The reaction of ions is thought to play a crucial role in the synthesis of complex organic species in both the interstellar medium (ISM) and the atmospheres of planets and their satellites Larsson et al. (2012). As increasingly complex species are identified in such environments, the potential impact of isomers increases as well. The existence of isomeric ions in the ISM has been known for a long time and in some cases, such as HCO⁺ and HOC⁺, a determination has been made of their relative abundances Woods et al. (1983). As isomers have different spectroscopic and chemical properties, and, in many cases, isomerization barriers are too high to be overcome in interstellar conditions, they must be treated separately.

Notably, the relative abundance of isomers in a given environment cannot usually be determined by their thermodynamic properties, with the feasibility of formation and destruction mechanisms often more important than relative energies. Furthermore, while isomers often have quite different dipole moments and other spectroscopic properties and therefore have distinct radioastronomic profiles Woods et al. (1983), in situ measurements of ion abundances in planetary and satellite atmospheres often employ mass spectrometers which bring extreme sensitivity and time/localisation specific information, but cannot distinguish between isomers. The novel Atacama Large Interferometer Array (ALMA) has enabled data collection on the density as a function of altitude for HCN and HNC Cordiner et al. (2019) and the unprecedented resolution of this device should also allow for the retrieval of data about the abundance and distribution of isomeric ions in different astronomic objects. However, at present, the relative abundance of isomers in such environments has proven difficult to assess. So, in order to develop accurate models to predict these abundances, it is important to have reliable data on the reactivity of ionic isomers with common interstellar and atmospheric molecules.

In recent years, efforts have been made to selectively generate relevant isomers Fathi et al. (2016), with VUV photoionization in particular proving an effective tool Polášek et al. (2016) while noble gas tagging has been successful in characterizing the presence and the relative abundance of ions produced by electron impact ionization Brünken et al. (2019).

Like Earth, Titan has a dense nitrogen-dominated atmosphere Hörst (2017), which is one of the most chemically complex atmospheres in the solar system, as demonstrated by the data from the ion and neutral mass-spectrometer (INMS) of the Cassini mission. Titan’s atmospheric chemistry is driven by the ionization and/or dissociation of its two primary components, N₂ and CH₄, through a combination of extreme ultraviolet (EUV) radiation Imanaka and Smith (2007) and magnetospheric electrons. The resultant ions and radicals can then react to form more complex species Vigren et al. (2012), Westlake et al. (2014), including large hydrocarbons and nitrogen-bearing compounds Vinatier et al. (2007), Nixon et al. (2018), with cations of masses up to m/z 99 having been identified at altitudes of 950 km above the surface while cations of masses up to m/z 350 were detected by the Cassini plasma spectrometer ion beam sensor (CAPS-IBS) Vuitton et al. (2007).

Reactions of nitrogen-containing ions with hydrocarbons, especially unsaturated hydrocarbons such as C₂H₂ and C₂H₄, have been suggested as the basis for the formation of some of the complex ions that have been detected in Titan’s atmosphere Westlake et al. (2014), Vuitton et al. (2019). However, protonated nitriles, a major part of the nitrogen-containing ions in Titan’s atmosphere, are less reactive with their major reaction pathway in Titan’s atmosphere being dissociative recombination Vuitton et al. (2007). However, this is not the case for the methylenimine cation H₂CNH^+• and its isomer the aminomethylene ion HCNH₂ ^+•, which are both reactive radical cations and could form the basis of chain elongation reactions with unsaturated and saturated hydrocarbons that produce larger ions, from which neutral species can then be generated by dissociative recombination. Though the m/z 29 signal recorded by INMS is regarded as mostly arising from the presence of C₂H₅ ⁺, models predict a density of HCNH₂ ^+• and its isomers amounting to 1.1 × 10⁻² cm⁻³ in Titan’s ionosphere Vuitton et al. (2007).

Although the H₂CNH^+• and HCNH₂ ^+• ions have not yet been identified in the ISM, neutral methanimine (i.e. methylene imine, H₂CNH) is ubiquitous, having been detected in several objects including giant molecular clouds as well as both high mass and solar-type protostellar systems Dickens et al. (1997), Suzuki et al. (2016), Widicus Weaver et al. (2017), Ligterink et al. (2018), Bogelund et al. (2019). Methanimine, which could form either on the surface of dust grains Bernstein et al. (1995), Theule et al. (2011) or via gas phase reactions Suzuki et al. (2016), could serve as the basis for synthetic routes to form more complex nitrogen-containing molecules, including biomolecule precursors such as glycine and its corresponding α-aminonitrile Basiuk and Bogillo (2002), Aponte et al. (2017).

