Experimental and Computational Studies on the Reactivity of Methanimine Radical Cation (H₂CNH^+•) and its Isomer Aminomethylene (HCNH₂^+•) With C₂H₂

Abstract

Experimental and theoretical studies are presented on the reactivity of the radical cation isomers H₂CNH^+• (methanimine) and HCNH₂^+• (aminomethylene) with ethyne (C₂H₂). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors as well as via direct photoionization of methanimine. Reactive cross sections (in absolute scales) and product branching ratios are measured as a function of photon and collision energies. Differences between isomers’ reactivity are discussed in light of ab-initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C₃NH₄]⁺. Theoretical calculations show that while for the reaction of HCNH₂^+• with acetylene any of the three lowest energy [C₃NH₄]⁺ isomers can form via barrierless and exothermic pathways, for the H₂CNH^+• reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH₂CHCNH⁺), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.

1 Introduction

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.

2 Experimental Methodology

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:

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.

3 Theoretical Methodology

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.

4 Experimental Results

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.

4.1 Generation of H₂CNH^+• From Photoionization of Methanimine

H₂CNH was synthesized by dehydrocyanation of aminoacetonitrile performed under vacuum, following an improved version of the original method Guillemin and Denis (1988) and the interested reader is referred to Gans et al. (2019) for details of the chemical procedure. After synthesis, methanimine was stored under liquid nitrogen, and transferred to dry ice for introduction in the ion source of the CERISES set-up using its vapor pressure at dry ice temperature (T ∼ −78°C).

The photoionization efficiency (PIE) curves (i.e. ion yields as a function of photon energy) for the parent radical cation H₂CNH^+• (m/z 29) and the photo-dissociation product HCNH⁺ (m/z 28) are reported in Figure 1, where the observed Appearance Energies (AEs) are given. For H₂CNH^+•, an Ionization Energy (IE) of 10.01 ± 0.08 eV was obtained using the method of linear threshold extrapolation of the PIE curve at m/z 29 Chupka (1971), Traeger and McLoughlin (1981), Castrovilli et al. (2014), a value that is fully consistent with the adiabatic ionization energy of methanimine of 9.99 eV as determined via mass-selected threshold photoelectron spectroscopy Holzmeier et al. (2013). For the HCNH⁺ photo-fragment, an experimental AE = 11.02 ± 0.08 eV was estimated taking the first point above noise level of the PIE curve at m/z 28, after signal correction due to “spillover” of the more abundant peak at m/z 29. This is also consistent with the onset of dissociative photoionization of methanimine via H^• loss, observed at 11.13 eV Holzmeier et al. (2013).

FIGURE 1

We emphasize that the alternative generation method starting from neutral methanimine is explored as a way to produce the methanimine radical cation isomer free from interference from isobaric contaminants (C₂H₅⁺, H¹³CNH⁺ and ¹³CCH₄^+•), as is detailed in Section 4.3. Conversely, since the methanimine sample is free from heavier mass contaminants, the only expected interference at m/z 29 is limited to the ¹³C contribution of the HCNH⁺ photo-fragment at m/z 28. However, from Figure 1 it can be clearly seen that this fragment is less intense than the m/z 29 cation at least up to a photon energy of ∼13.1 eV. As a consequence, any contamination due to H¹³CNH⁺ at m/z 29 should account for less than 1% of the m/z 29 signal below E_phot = 13.1 eV.

4.2 Reactions of H₂CNH^+• and HCNH₂^+• With C₂H₂

The reaction of both isomers yields products at m/z 54, 39 and 28, while for the H₂CNH^+• isomer an additional minor product was observed at m/z 27. The most probable product channels are indicated as Eqs 3–11, and the reaction enthalpies estimated from literature values are summarized and compared with results from our theoretical calculations in Table 1.

Although the m/z 39 product could alternatively correspond to the formation of HCCN^+• or HCNC^+• ions formed together with a CH₄ molecule, these processes are not only endothermic by about 2 eV [on the basis of the calculated energies of HCCN^+• and HCNC^+•Harland and McIntosh (1985), Goldberg et al. (1995), Holmes et al. (2006)] but also very difficult to account for from a mechanistic point of view. Hence this possibility is not considered any further and the corresponding products are not included in Table 1.

