Abiotic Formation of Carboxylic Acids (Rcooh) in Interstellar and Solar System Model Ices

Abstract

The present laboratory study simulated the abiotic formation of carboxylic acids (RCOOH) in interstellar and solar system model ices of carbon dioxide (CO 2 )-hydrocarbon mix C n H 2n+2 (n = 1-6). The pristine model ices were irradiated at 10 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray particles. The chemical processing of the ices was monitored by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases on line and in situ in order to extract qualitative (carriers) and quantitative (rate constants and yields) information on the newly synthesized species. Carboxylic acids were identified to be the main carrier, together with carbon monoxide (CO) and a trace of formyl (HCO) and hydroxycarbonyl (HOCO) radicals at 10 K. The upper limit of acid column density at 10 K was estimated as much as (1.2 ± 0.1) × 10 17 molecules cm −2 at doses of 17 ± 2 eV molecule −1 , or the yield of 39% ± 4% from the initial column density of carbon dioxide. The temporal column density profiles of the products were then numerically fit using two independent kinetic schemes of reaction mechanisms. Finally, we transfer this laboratory simulation to star-forming regions of the interstellar medium, wherein cosmic-ray-induced processing of icy grains at temperatures as low as 10 K could contribute to the current level of chemical complexity as evidenced in astronomical observations and in extracts of carbonaceous meteorites.

On the formation of niacin (vitamin B3) and pyridine carboxylic acids in interstellar model ices

“…Employing in situ infrared spectroscopy, the formation of radiation-induced mono-and disubstituted carboxylic acids was monitored along with the HOCO radical, and optimized kinetic fittings were obtained. From this, the rate constant for the addition of a single carboxyl group was determined to be (4.4 ± 2.6) × 10 -4 s -1 , comparable to the 1.1× 10 -4 s -1 found for the formation of generic alkyl carboxylic acids in hydrocarbon-carbon dioxide ice mixtures [21]. Addition of a second carbon dioxide molecule to a monosubtituted pyridine carboxylic acid was found to proceed at a rate of (2.7 ± 1.6) × 10 -4 cm 2 s -1 , slower than the initial addition of carbon dioxide to pyridine due to the effects of the highly electronegative carboxyl group.…”

“…Similar to the mechanisms extracted in previous studies of carboxylic acid formation in interstellar model ices [21,39], the initial step in the formation of pyridine carboxylic acids in this study is the loss of a hydrogen atom, as described in reaction (9), from pyridine. This reaction is essentially a carbon-hydrogen bond cleavage and is highly endogenic by 439 kJ mol -1 (4.55 eV) for the o-pyridinyl radical and by 468 kJ mol -1 (4.85 eV) for the m-and p-pyridinyl radicals [53].…”

“…This reaction is essentially a carbon-hydrogen bond cleavage and is highly endogenic by 439 kJ mol -1 (4.55 eV) for the o-pyridinyl radical and by 468 kJ mol -1 (4.85 eV) for the m-and p-pyridinyl radicals [53]. The hydrogen atoms produced possess up to a few eV of kinetic energy [17,21]. Hydrogen atoms with sufficient kinetic energy are then capable of overcoming the entrance barrier of 106 kJ mol -1 (1.10 eV) required to form the trans-carboxyl radical (HOCO (reaction (5)).…”

“…On the other hand, complex organic molecules and carboxylic acids (RCOOH) in particular have been shown to be formed in laboratory experiments in interstellar analog ices upon exposure of these model ices to ionizing radiation such as ultraviolet photons, galactic cosmic rays, and energetic electrons simulating the secondary electrons generated via the interaction of galactic cosmic ray particles with interstellar ices [20,21]. More specifically, formic acid (HCOOH) [22], acetic acid (CH 3 COOH) [17], and complex organic carboxylic acids with 'R' being an organic alkyl group (RCOOH) [21] were formed in water-carbon monoxide, methane-carbon dioxide, and carbon dioxide-hydrocarbon ices, respectively, upon exposure to energetic electrons. The initial step in the reaction was found to be the cleavage of the oxygen-hydrogen (reaction (1)) or carbonhydrogen (reaction (2) and (3)) bonds upon decomposition of water, methane, and a generic hydrocarbon, respectively.…”

“…These simulations yielded an imparted energy of 4.7 ± 0.5 keV per electron at an average penetration depth of 390 ± 20 nm (less than the thickness of the ice to avoid interaction of the electrons with the silver wafer) translating to an average of 4.0± 0.1 eV absorbed per molecule in the deposited ice. This value corresponds to an average linear energy transfer (LET) of 12 keV µm -1 , on the order of magnitude 10-20 MeV cosmic rays transfer to interstellar ices ([38,39]). The irradiated ice remained at 10 K for an additional 60 minutes before being heated to 293 K at a rate of 0.5 K minute -1 .…”