Due to its low ionization energy of 9.99 eV Holzmeier et al. (2013), methanimine can be ionized by VUV photons in the ISM. A possible pathway for the synthesis of larger nitrogen-containing species could therefore be the ionization of H₂CNH followed by the reaction of the resulting radical cation with an unsaturated hydrocarbon molecule, such as ethene or ethyne. The resultant ion could then either undergo dissociative recombination to form a complex neutral molecule through elimination of a hydrogen atom, or react further with other interstellar compounds to generate an even larger ionic species. Since methanimine has been detected also in ion-rich regions [e.g., at the “radical-ion peak” in the Orion molecular cloud Dickens et al. (1997)], such a course of reaction does not seem unlikely.

This paper presents a reactivity study of both H₂CNH^+• and HCNH₂ ^+• with ethyne using synchrotron radiation and apt neutral precursors to selectively generate the charged species. It aims to establish the occurrence of product channels leading to the formation of new C-C or C-N bonds through chain elongation reactions with ethyne, thereby leading to an increase in chemical complexity.

In Section 2 and Section 3 we present the experimental and theoretical methodologies. In Section 4 we provide a detailed description of the experimental results while in Section 5 the computational results are presented alongside proposed reaction mechanisms. Experimental results are discussed and interpreted in light of calculations of the most probable reaction mechanisms in Section 6. Finally, the conclusions are summarized in Section 7, where implications of present results on the formation of complex N-containing ions in Titan’s atmosphere and in the ISM are briefly addressed.

Experiments were performed using the CERISES apparatus, an associated experiment of the DESIRS beamline Nahon et al. (2012) of the SOLEIL synchrotron radiation facility. Since the experimental set-up has been described in detail previously Alcaraz et al. (2004), Cunha de Miranda et al. (2015), only the most relevant details will be given here. CERISES is a guided ion beam tandem mass spectrometer composed of two octopoles located between two quadrupole mass filters in a Q1-O1-O2-Q2 configuration that permits investigation of bimolecular reactions of mass-selected ions occurring in a 4 cm long reaction cell located after the first octopole. Absolute reaction cross sections (CSs) and branching ratios (BRs) as a function of both photon (E _phot ) and collision (E _CM ) energies are derived by measuring the yields of parent- and product-ions.

The HCNH₂ ^+• and H₂CNH^+• isomers were produced by dissociative photoionization of the gaseous precursors cyclopropylamine (c-C₃H₅NH₂) and azetidine (c-CH₂CH₂CH₂NH) respectively, according to the following reactions: c − C 3 H 5 NH 2 + h ν → HCNH 2 + • + C 2 H 4 (1) c − CH 2 CH 2 CH 2 NH + h ν → H 2 CNH + • + C 2 H 4 (2)

These commercially available gaseous precursors were introduced into the ion source at roughly 10⁻⁶ mbar [see Richardson et al. (2021), Sundelin et al. (2021) for a complete description of the generation of the ions]. In addition, photoionization of methanimine (H₂CNH) was used to selectively generate the H₂CNH^+• isomer, with the details of the ion generation processes presented in Section 4.

The vacuum ultraviolet (VUV) radiation for photoionization is provided by the undulator-based DESIRS beamline Nahon et al. (2012), operating between 5 and 40 eV. Photons at the desired energies are selected and scanned simultaneously with the undulator peak energy by a normal incidence monochromator equipped with a low dispersion uncoated SiC grating (200 grooves/mm) optimized to provide photon flux up to 10¹³ photons/s. In the present experiments, the photon energies required to produce the [CH₃N]^+• isomer ions are in the region 9.5–14.0 eV. The monochromator slits were set in the range 300–600 μm, corresponding to a resolution of 20–40 meV, in order to balance the maximization of signal with avoiding detector saturation. The monochromator was operated at its first order of diffraction. Higher orders were removed from the incident beam using a gas filter Mercier et al. (2000) installed on the beam line and filled with Argon (at a pressure of 0.2 mbar) to efficiently remove any photons with E _phot larger than 15.7 eV. The absolute scale of the photon energy was checked using sharp absorption lines of atomic Argon around 11.828 and 14.304 eV Minnhagen (1973), Kramida et al. (2020) that were observed with systematic shifts of about 10–20 meV above their tabulated values.

