An Infrared Spectroscopic Study Toward the Formation of Alkylphosphonic Acids and Their Precursors in Extraterrestrial Environments

Abstract

The only known phosphorus-containing organic compounds of extraterrestrial origin, alkylphosphonic acids, were discovered in the Murchison meteorite and have accelerated the hypothesis that reduced oxidation states of phosphorus were delivered to early Earth and served as a prebiotic source of phosphorus. While previous studies looking into the formation of these alkylphosphonic acids have focused on the iron-nickel phosphide mineral schreibersite and phosphorous acid as a source of phosphorus, this work utilizes phosphine (PH 3), which has been discovered in the circumstellar envelope of IRC +10216, in the atmosphere of Jupiter and Saturn, and believed to be the phosphorus carrier in comet 67P/Churyumov-Gerasimenko. Phosphine ices prepared with interstellar molecules such as carbon dioxide, water, and methane were subjected to electron irradiation, which simulates the secondary electrons produced from galactic cosmic rays penetrating the ice, and probed using infrared spectroscopy to understand the possible formation of alkylphosphonic acids and their precursors on interstellar icy grains that could become incorporated into meteorites such as Murchison. We present the first study and results on the possible synthesis of alkylphosphonic acids produced from phosphine-mixed ices under interstellar conditions. All functional groups of alkylphosphonic acids were detected through infrared spectroscopically, suggesting that this class of molecules can be formed in interstellar ices.

An interstellar synthesis of phosphorus oxoacids

“…In the infrared spectra of irradiated, phosphine-bearing ices, critical functional groups of phosphorus oxoacids were identified at 5 K and also in the residues that remained after annealing the exposed ices to 300 K (Supplementary Tables 1 and 2 ) 25 . These bands include stretching modes of phosphorus‒oxygen single and double bonds [ν(P‒O) (800–950 cm −1 ), ν(P=O) (1140–1300 cm −1 )], the deformation mode of the OPOH functional group [δ(O=P‒OH) (1550–1710 cm −1 )], and the P‒OH moiety [ν(O‒H) (2170 and 2700 cm −1 ).…”

Exploiting Photoionization Reflectron Time-of-Flight Mass Spectrometry to Explore Molecular Mass Growth Processes to Complex Organic Molecules in Interstellar and Solar System Ice Analogs

“…Finally, PI-ReTOF-MS can be expanded beyond astrochemical questions to explore fundamental problems of chemical bonding through the preparation of novel molecules relevant to astrobiology and inorganic chemistry. This has been accomplished via the detection of organics carrying the peptide bond (formamide, HCONH 2 ; urea, H 2 NCONH 2 ; N -methylformamide, HCONHCH 3 ) and of phosphorus-containing species like phosphorus oxoacids , along with their alkyl-substituted counterparts like alkylphosphoric and alkylphosphonic acids . Recently identified exotic phosphorus-carrying molecules like formylphosphine (HCOPH 2 ) and phosphino formic acid (PH 2 COOH) ,− along with two P 3 N 3 isomers, cyclotriphosphazene and 1,3,5-triphosha-2,4,6-triazabicyclo[2.2.0]­hexa-2,5-diene, the inorganic analogs of benzene and Dewar-benzene, highlight the potential of PI-ReTOF-MS.…”

Origin of alkylphosphonic acids in the interstellar medium

“…While phosphorus-bearing minerals such as schreibersite have been investigated as a source of alkylphosphonic acids (5,(9)(10)(11), the unique potential of an alternative formation scenario, the synthesis of alkylphosphonic acids in phosphorusdoped ices condensed on interstellar grains in cold molecular clouds through interaction with ionizing radiation, has not been fully explored yet. Functional groups, which might be linked to alkylphosphonic acids, have been proposed in processed phosphine (PH 3 )-rich ices by infrared spectroscopy (7). However, any discrete alkylphosphonic acids were not identified in this study (7) because the overlapping absorption bands of these complex organic molecules prevent their unambiguous detection (12).…”

“…A polished silver mirror was mounted to a rotatable cold head that reached temperatures of 4 . Using laser interferometry, the thickness of the ice was determined to be 750 ± 50 nm (29), while the composition ratio used the integrated absorption coefficients of the infrared vibrational bands taken from a Nicolet 6700 Fourier transform infrared spectrometer (see "Infrared and conversion yields" in the Supplementary Materials) (7). The ices were irradiated with 5-keV electrons for 1 hour at 2.0 × 10 12 electrons cm −1 s −1 ; the CASINO simulation (30) calculated the average penetration depth to be 270 nm with a maximum penetration depth of 630 nm and resulted in an average dose of 27 ± 6 eV molecule −1 .…”

“…While a purely terrestrial hypothesis might state that no "phosphorus problem" exists and that organisms could have scavenged the limited phosphorus from geologic phosphates (2), an extraterrestrial view advocates that meteoric minerals were delivered to Earth during the heavy bombardment period and, after aqueous processing, formed soluble phosphites that were available to the first organisms (5). An alternative to these hypotheses considers key chemical reactions essential to produce complex, biologically relevant organic molecules in astrophysical environments such as in interstellar ices in deep space before their delivery to Earth (6,7).…”