Trivalent phosphorus, linked to anaerobic conditions on Earth, has also been found in the atmospheres of giant planets and in the distant interstellar medium. Phosphates are present in certain meteorites and on Saturn's moon Enceladus. How did phosphorus make its way between these environments and ultimately became central to life on Earth? Studying here, in terrestrial laboratories, various properties of unfamiliar molecules that may wander interstellar space is an intriguing path. ...a path being followed by scientists from the Institute of Physical Chemistry, Polish Academy of Sciences.
Humans have always been fascinated by the night sky, looking into the vast expanse of space where stars glitter like distant diamonds, finding inspiration and wondering about their own existence. This fascination has only grown as our ability to look and even travel into space has advanced. Telescopes, not only ground-based, have been built to peer into the Universe at many different wavelengths of electromagnetic radiation.
Satellites and other spacecraft have been sent short distances to explore Solar System bodies or even launched with trajectories bound to explore beyond the influence of our own star. The molecules astronomers observe are those that can survive the harsh conditions of the rarefied interstellar medium (ISM) or those that find protection in environments such as dense planetary atmospheres.
Unsaturated organic nitriles (molecules terminated with the group -CN) play an important role in the chemistry of ISM. HCN (hydrogen cyanide), HCCN (cyanomethylene), and HCCCN (cyanoacetylene) or vinyl cyanide (CH2CHCN) are the examples of chemical compounds observed in numerous locations, mainly using radio telescopes. These nitrogen-containing species are thought to play a role in the eventual production of amino acids and proteins. In our region of the Galaxy, the abundance of phosphorus (which sits directly below nitrogen in the periodic table) is approximately two hundred times smaller than that of nitrogen.
This difference is reflected in the fact that only seven P-bearing compounds (CP, NCCP, CCP, HCP, PN, PO, and PH3) have been identified in the ISM to date, while more than one hundred N-bearing species are already known to be there. Nevertheless, phosphorus is more abundant on Earth than in the Universe as a whole and can be found in nucleotides, phospholipids, and nucleic acids which are critical to life as we know it. What phosphorus carriers have yet to be identified in the interstellar medium? How are these molecules transformed into the substances we eventually observe on Earth? What signatures can we use to identify them in different remote environments? How do they end up concentrated on planets like Earth?
The questions to be answered remain formidable and endless. The challenge of adding to our understanding how certain unusual, highly reactive molecules containing a phosphorus atom can be identified in space has been taken up in Warsaw by the team from the Institute of Physical Chemistry: Dr. Arun-Libertsen Lawzer, Dr. Thomas Custer, doctoral student Elavenil Ganesan, led by Prof. Robert Kolos. They work in collaboration with Prof. Jean-Claude Guillemin of the Ecole Nationale Superieure de Chimie de Rennes (France).
Their recent paper explores the photochemistry of phosphabutyne (CH3CH2CP). Embedded in inert ice and exposed to ultraviolet light, the molecule was shown to undergo both isomerization (rearrangement of atoms) and loss of hydrogen. Two important products observed were phosphabutadiyne (HC3P) and vinylphosphaethyne (H2CCHCP). Their nitrogen-bearing analogues cyanoacetylene (HC3N) and vinyl cyanide (H2CCHCN) are already recognized as important and abundant interstellar molecules.
Both HC3P and H2CCHCP are very reactive and therefore unstable in typical laboratory conditions. Their formation was now made possible through the use of a cryogenic technique, where a small amount of phosphabutyne was frozen at around 10 Kelvin into an ice made of argon. Phosphabutyne molecules were effectively trapped between argon atoms, just as were the reactive species formed from them, like HC3P.
This isolation, i.e. separation with Ar atoms, made the photoproduced molecules stable and ready for spectroscopic characterization. Probing with infrared light revealed the frequencies of molecular vibrations, unique to each of the products. Quantum chemical computations helped in matching these frequencies with specific chemical compounds. In addition to HC3P and H2CHCP, several exotic isomers of the initial molecule could be seen, as well as the smaller products: ethynylphosphinidene (HCCP) and phoshaethyne (HCP).
"We were after the completely unexplored infrared spectroscopy of HC3P and CH2CHCP. Thus far, only the microwave, i.e. purely rotational spectra of these two have been reported. Characterising the molecular vibrations of such exotic, phosphorus-bearing molecules is important to the burgeoning field of infrared astrospectroscopy." - says prof. Kolos, while Dr. Lawzer specifies: "In the case of HC3P, we measured as many as five vibrational frequencies, which should be beneficial for future remote detections".
The study uncovers vibrational signatures of thus far poorly characterised or unknown molecules and indicates how ultraviolet light can degrade certain phosphorous derivatives in a chemically inert icy environment, a first step to understanding the reactions occurring in the ISM. "Advances in instrumentation allow us to identify molecules at ever lower abundances. HC3P, the phosphorus analogue of the famous astromolecule HC3N, looks like a candidate for detection with the James Webb Space Telescope" - remarks Dr. Custer. With time we should learn whether such phosphorus carriers are out there and whether they are important for the origin of life.
Research Report:Isomerisation of phosphabutyne and a photochemical route to phosphabutadiyne (HC3P), a phosphorus analogue of cyanoacetylene
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Institute of Physical Chemistry, Polish Academy of Sciences
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