The resolution of the mass filters Q1 and Q2 were adjusted in order to provide a suitable compromise between signal and separation of adjacent masses. We estimate that in this way we are able to remove 99.99% of neighboring ions for Q1 and 99.90% of neighboring ions for Q2 at the lowest collision energy, with a loss of performance reaching 98.80% of neighboring ions at the highest acceleration potentials of 20 V.

The collision energy in the lab is dependent on the reagent ion charge (+1 in our case) and on the difference between the ion source and the reaction cell potentials. The retarding potential method was applied to the reagent ion beam Teloy and Gerlich (1974) to determine the maximum of the first derivative of the reagent ion yield, which defines the zero of the kinetic energy. In this way we have estimated an average reagent ion beam FWHM of ∼0.4 eV, equivalent to ∼0.19 eV in the center-of-mass frame. By changing the potentials of the reaction cell and all subsequent elements, we were able to scan a collision energy range from ∼0.06 to ∼10 eV in the center of mass frame (E _CM ).

C₂H₂ was stored in a dilute mixture with acetone, which was removed using a coil cooled with dry ice before insertion into the instrument. C₂H₂ was introduced in the reaction cell at a dynamic pressure of up to 2.0 × 10⁻⁷ bar, which guarantees operation close to a single collision regime, keeps secondary reactions at a reduced level and limits the attenuation of the reagent ion to less than 10%, while still providing an accurate pressure measurement and a dynamic range on the absolute cross sections determination in the range 0.01–100 Å². The absolute pressure in the reaction cell was measured using an MKS 398H differential manometer instrument. All data were collected with either reaction gas in the cell or in the surrounding chamber, therefore allowing the removal of any contribution resulting from collisions occurring outside the reaction cell, corresponding to no more than 5% of product ion counts. The absolute cross section acquisitions were performed in the so-called multi-scan mode, which involves recording signals for all ionic species at a single point before moving to the next point. This method has the benefit of drastically reducing the effects of potential drifts of source pressure, reaction cell pressure and photon flux.

The mechanisms for the reactions of H₂CNH^+• and HCNH₂ ^+• were studied using GAUSSIAN 09, Revision D.01 Frish et al. (2013). Geometries for intermediate structure, i.e. minima and transition states (TSs), were calculated at the MP2/6-31G(d) and MP2/6-311++G(d,p) levels. The identity of TS and minima were checked by frequency calculations and zero-point energy corrections were applied to the obtained energies. IRC calculations were performed at the MP2/6-31G(d) level to ensure that the TS connect the correct minima. Single point energy calculations were carried out for all stationary points at the CCSD(T)/6-311++G(d,p) level with zero-point energy corrections taken from the MP2/6-311++G(d,p) level. Reactants and product geometries and energies were also calculated at the same level (including zero-point energy corrections) and the reactants’ energies subtracted from the optimizations of the TS and minima, resulting in the relative energies E _rel . Cartesian coordinates, structures and electronic energies of reagents, products, minima and TS are reported in the Supplementary Material.

The characterization of the fragmentation pathways for generation of the two isomers via dissociative photoionization of cyclopropylamine (c-C₃H₅NH₂) and azetidine (c-CH₂CH₂CH₂NH) has been described in previous publications Richardson et al. (2021), Sundelin et al. (2021) to which the interested reader is referred to for details. In the following subsection we will focus on the formation of the H₂CNH^+• isomer through direct ionization of methanimine.

Our calculations show that HCNH₂ ^+• is slightly more stable (by 18.9 kJ/mol) than H₂CNH^+•, consistent with a previous literature value Nguyen et al. (1994) in which the difference in the heat of formations of the two isomers, calculated at the UQCISD(T)/6-311++G(3df, 2p) and RCCSD(T)/cc-pVTZ levels of theory, is equal to 16 kJ/mol. Given the open shell nature and the presence of a charge such differences in the calculations are not surprising. All enthalpies are calculated with the GAUSSIAN suite of programs at the CCSD(T)/6-311++G(d,p) level of theory. Unless explicitly noted otherwise, relative enthalpies are given (in kJ/mol and in parenthesis after each structure) with respect to the sum of the separated reactants having the lowest energy, i.e. HCNH₂ ^+• plus C₂H₂.

