Laboratory rotational spectroscopy and interstellar search for the protein precursor 4-oxobutanenitrile (HCOCH$_2$CH$_2$CN)

Understanding the presence and distribution of prebiotic precursors in the interstellar medium (ISM) is key to tracing the chemical origins of life. Among them, 4-oxobutanenitrile (\ch{HCOCH2CH2CN}) has been identified in laboratory simulations as a plausible intermediate in the formation of glutamic acid, a proteinogenic amino acid. Here, we report its gas-phase rotational spectrum, measured using two complementary techniques: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy (2$-$18 GHz) and free-jet millimeter-wave (FJ$-$AMMW) absorption spectroscopy (59.6$-$80 GHz). Quantum chemical calculations revealed nine low-energy conformers, of which the TC conformer was assigned based on the measured spectra. The resulting spectroscopic parameters were used to search for the molecule in the ultradeep spectral survey of the G+0.693-0.027 molecular cloud, located in the Galactic Center. No signal attributable to 4$-$oxobutanenitrile was detected. A stringent upper limit to its column density was derived ($N<$ 4 $\times$10$^{12}$ cm$^{-2}$), corresponding to a molecular abundance of $<$ 2.9 $\times$10$^{-11}$ relative to H$_{2}$. This upper limit lies well below the observed abundances of simpler structurally related species containing $-$HCO and $-$CN groups, underscoring the challenge of detecting increasingly complex prebiotic molecules in the ISM and the need for future, more sensitive astronomical facilities.

💡 Research Summary

This paper presents a comprehensive laboratory and astronomical investigation of 4‑oxobutanenitrile (HCOCH₂CH₂CN), a molecule proposed as a key prebiotic intermediate in the formation of the proteinogenic amino acid glutamic acid. The authors first performed an extensive conformational analysis using density‑functional theory (B3LYP‑D3BJ/def2‑TZVPP). By scanning the two pivotal dihedral angles (θ = C1‑C2‑C3‑C4 and ϕ = C2‑C3‑C4=O), they identified nine low‑energy conformers. The global minimum (designated TC) features a trans‑oriented alkyl chain (θ ≈ 180°) and a planar carbonyl group (ϕ ≈ 0°). The remaining eight conformers form four enantiomeric pairs (G‑C/G‑+C, G‑A/G‑+A, T‑A/T‑+A) lying 4–11 kJ mol⁻¹ above TC, with interconversion barriers as low as ~4 kJ mol⁻¹.

Two complementary high‑resolution spectroscopic techniques were employed to record the gas‑phase rotational spectrum of 4‑oxobutanenitrile. Chirped‑pulse Fourier‑transform microwave (CP‑FTMW) spectroscopy covered 2–18 GHz, yielding 87 hyperfine‑resolved lines (including the ¹⁴N quadrupole splitting). Free‑jet absorption millimetre‑wave (FJ‑AMMW) spectroscopy spanned 59.6–80 GHz, providing an additional 21 lines (34 hyperfine components). All observed transitions belong exclusively to the TC conformer; no signatures of the higher‑energy conformers were detected, consistent with the low interconversion barriers that enable complete relaxation to the global minimum under supersonic expansion conditions. The dipole moment components were experimentally confirmed as |μ_a| = 2.60 D and |μ_b| = 1.54 D (μ_c = 0).

A global fit of the combined data set produced precise rotational constants (A₀ = 16000.7925 MHz, B₀ = 1622.40376 MHz, C₀ = 1500.80587 MHz) and quartic centrifugal distortion parameters (D_J, D_JK, D_K, d₁, d₂). These constants enable frequency predictions with uncertainties of 0.05–0.1 km s⁻¹ up to ~80 GHz, sufficient for astronomical searches in sources with typical line widths of 15–20 km s⁻¹, such as the Galactic‑Center molecular cloud G+0.693‑0.027.

Using the ultra‑deep spectral survey of G+0.693‑0.027 obtained with the IRAM 30 m telescope (covering 84 GHz with rms ≈ 0.5 mK), the authors searched for the strongest μ_a‑type transitions of the TC conformer. No emission was found at the predicted frequencies. Consequently, a 3σ upper limit on the column density was derived: N(HCOCH₂CH₂CN) < 4 × 10¹² cm⁻², corresponding to an abundance relative to H₂ of < 2.9 × 10⁻¹¹. This limit is more than an order of magnitude lower than the abundances of simpler, structurally related species containing –HCO or –CN groups that have been detected in the same cloud (e.g., CH₃CN, HCOCH₃).

The study highlights several key points. First, it provides the first complete set of laboratory spectroscopic parameters for 4‑oxobutanenitrile, confirming that only the TC conformer survives under typical jet‑cooling conditions. Second, the derived astronomical upper limit demonstrates the current difficulty of detecting increasingly complex prebiotic molecules in the interstellar medium, even in chemically rich environments like G+0.693‑0.027. Third, the authors argue that future facilities with higher sensitivity (e.g., ngVLA, SKA‑mid) or additional laboratory measurements at higher frequencies (> 100 GHz) will be essential to push detection limits further. Overall, the work exemplifies the synergy between laboratory molecular spectroscopy and radio‑astronomical observations in advancing our understanding of prebiotic chemistry in space.