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We are now on a clear trajectory for improvements in exoplanet observationsthat will revolutionize our ability to characterize their atmosphericstructure, composition, and circulation, from gas giants to rocky planets.However, exoplanet atmospheric models capable of interpreting the upcomingobservations are often limited by insufficiencies in the laboratory andtheoretical data that serve as critical inputs to atmospheric physical andchemical tools. Here we provide an up-to-date and condensed description ofareas where laboratory and/or ab initio investigations could fill critical gapsin our ability to model exoplanet atmospheric opacities, clouds, and chemistry,building off a larger 2016 white paper, and endorsed by the NAS ExoplanetScience Strategy report. Now is the ideal time for progress in these areas, butthis progress requires better access to, understanding of, and training in theproduction of spectroscopic data as well as a better insight into chemicalreaction kinetics both thermal and radiation-induced at a broad range oftemperatures. Given that most published efforts have emphasized relativelyEarth-like conditions, we can expect significant and enlightening discoveriesas emphasis moves to the exotic atmospheres of exoplanets.

Context. CH3CN (methyl cyanide) is one of the simplest and most abundant interstellar complex organic molecules (iCOMs), and has been detected in young solar analogues, shocked regions, protoplanetary discs, and comets. CH3CN can therefore be considered a key species to explore the chemical connections between the planet-forming disk phase and comets. However, for such comparison to be meaningful, kinetics data for the reactions leading to CH3CN formation and destruction must be updated. Aims. Here we focus on the destruction of methyl cyanide through collisions with He+.. We employed a combined experimental and theoretical methodology to obtain cross sections (CSs) and branching ratios (BRs) as a function of collision energy, from which we calculated reaction rate coefficients k(T) in the temperature range from 10 to 300 K. Methods. We measured CSs and BRs using a guided ion beam setup, and developed a theoretical treatment based on an analytical formulation of the potential energy surfaces (PESs) for the charge exchange process. The method employs a Landau Zener model to obtain reaction probabilities at crossings between the entrance and exit PESs, and an adiabatic centrifugal sudden approximation to calculate CSs and k(T), from subthermal to hyper-thermal regimes. Results. k(T) and experimental BRs differ from those predicted from widely used capture models. In particular, the rate coefficient at 10 K is estimated to be almost one order of magnitude smaller than what is reported in the KIDA database. In addition, the charge exchange is completely dissociative and the most abundant fragments are HCCN+/CCNH+ , HCNH+ and CH2+. Conclusions. Our results, combined with a revised chemical network for the formation of CH3CN, support the hypothesis that methyl cyanide in protoplanetary discs could be mostly the product of gas-phase processes rather than grain chemistry, as currently proposed. These findings are expected to have implications in the comparison of the abundance ratios of N-bearing molecules observed in discs with cometary abundance ratios. [ABSTRACT FROM AUTHOR]

The gas phase reactivity of the radical cation isomers H2CNH˙+ (methanimine) and HCNH2˙+ (aminomethylene) with propene (CH3CHCH2) has been investigated by measuring absolute reactive cross sections and product branching ratios, under single collision conditions, as a function of collision energy (in the range ∼0.07–11.80 eV) using guided ion beam mass spectrometry coupled with VUV photoionization for selective isomer generation. Experimental results have been merged with theoretical calculations to elucidate reaction pathways and structures of products. The H2CNH˙+ isomer is over a factor two more reactive than HCNH2˙+. A major channel from both isomers is production of protonated methanimine CH2NH2+via hydrogen-atom transfer reaction but, while H2CNH˙+ additionally gives charge and proton transfer products, the HCNH2˙+ isomer leads instead to protonated vinylimine CH2CHCHNH2+, produced alongside CH3˙ radicals. The reactions have astrochemical implications in the build up of chemical complexity in both the interstellar medium and the hydrocarbon-rich atmospheres of planets and satellites. [ABSTRACT FROM AUTHOR]

The gas-phase reaction between the ethyl cation (C2H5+) and ethyne (C2H2) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C2H5Br using tuneable VUV radiation in the range 10.5–14.0 eV is employed to generate C2H5+, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C4H7]+ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼ 0.1 eV: (a) C3H3++CH4, with BR = 0.76 ± 0.05 and (b) C4H5++H2, with BR = 0.22 ± 0.02. A third channel giving C2H3+ in association with C2H4 is shown to emerge at both high internal excitation of C2H5+ and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3 × 10 − 11 − 5.2 × 10 − 10   cm 3 · molecule − 1 · s − 1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH3CH2CHCH+, from which barrierless and exothermic pathways exist for the production of both cyclic c−C3H3+ and linear H2CCCH+ isomers of the main product channel. For the minor C4H5+ product, two isomers are energetically accessible: the three-member cyclic isomer c−C3H2(CH3)+ and the higher energy linear structure CH2CHCCH2+, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films). [ABSTRACT FROM AUTHOR]