For both isomers, the reaction mechanisms proceed via the formation of covalently bound adducts through a radical-terminal approach, with the barriers for the non-radical terminal attack being endothermic by more than 45 kJ/mol in both cases.

HCNH ₂ ^+• isomer (pathways are described graphically in Figures 6, 7, while reaction enthalpies are compared with literature values in Table 1): for this isomer, an initial van der Waals cluster A1 (−27.9) can proceed to a covalently-bound adduct A2 (−227.6) via the transition state (TS) A1-2 (−27.4). From A2, a series of pathways are possible, all of which are shown diagrammatically in Figure 7 as well as being described here.

The most favorable pathway, from the thermochemical point of view, to form the observed major product at m/z 54 proceeds from a rotamer of A2, named A3 (−217.1), with the two structures being separated by the TS A2-3 (−191.0). A3 can then undergo a [1,4] H-shift (from the N atom to the terminal C) to give A4 (−150.1) via the TS A3-4 (−71.7). From A4 the ejection of an H^• via A4-P (−49.7) gives the m/z 54 product in the form of the lowest energy isomer H₂CCHCNH⁺ (protonated vinyl cyanide) in combination with H^• (−107.5).

While producing a higher energy C₃H₄N⁺ isomer, the lowest energy pathway for m/z 54 proceeds through the cyclic intermediate A7 (−154.2) which can be reached via A2-7 (−147.6). A7 can then barrierlessly eject the H^• from the central carbon to give a three-membered ring aminic ion c-CHCHC(NH₂)⁺ (−73.6).

A different C₃H₄N⁺ isomer at m/z 54 can be formed from A3 by the direct ejection of H^• via the TS A3-P (−47.8) to give a linear aminic ion HCCCHNH₂ ⁺ plus H^• (−65.3). A fourth pathway leading to the formation of a product at m/z 54 proceeds from A2, which can undergo a [1,3] H-shift via the TS A2-9 (−43.1) to give A9 (−222.8), which can then eject H^• via TS A9-P (−32.8) to give the already mentioned H₂CCHCNH⁺ plus H^•. All these pathways are fully submerged, hence they can all contribute to the observed m/z 54 product.

Though not included in Figure 7, our calculations have identified another pathway, higher in energy with respect to the ones above, that leads to the production of a fourth high energy C₃H₄N⁺ isomer, the linear CH₂CCNH₂ ⁺ (protonated ketenimine). This pathway (shown in Supplementary Figure S30 of the Supplementary Material) starts from A9 and proceeds via successive interconversions into the rotamers A10 (−230.5), via TS A9-10 (−226.7), and then into A11 (−180.1), via TS A10-11 (−188.9), with the lower energy of the TS being due to the zero-point correction. A11 can undergo a hydrogen migration to give A12 (−226.2), via TS A11-12 (−60.5), which can finally eject H^• to give CH₂CCNH₂ ⁺ plus H^• (−23.6) via TS A12-P (−3.3).

The next major product from the HCNH₂ ^+• isomer is at m/z 28. A possible pathway has been identified stemming from A4. This intermediate can interconvert to give its rotamer A5 (−138.8) via the TS A4-5 (−157.6). Note that the lower energy of the TS compared to A4 is due to the zero point correction. A5 can then cleave the central C-C bond to give a van der Waals complex of the products, A6 (−49.5), via the TS A5-6 (−30.9). The van der Waals adduct can subsequently fragment, with no energy barrier, into the products HCNH⁺ (protonated hydrogen cyanide) plus C₂H₃ ^• (−9.0).

Finally, a pathway should be accounted for the extremely minor product detected at m/z 39 and attributed to c-C₃H₃ ⁺. It starts from the cyclic A7 intermediate, which can cleave the C-N bond to give the van der Waals complex A8 (−46.1) via the TS A7-8 (−37.7). A8 can then fragment barrierlessly into the products c-C₃H₃ ⁺ and NH₂ ^+• (+3.9). The small yield of the m/z 39 product is compatible with the calculated endothermicity of the channel.

