Hydrogenation of ethylamine under interstellar conditions: whenhydrogenation yields unsaturated products

Ethylamine (C₂H₅NH₂) has been tentatively detected in the interstellar medium towards G+0.693-
0.027 and Sgr B2 [1], and was identified alongside β-alanine in comet 81P/Wild 2 [2]. As a
potential precursor to alanine through the CH₃CHNH₂ radical [3], understanding the surface
reactivity of ethylamine under interstellar conditions is directly relevant to the question of amino
acid formation in space.
We present the first experimental study of ethylamine hydrogenation under dark molecular cloud
conditions, performed using the VENUS apparatus at LIRA CY, Cergy Paris Université [4].
Ethylamine and H atoms are co-deposited onto a gold surface held at 10 K inside a UHV chamber
(P ~ 10⁻10 mbar). H atoms are generated by microwave dissociation of molecular hydrogen. After
deposition, a Temperature-Programmed Desorption (TPD) experiment is performed by heating the
surface at 12 K/min, and desorbing species are detected by a quadrupole mass spectrometer (QMS)
operated at 30 eV ionization energy. Products are identified by deconvolving the QMS signal into
contributions from candidate species, each characterized by an independently determined cracking
pattern.
Figure 1 compares the desorption profile at m/z = 30 for pure ethylamine and for ethylamine co-
deposited with H atoms. The strong reduction in signal demonstrates that a significant portion of the
initial ethylamine is consumed upon H-atom exposure. Attempting to fit the co-deposition signal
with ethylamine alone leaves large systematic residuals across multiple m/z channels,
demonstrating the presence of new molecular species.
Figure 2 shows the result of the full deconvolution. The cracking patterns of CH₄, NH₃, CH₃CN,
and CH₃NH₂ were determined by independent pure depositions; that of CH₃CHNH (ethanimine)
was derived directly from the residual signal. The main products identified are CH₄ (desorbing at
~66 K), NH₃ (~90 K), CH₃CN (~129 K), and CH₃CHNH (~135 K), with CH₃NH₂ and HCN as
minor species.
The product distribution reveals a clear selectivity. All identified carbon-containing products either
retain the full C–C–N skeleton of ethylamine (CH₃CHNH, CH₃CN) or result from its complete
fragmentation (CH₄, NH₃). No products retaining only a partial skeleton, such as methylamine
(CH₃NH₂) or ethane, are detected in significant quantities, despite NH₃ and CH₄ being among the
main products. This suggests that single C–C or C–N bond cleavage is not a dominant pathway:
when the carbon skeleton breaks, it breaks entirely.
The observed products are consistent with H abstraction on ethylamine being an accessible
pathway, which would make the CH₃CHNH₂ radical a possible intermediate, but its existence and
the precise reaction mechanism remain to be confirmed. Ongoing quantum chemical calculations
aim to determine the energy barriers along the relevant pathways and establish whether this radical is indeed formed, with direct implications for the proposed role of ethylamine as a precursor to
alanine in interstellar environments.
References: [1] Zeng et al., ApJL 920, L27 (2021) — [2] Glavin et al., Meteoritics & Planet. Sci.
43, 399 (2008) — [3] Förstel et al., ApJ 845, 83 (2017) — [4] Congiu et al., Rev. Sci. Instrum. 91,
12 (2020)
Figures: (1) m/z = 30 desorption profile, pure ethylamine vs. ethylamine + H; (2) Full TPD
deconvolution with identified products.