also significant and assigned the 2-, 3-, and 4-N-protonated-picolyl radical cation for each case. H-Abstraction from propene is not competitive under experimental conditions. Potential energy schemes, at the M06-2X/6-31(2df,p) level of theory and basis set, are described to assist in rationalising observed product branching ratios and elucidating possible reaction mechanisms. Reaction barriers to the production of vinylpyridinium (m/z 106) + CH₃ are the lowest identified for the 3- and 4-dehydro-N-pyridinium reactions, in support of the observed dominance of the m/z 106 ion signal. Ethylene loss via ring-mediated H-transfer along the propyl group is found to be the lowest energy pathway for the 2-dehydro-N-pyridinium reaction, suggesting a preference toward m/z 93 (N-protonated-picolyl radical cation) over the experimentally observed products. Entropic bottle-necks along the m/z 93 pathway however, associated with ring-mediated H-atom transfer, are responsible for the dominance of m/z 106 in the 2-dehydro-N-pyridinium + propene reaction. For all three isomers, computed barriers for all observed reaction channels were below the entrance channel, suggesting these reactions can contribute to molecular weight growth in extraterrestrial environments with accelerated reaction rates in low temperature regions of space.

Nitrogen matters: the difference between PANH and PAH formation

Because of the large stability of the nitrile group, the N-substituted aromatic molecule quinoline does not form in the phenyl + acrylonitrile reaction, in contrast to naphthalene formation in the isoelectronic phenyl + vinylacetylene reaction.