H ₂ CNH ^+• isomer (pathways are described graphically in Figures 6, 8, while reaction enthalpies are compared with literature values in Table 1): for this isomer the reagents (at +18.9 kJ/mol above the other isomer) come together to form an initial van der Waals complex B1 (−32.6) which can then evolve, by formation of a C-N bond, into the covalently-bound complex B2 (−146.4) via the TS B1-2 (−33.6), with the lower energy of the TS compared to the van der Waals complex arising from the zero-point energy correction. B2 can then interconvert into its rotamer B3 (−154.7) via TS B2-3 (−141.9) from which the various pathways then proceed, as shown diagrammatically in Figure 8 and described in the following.

The lowest energy pathway to form the m/z 54 product proceeds via a cyclisation to give a four-membered ring intermediate, B5 (−239.3), via B3-5 (−48.3). B5 can open to give the N-terminal structure A4, via B5-A4 (−108.6). The A4 intermediate is a common structure with the potential energy hypersurface of the other isomer, hence from there the reaction can proceed to give H₂CCHCNH⁺ (protonated vinyl cyanide) plus H^• (−107.5), as already described above. Other C₃H₄N⁺ isomers could be produced from B3 via an interconversion into a second rotamer B4 (−154.8) via the TS B3-4 (−151.4). The latter rotamer can undergo a [1,3] H-shift to give B9 (−240.2) via B4-9 (+32.8). B9 can then eject H^• to give H₂CCHNCH⁺ (protonated vinyl isonitrile) plus H^• (−65.2) via the TS B9-P (−24.4). While this pathway involves a barrier higher in energy than the reactants (i.e. B4-9 that is 13.9 kJ/mol above the reactants) it is potentially relevant at higher photon energies where the HCNH₂ ^+• isomer might be formed with sufficient internal energy to both overcome the barrier and to potentially inhibit the alternative low energy pathway going via the cyclisation to give B5.

There are also two higher-energy pathways leading to another C₃H₄N⁺ isomer at m/z 54 that proceed via B9. For clarity reasons they have not been included in Figure 8, but they are shown in Supplementary Figure S31 and are described briefly here. The first pathway involves a [1,2] H-shift to give B11 (−277.1) via a TS B9-11 (−35.5), with B11 then ejecting H^• to give CH₂CNCH₂ ⁺ (−24.4) via B11-P (+3.7). The second involves a [1,3] H-shift from B9 to give B12 (-276.1) via TS B9-12 (+10.2), which then also eject H^• to give CH₂CNCH₂ ⁺, this time via TS B12-P (+3.2).

The m/z 28 product, corresponding to HCNH⁺ formed in association with C₂H₃ ^•, can proceed from the A4 intermediate, as already described for the HCNH₂ ^+• isomer, but it can also form from B3 via a [1,4] H-shift to give B6 (−133.1) via the TS B3-6 (−31.7). B6 can then interconvert into its rotamer B7 (−138.7), via the TS B6-7 (−98.5), which can then cleave the central C-N bond to give B8 (−69.8), a van der Waals complex of the products, via B7-8 (6.4). B8 can then dissociate barrierlessly into HCNH⁺ and C₂H₃ ^•.

As A4 is a common structure, the whole of the HCNH₂ ^+• PES is also accessible for the H₂CNH^+• isomer, therefore creating a potential pathway to a m/z 39 product through a series of rearrangements via A2, as previously described for the HCNH₂ ^+• isomer. However, due to the large number of interconversions, any contribution from this pathway is expected to be extremely minor despite being fully submerged with respect to the reactants.

While not included in Figure 8, there is also a pathway to interconvert between B5 and the rotamers B6 and B7 and a diagram is shown in Supplementary Figure S32. B5 can undergo a [1,2] H-shift to give B10 (−231.4) via B5-10 (−71.7). This can then ring-open via C-C bond cleavage to give B6 via B10-6 (−48.3) or B7 via B10-7 (−24.3). For the very minor m/z 27 product, only observed at high photon and high collision energies, two endothermic pathways have been found, one proceeding via intermediate B6 without a barrier to give C₂H₃ ⁺ plus HCNH (+145.1) and the other one going from B9 to give C₂H₃ ⁺ plus H₂CN (+104.0), also with no further barrier. Since the energies of both products are out of scale they have not been reported in Figure 8.