Radiotherapy is one of the most widespread and efficient strategies to fight malignant tumors. Despite its broad application, the mechanisms of radiation-DNA interaction are still under investigation. Theoretical models to predict the effects of a particular delivered dose are still in their infancy due to the difficulty of simulating a real cell environment, as well as the inclusion of a large variety of secondary processes. This work reports the first experimental study of the ion–molecule reactions of the H2O˙+ and OH+ ions, produced by photoionization with synchrotron radiation, with a furan (c-C4H4O) molecule, a template for deoxyribose sugar in DNA. The present experiments, performed as a function of the collision energy of the ions and the tunable photoionization energy, provide key parameters for the theoretical modelling of the effect of radiation dose, like the absolute cross sections for producing protonated furan (furanH+) and a radical cation (furan˙+), the most abundant products, which can amount up to 200 Å2 at very low collision energies (<1.0 eV). The experimental results show that furanH+ is more fragile, indicating how the protonation of the sugar component of the DNA may favor its dissociation with possible major radiosensitizing effects. Moreover, the ring opening of furanH+ isomers and the potential energy surface of the most important fragmentation channels have been explored by molecular dynamics simulations and quantum chemistry calculations. The results show that, in the most stable isomer of furanH+, the ring opening occurs via a low energy pathway with carbon–oxygen bond cleavage, followed by the loss of neutral carbon monoxide and the formation of the allyl cation CH2CHCH2+, which instead is not observed in the fragmentation of furan˙+. At higher energies the ring opening through the carbon–carbon bond is accompanied by the loss of formaldehyde, producing HCCCH2+, the most intense fragment ion detected in the experiments. This work highlights the importance of the secondary processes, like the ion–molecule reactions at low energies in the radiation damage due to their very large cross sections, and it aims to provide benchmark data for the development of suitable models to approach this low collision energy range. [ABSTRACT FROM AUTHOR]

Methanol is a key species in astrochemistry as its presence and reactivity provides a primary route to the synthesis of more complex interstellar organic molecules (iCOMs) that may eventually be incorporated in newly formed planetary systems. In the interstellar medium, methanol is formed by hydrogenation of CO ices on grains, and its fate upon collisions with interstellar ions should be accounted for to correctly model iCOM abundances in objects at various stages of stellar evolution. The absolute cross sections (CSs) and branching ratios (BRs) for the collisions of He˙+ ions with CH3OH are measured, as a function of the collision energy, using a Guided Ion Beam Mass Spectrometer (GIB-MS). Insights into the dissociative electron (charge) exchange mechanism have been obtained by computing the entrance and exit multidimensional Potential Energy Surfaces (PESs) and by modelling the non-adiabatic transitions using an improved Landau–Zener–Stückelberg approach. Notably, the dynamical treatment reproducing the experimental findings includes a strong orientation effect of the system formed by the small He˙+ ion and the highly polar CH3OH molecule, in the electric field gradient associated to the strongly anisotropic intermolecular interaction. This is a stereodynamical effect that plays a fundamental role in collision events occurring under a variety of conditions, with kinetic energy confined within intervals ranging from the sub-thermal to the hyper-thermal regime. [ABSTRACT FROM AUTHOR]

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 reactionCH3+ +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 [ABSTRACT FROM AUTHOR]

Supersonic expansion studies of oxygen reveal that the extent of oxygen molecule alignment depends on the velocity of the molecules during the expansion. The slower molecules exhibit a lesser degree of alignment than the fast moving molecules. The variation in the gas mixture also effects the alignment. This alignment dependency enables the production of rotationally aligned molecules.

The comments of C.D. Pibel and J.B. Halpern on a previous study of molecular oxygen in supersonic seeded beams are flawed. One of their main findings is that further constraints arise when the beam velocity direction is considered as a good quantization axis for the total angular momentum. However, they failed to realize that these constraints are too severe to account for the paramagnetism data measured under extreme velocity and pressure conditions.

The reactivity of biphenylium cations C12H9+ with benzene C6H6 is investigated in a joint experimental and theoretical approach. Experiments are performed by using a triple quadruple mass spectrometer equipped with an atmospheric pressure chemical ion source to generate C12H9+ via dissociative ionization of various isomers of the neutral precursor hydroxybiphenyl (C12H10O). C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C18H15+, C18H13+, C17H12+, and C8H7+. The dependence of product ion yields on the kinetic energy of reagent ions, as well as further experiments performed using partial isotopic labelling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated terphenyl C18H15+. Its formation is found to be exothermic and barrierless and, therefore, might occur under the low pressure and temperature conditions typical of planetary atmospheres and the interstellar medium. Theoretical calculations have focussed on the channel leading to C8H7+ plus C10H8, identifying, as the most probable fragments, the phenylethen-1-ylium cation and naphthalene, thus suggesting that the pathway leading to them might be of particular interest for the synthesis of polycyclic aromatic hydrocarbons. Both experiments and theory agree in finding this channel exoergic but hampered by small barriers of 2.7 and 3.7 kcal mol-1 on the singlet potential energy surface. [ABSTRACT FROM AUTHOR]

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C10H7+ with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C16H13+, C16Hn+ (with n=10-12), and C15H10+. From pressure-dependent measurements, absolute cross sections of 1.0±0.3 and 2±0.6 Å2 (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C16H12+ and C15H10+ ions, respectively. From cross section values a phenomenological total rate constant k=(5.8±1.9)×10-11 cm3 s-1 at an average collision energy of about 0.27 eV can be estimated for the process C10H7++C6H6→all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H2, CH3). A major signal relevant to the fragmentation of the initial adduct C16H13+ belongs to C15H10+. Since it is not obvious how CH3 loss from C16H13+ can take place to form the C15H10+ radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the π system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol-1. From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol-1 with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the 'naphthyl' part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases. [ABSTRACT FROM AUTHOR]

The reaction of N2O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C6H5+, and its perdeuterated derivative C6D5+ are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C5(H,D)5+, C6(H,D)5O+, and C3(H,D)3+ are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C6H5·N2O)+, which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C6H5ON2+ (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring π system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N2 quite easily, thus affording the phenoxyl cation 3. This is only the first of several C6H5O+ isomers (4–6 and 8–12), which can stem from 3 upon different cleavages and formations of C–C bond and/or H shifts. As regards the formation of C5H5+, among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C3H3+ is generally hindered and its detection depends again on the availability of some extra initial energy. [ABSTRACT FROM AUTHOR]