Spectroscopic Characterization of 3-Aminoisoxazole, a Prebiotic Precursor of Ribonucleotides

Abstract

The processes and reactions that led to the formation of the first biomolecules on Earth play a key role in the highly debated theme of the origin of life. Whether the first chemical building blocks were generated on Earth (endogenous synthesis) or brought from space (exogenous delivery) is still unanswered. The detection of complex organic molecules in the interstellar medium provides valuable support to the latter hypothesis. To gather more insight, here we provide the astronomers with accurate rotational frequencies to guide the interstellar search of 3-aminoisoxazole, which has been recently envisaged as a key reactive species in the scenario of the so-called RNA-world hypothesis. Relying on an accurate computational characterization, we were able to register and analyze the rotational spectrum of 3-aminoisoxazole in the 6-24 GHz and 80-320 GHz frequency ranges for the first time, exploiting a Fourier-transform microwave spectrometer and a frequency-modulated millimeter/sub-millimeter spectrometer, respectively. Due to the inversion motion of the -NH₂ group, two states arise, and both of them were characterized, with more than 1300 lines being assigned. Although the fit statistics were affected by an evident Coriolis interaction, we were able to produce accurate line catalogs for astronomical observations of 3-aminoisoxazole.

Keywords: astrochemistry; large amplitude motion; origin of life; prebiotic molecules; rotational spectroscopy.

Figure 1

Molecular structure and atom labeling of 3-aminoisoxazole. On the right, a side view of the molecule shows the hydrogen position with respect to the ring plane.

Figure 2

Hyperfine structure of the $J_{K_a,K_c}=3_{1,3}\leftarrow 2_{0,2}$ transition of 3-aminoisoxazole recorded using the COBRA-FTMW spectrometer. The 7 MHz scan has been registered with 10,000 shots every 100 kHz. All lines appear as doublets due to Doppler effect. The labels refer to the spectral lines assigned and collected in the Supplementary Material.

Figure 3

Simulated (based on FTMW results, top panel) and experimental (bottom panel) typical pattern of the $K_a=0,1$ transitions in the millimeter-wave region. A 20 MHz scan is shown for each transition. The two inversion states are easily distinguishable.

Figure 4

Graphical representation of the included and rejected transitions of the $0^{+}$ (left panel) and $0^{-}$ (right panel) states in the fitting procedure.