Comparison of the two sources of H₂CNH^+• shows a high level of qualitative agreement with regards to the trends as a function of both E _phot and E _CM , as well as broad quantitative agreement for both CSs and BRs of the various products. Exact quantitative comparison is not possible due to the different scaling of internal energy with photon energy. Similarly, comparison of the data collected with the two different isomers indicates a high level of selectivity for both isomers over the whole photon energy range.

For both isomers the major product is at m/z 54, corresponding to C₃H₄N⁺. Previous computational studies Wang et al. (2015), Heerma et al. (1986) have provided strong evidence for the existence of several C₃H₄N⁺ isomers in the gas phase, the most stable of which being protonated vinyl cyanide H₂CCHCNH⁺. The next most stable is a three-membered ring structure c-(CHCHC)–NH₂ ⁺ that is 22.2 kJ/mol higher in energy Wang et al. (2015), followed by the linear aminic structure HCCCHNH₂ ⁺ [37.7 kJ/mol above the most stable isomer Wang et al. (2015)]. At higher energies there is protonated vinyl isonitrile, H₂CCHNCH⁺ [49.8 kJ/mol above the most stable isomer Wang et al. (2015)], protonated ketenimine H₂CCCNH₂ ⁺ [57.4 kJ/mol above the most stable isomer Wang et al. (2015)] and CH₂CNCH₂ ⁺ [61.1 kJ/mol above the most stable isomer Wang et al. (2015)]. Although our experimental technique can not distinguish between the different C₃H₄N⁺ isomers, ab-initio calculations can help in differentiating among the pathways leading to them.

For the reaction of HCNH₂ ^+• the m/z 54 product is approximately constant in cross section as a function of the photon energy while it decreases with increasing collision energy. These findings can be rationalized by the fact that the m/z 54 formation pathways have only submerged barriers and proceed via complex-mediated mechanisms, as confirmed by our calculations. In fact, not just one, but four different pathways with fully submerged barriers have been found (see Section 5) to yield the three lowest energy isomers CH₂CHCNH⁺, c-CHCHC(NH₂)⁺ and HCCCHNH₂ ⁺. Hence, the observed m/z 54 product is expected to correspond to a combination of the three isomers.

For the H₂CNH^+• isomer, the CS trend with E _phot for the m/z 54 product can be explained through a combination of multiple channels. At low internal energies, a barrierless pathway proceeds via the cyclized adduct B5 but this decreases in relevance as the internal energy increases due to the inhibition of the cyclisation. At higher E _phot , a pathway with a slight barrier via B9 opens, with this pathway giving the plateau that is observed at higher internal energies. Hence, for H₂CNH^+•, in the absence of internal excitation, the observed m/z 54 product is exclusively due to protonated vinyl cyanide H₂CCHCNH⁺.

As a function of the collision energy, the m/z 54 channels for both isomers show an initial decrease in CS section, indicative of barrierless pathways as suggested by our calculations. This is due to the higher collision energy inhibiting the formation of the van der Waals clusters (A1 and B1) in the entrance channels.

For the reaction of HCNH₂ ^+•, the m/z 28 and 39 products are approximately constant in cross section as a function of E _phot . This is in agreement with the computations that identify fully submerged pathways for the m/z 28 product and for formation of the van der Waals complex of the products for the m/z 39 channel, with the barrierless fragmentation of this complex lying approximately equal in energy with the reactants. The lower cross section of the m/z 28 product for this isomer than for H₂CNH^+• is expected to be due to the presence of the multiple lower-lying channels to give products at m/z 54 which are discussed above.

For the reaction of H₂CNH^+•, the m/z 28 channel corresponds to a combination of collision induced dissociation, especially at higher collision energies, with the two fully submerged pathways (see blue and red lines in Figure 8) leading to a CS trend that is independent of the photon energy at low collision energies. It is not clear what contribution there might be at low collision energies from the fragmentation of the parent at higher photon energies.

The small signal observed for the m/z 39 product for H₂CNH^+• is expected to indicate the very small level of interconversion between the isomers via A4. While this interconversion is barrierless for both ions, the fact that this requires passage over twelve TSs and intermediate structures is expected to lead to a significant level of kinetic inhibition, especially compared to other barrierless pathways present.