4.3 Considerations About Impurities in the Reagent Ions

When selecting m/z 29 from the neutral precursors (cyclopropylamine and azetidine), there is no way to discriminate against the presence of isobaric contaminants, namely C₂H₅⁺, H¹³CNH⁺ and ¹³CCH₄^+•. Calculations and experimental evidence from our previous works Richardson et al. (2021), Sundelin et al. (2021) show that dissociative photoionization of azetidine to give H₂CNH^+• plus C₂H₄ has an experimental AE (10.2 ± 0.1 eV) very close to the fragmentation channel giving HCNH⁺ plus C₂H₅^+•. Additionally, the azetidine radical cation can fragment into C₂H₄^+• plus H₂CNH at photon energies above ∼11.3 eV. Hence both H¹³CNH⁺ and ¹³CCH₄^+• might be present as impurities when mass selecting reagent ions at m/z 29 from azetidine. For cyclopropylamine, dissociative photoionization to HCNH₂^+• (plus C₂H₄) has a similar AE (10.2 ± 0.1 eV) also very close to fragmentation into HCNH⁺ plus C₂H₅^•. Additionally, the formation of the isobaric cation C₂H₅⁺ is possible at photon energies higher than ∼12.4 eV. Since fragmentation into C₂H₄^+• plus HCNH₂ opens up at very high photon energies, any contamination due to ¹³CCH₄^+• is not relevant in this case. Hence, for cyclopropylamine the relevant contaminants at m/z 29 are H¹³CNH⁺ and C₂H₅⁺.

In the following the reactivity of such isobaric impurities with C₂H₂ will be addressed.

4.3.1 Contamination due to C₂H₅⁺

Literature data Kim et al. (1977), Anicich (1993), Anicich (2003) indicate that C₂H₅⁺ reacts with C₂H₂ with an overall rate k = 1.9 × 10⁻¹⁰ cm³⋅molec⁻¹⋅s⁻¹ to give the following products (with BRs indicated in parenthesis):

A recent re-evaluation Richardson et al. (n.d.) of this reaction using the same set-up employed in this work where C₂H₅⁺ was generated via VUV dissociative photoionization of C₂H₅Br at a photon energy of 11.3 eV and at the lowest achievable collision energy (E_CM ∼ 0.08 eV) gives quite different product branching ratios. Most notably, Eq. 14 is found to be the major channel (BR = 0.71), while Eq. 15 is the second most abundant channel (BR = 0.22), with minor amounts of the other products (C₄H₇⁺ and C₂H₃⁺) accounting for the remaining product flux.

Since Eq. 14 from C₂H₅⁺ leads to a product of the same mass as Eq. 7, contamination at m/z 39 will emerge as the photon energy is increased when working with HCNH₂^+• from cyclopropylamine.

4.3.2 Contamination due to H¹³CNH⁺ and ¹³CCH₄^+•

As already mentioned, a contribution from H¹³CNH⁺ at all photon energies is expected for both azetidine and cyclopropylamine experiments. However, literature reports indicate that HCNH⁺ is unreactive with C₂H₂ [k < 1.0 × 10⁻¹³ cm³⋅molec⁻¹⋅s⁻¹ at room temperature Anicich et al. (2000), Anicich and McEwan (2001), Milligan et al. (2001)], as expected from thermochemical considerations, since the most feasible channel (i.e. proton transfer to give C₂H₃⁺ plus HCN) is endothermic by approximately 0.75 eV. In our experiment, this channel could potentially open but only at high photon energies and should be accounted for when looking at data collected at m/z 27 under such conditions. Contamination due to ¹³CCH₄^+• is expected only when working with azetidine above ∼11 eV photon energy. Literature data Anicich (1993) indicate that the ethylene radical cation C₂H₄^•+ reacts with C₂H₂ with an overall rate k = 8.4 × 10^–10 cm³⋅molec⁻¹⋅s⁻¹ to give the following products:

The reaction of ¹³CCH4^+• should therefore lead to products at m/z 39 and 40 (i.e. C₃H₃⁺ and ¹³CC₂H₃⁺ from Eq. 16) plus a minor amount of m/z 54 (¹³CC₃H₅⁺) via Eq. 17.

To further check for the reactivity of ¹³CCH₄^+• contaminant, control experiments have been performed by selecting m/z 28 ions (a combination of HCNH⁺ and C₂H₄^•+) from dissociative ionization of azetidine and reacting them with C₂H₂. Products observed at m/z 39 and 53 are indicative of the occurrence of Eqs 16, 17 and the BR is consistent with the literature value reported above. Hence contamination due to ¹³CCH₄^+• might overlap with products at m/z 39 and 54 in the reaction of H₂CNH^+• generated from azetidine. As already discussed in Sundelin et al. (2021), since the ratio of the photodissociation yields for m/z 28 and 29 at the explored photon energies is in the range 0.3–1.0, only a small contamination of ¹³CCH₄^+• should be present in the reagent beam.

4.4 Experimental Results: Data as a Function of the Photon Energy (E_phot)

Product cross sections (CSs) as a function of E_phot for the reactions of HCNH₂^+• and H₂CNH^+• isomers generated through dissociative photoionization of cyclopropylamine and azetidine are given in Figure 2, while in Figure 3 analogous data are presented for H₂CNH^+• generated via photoionization of methanimine. In both figures the large uncertainties at the lowest photon energies are due to the low parent ion intensity near threshold Richardson et al. (2021). Trends as a function of E_phot give qualitative indications on how the reactivity is affected by changes in the internal energy content of the reagent ions.

FIGURE 2

FIGURE 3

The reaction of HCNH₂^+• (top part of Figure 2) gives a major product at m/z 54 with no significant dependence on E_phot and, by extension, on the internal energy of the ion, at least up to ∼12.5 eV. The decrease in CS at E_phot > 12.5 eV is an artefact due to the increasing contamination of C₂H₅⁺ in the parent beam (as discussed in Section 4.3), which reduces the relative amount of HCNH₂^+• ion available for reaction. The other products at m/z 28 and 39 are minor, but they also do not show any significant change as a function of the photon energy, with the rise in the m/z 39 product above E_phot > 12.5 being due to the emergence of the C₂H₅⁺ contaminant.

The reaction with H₂CNH^+• from azetidine (bottom part of Figure 2) also gives a major product at m/z 54, but differently from the previous isomer, its CS shows a marked decrease with increasing E_phot before plateauing, thus pointing to a barrierless pathway somehow inhibited by the increase in internal energy of the reactant ion. The other main channel is at m/z 28, with the CS showing no change with increasing E_phot, making it the major product at E_phot above ∼11.7 eV. The m/z 39 channel shows a very small cross section up to E_phot = 11.4–11.5 eV, though above this energy a slight increase is observed which might be the result of the minor ¹³CCH₄^+• impurity (see discussion in Section 4.3). Being very minor, this channel is discussed here only due to its relevance at higher collision energies. The reaction with H₂CNH^+• shows an additional minor product at m/z 27, whose CS increases with increasing photon energy.

CSs for the reaction of H₂CNH^+• from methanimine (Figure 3) show the same four products, with channels at m/z 54 and 28 being the major ones at all photon energies, and those at m/z 39 and 27 being minor throughout the whole photon energy range, in good agreement with the data collected using azetidine. As already mentioned, products observed using methanimine as precursor do not suffer from the contamination issues mentioned in Section 4.3, however a quantitative comparison as a function of E_phot should be performed with care, since the photon energy scales are not directly related. In fact, while in the case of methanimine photoionization the excess energy carried by the photon will contribute to increase the internal energy of the reacting H₂CNH^+•, in the case of dissociative photoionization from azetidine the excess energy might be dissipated also as internal and kinetic energy of the neutral co-fragment. Therefore, the change in internal energy of H₂CNH^+• with increasing E_phot is expected to be smaller in the azetidine experiment than for the methanimine precursor. CS for product at m/z 54 decreases sharply with increasing E_phot up to about 10.8 eV, after which it levels off and is approximately constant. Possible explanations of the observed trend are discussed in Section 6, here we only highlight the good qualitative agreement with the decrease as a function of E_phot observed with the dissociative photoionization of azetidine.