In general, products at m/z 28 and 39 from both isomers show an initial decrease in CS section with increasing collision energy, indicative of barrierless pathways as suggested by our calculations. This is due to the higher collision energy inhibiting the formation of the van der Waals clusters (A1 and B1) in the entrance channels. For both isomers, above ∼0.7 eV, the m/z 28 product shows a gradual rise with increasing collision energy which is probably due to the collisional fragmentation of the parent ion into HCNH⁺ and H^•. The very minor m/z 39 channel also shows a higher collision energy feature for both isomers, with appearance energies of ∼1 eV for HCNH₂ ^+• and ∼4 eV for H₂CNH^+•. This is believed to correspond to a mechanism to give either c-C₃H₃ ⁺ or its high energy isomer CH₂CCH⁺ via a significant barrier, though no individual channel has been identified.

The m/z 27 channel is only observed with the H₂CNH^+• isomer, where it shows an increase with increasing photon energy, a trend that relates well with the endothermic channels Eqs 3 and 4 identified both in literature and our calculations. As a function of the collision energy, this channel shows a steady increase, again indicative of an endothermic proton transfer process which can proceed directly at higher collision energies.

Absolute cross sections for the reaction of methanimine (H₂CNH^+•) and aminomethylene (HCNH₂ ^+•) radical cations with C₂H₂ have been measured as a function of photon and collision energies under a single collision regime using a guided ion beam tandem mass spectrometer with VUV ionization for ion generation.

Through comparison of H₂CNH^+• formed both via direct and dissociative ionization and HCNH₂ ^+• formed exclusively through dissociative ionization, we are able to conclude that both isomers are formed selectively with a high level of correspondence between the two H₂CNH^+• sources. Ab-initio calculations of the most relevant stationary points on the potential energy surfaces allow for the rationalization of experimental results.

Our experiments clearly show that the reaction of both isomers with C₂H₂ lead to the formation of C₃H₄N⁺ products (detected at m/z 54), and by extension to an elongation of the main skeleton of the ion. Measured CSs show a negative dependence on the collision energy, indicative of barrierless and exothermic processes that are feasible at low temperatures. Thus, the title reactions can be viable intermediate steps to produce larger ions in the ISM as well as ionospheres of planets and their satellites.

But what about the structures of the C₃H₄N⁺ products? Theoretical calculations show that, for the reaction of HCNH₂ ^+•, the lowest energy isomer (H₂CCHCNH⁺, protonated vinyl cyanide, a.k.a. acrylonitrile) as well as two higher energy isomers, namely c-CHCHC(NH₂)⁺ and HCCCHNH₂ ⁺, can be generated via barrierless and exothermic pathways. It should, however, be noted that our experiment cannot yield any information about the branching ratios for the formation of the different C₃H₄N⁺ isomers. Since all of these pathways involve submerged barriers, no preference for the production of individual ionic products could be inferred from our calculations starting from the HCNH₂ ^+• isomer. Conversely, for the H₂CNH^+• reagent, the only barrierless pathway is the one leading to the production of protonated vinyl cyanide, which somewhat restricts the potential of the methanimine radical cation in the synthesis of interstellar nitrogen compounds.

Protonated vinyl cyanide has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan, where the title reaction can contribute to its formation. In particular, Cassini flyby missions to Titan’s upper atmosphere have detected the presence of a strong signal at m/z 54 that has been tentatively assigned to protonated vinyl cyanide on the basis of the large proton affinity of the neutral Vuitton et al. (2007). More recently, the spectroscopic detection of vinyl cyanide in Titan’s atmosphere has been confirmed using ALMA data Palmer et al. (2017). In the ISM, while vinyl cyanide is one of the most abundant nitriles detected in several environments such as the circumstellar shell of evolved carbon stars and dark clouds, the presence of its protonated form has been proposed on the basis of laboratory spectra but still not confirmed Martinez et al. (2013). It is therefore noteworthy that our title reaction, starting from the methanimine radical cation (H₂CNH^+•), could lead to protonated vinyl cyanide, from which acrylonitrile can be formed either through dissociative recombination with electrons Vigren et al. (2009) or proton transfer to species such as NH₃ Taquet et al. (2016).