Similarly to the azetidine experiment, the CS for the m/z 28 product shows no notable change as a function of E_phot. The CS for the very minor product at m/z 39 also shows no significant dependence on E_phot. This is at odds with the azetidine data in which a very slight increase was observed at higher photon energies, further supporting the idea that this increase is due to the emergence of a pathway with the ¹³CCH₄^+• impurity. Finally, CS for the other minor channel at m/z 27 shows a similar rise with increasing E_phot as observed with the azetidine precursor.

The BRs for the various product channels, obtained from the absolute cross section measurements, are summarized in Table 2 for both isomers. In the same Table estimates of the total (i.e. summed over all the product channels) rate constants at a fixed average energy k_tot (E_ave) have been given. For details on how cross sections have been converted into rate constants see our previous paper Sundelin et al. (2021). For the HCNH₂^+• isomer, BRs and k_tot values were obtained via dissociative photoionization of cyclopropylamine at E_phot = 12.0 eV, while for the H₂CNH^+• the methanimine data at E_phot = 11.1 eV were used. For both isomers the collision energy was fixed at E_CM = 0.08 ± 0.01 eV, corresponding to an average energy E_ave = 0.10 ± 0.01 eV Ervin and Armentrout (1985), Nicolas et al. (2002). The ratio between k_tot and the Langevin collision rate constant (k_L), also reported in Table 2, is a useful estimate of the overall efficiency of the reactions, which proceed at a rate of ∼10% of the Langevin limit for both isomers.

4.5 Experimental Results: Data as a Function of the Collision Energy (E_CM)

The CSs as a function of E_CM for the reaction of both HCNH₂^+• and H₂CNH^+• isomers generated through dissociative photoionization of cyclopropylamine and azetidine are shown in Figure 4, while in Figure 5 analogous data are presented for H₂CNH^+• generated via photoionization of methanimine.

FIGURE 4

FIGURE 5

For the HCNH₂^+• isomer, the m/z 54 product shows a marked decrease with increasing E_CM, indicative of a barrierless pathway most likely proceeding via a complex-mediated mechanism. The m/z 28 and 39 channels show a combination of two trends, with a similar decrease below 1 eV followed by a gradual increase with increasing E_CM, indicating the presence of a barrierless pathway, mostly contributing at low collision energy, superimposed on an endothermic process (or a process involving a barrier) appearing at high E_CM. It should be noted that CS at m/z 28 are affected by collisional fragmentation of the parent which is discussed in more detail below.

For the H₂CNH^+• isomer with the azetidine precursor (bottom of Figure 4), CSs were measured only for products at m/z 54 and 39, with the former decreasing with increasing E_CM and the latter showing a decrease with increasing E_CM, at low collision energies, followed by a ramp at very high collision energies. A more complete picture for the reactivity of the H₂CNH^+• isomer is obtained from the experiments using methanimine (see Figure 5) since CS as a function of E_CM have been measured for all of the four product channels. The trend for m/z 54 product, decreasing with increasing E_CM, confirms the results from the azetidine experiment. Also similar to the azetidine experiment, the m/z 39 product is minor at all collision energies and shows an initial decrease with increasing E_CM followed by an inversion in the E_CM trend at high energies. The m/z 28 channel shows a slight initial decrease followed by a gradual increase with increasing E_CM that resembles the trend observed for the HCNH₂^+• isomer. The m/z 27 product shows a similar increase with increasing E_CM as it does with photon energy, indicating that the barrier to this pathway can be overcome equally effectively with collision energy as internal energy. A detailed discussion with explanations of the trends as a function of collision energy is given in Section 6 on the basis of the results obtained from computations and presented in the next section.

5 Computational Results

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.

HCNH₂^+•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.

FIGURE 6

FIGURE 7

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.

H₂CNH^+•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.

FIGURE 8

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.

6 Discussion

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.

7 Conclusion

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 NH₃Taquet et al. (2016).

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