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martes, 23 de junio de 2020

Chemistry as the Origin of the Molecular Universe - Elena Jiménez Martínez

9.3
Chemistry as the Origin of the Molecular Universe.
The Lives of the Stars.







Apart from the poetic connotation that a “starry sky” has, the sky has been observed by Man, since he became sedentary, as a tool to determine periods of abundance for hunting and gathering, among others. On a clear night, one can see the infinity of stars that are distinguished from a faraway place of the Universe, our planet. This perspective is very limited, since we see an incredibly small part of the Universe. Only in the Milky Way, astronomers estimate that there may be around 300 billion stars. If the Universe has more than 100 trillion galaxies ... the number of stars is "astronomical"! 10.000.000.000.000.000.000.000 (1022). Are there really more stars in the Universe than grains of sand on every beach in the world, as Carl Sagan (1934-1996) said? Yes, with a simple calculation it results in 4.000.000.000.000.000.000.000 (4×1021) grains of sand in all beaches of the Earth. So, Carl Sagan was right and there are more stars in the Universe than grains of sand on the beaches of our planet. Few scientists have been as inspiring to public opinion as Carl Sagan has, especially among millennials and people who live motivated by curiosity and love for knowledge.
Since the revolution of observational Astronomy, we have become increasingly familiar with our sky and the "observable" objects in it. In particular, the space between two stars, that is, the interstellar medium, although it could be thought of as "empty", is not and is made up of approximately 99% of gas (mainly, hydrogen and helium) and a 1% of dust grains and ice. Although H2 is the most abundant gas, around 200 molecules have also been identified so far in the interstellar medium [1, 2]. These range from simple molecules such as water, H2O, essential for Life as we know it, to complex molecules such as fullerenes (C60 or C70), the third known stable molecular form of carbon, after graphite and diamond.
The vast majority of organic molecules detected contain, in addition to C and H atoms, O and/ or N atoms, and to a lesser extent S, P and Si. Many of these organic compounds are found on Earth and in our daily lives, for example, the alcohol present in alcoholic beverages (ethanol, CH3CH2OH) or acetone (CH3C(O)CH3) found in nail polish removers (Figure).

Simple organic molecules detected in the interstellar medium.

Some of these simple organic molecules are considered precursors of biological molecules such as sugars (which contain C, H, and O, for example, sucrose C12H22O11) or aminoacids (containing C, H, O, and N through amino (-NH2) and carboxyl (-C(O)OH) groups. For example, glycoaldehyde (HOCH2C(O)H) can react with a 3-carbon sugar to produce ribose and deoxyribose, important components of nucleotides, and are found in RNA and DNA, respectively. Glycolaldehyde was first detected in 2000 in the Sagittarius constellation at the center of our galaxy [3]. More recently, glycoaldehyde has been detected with the ALMA (Atacama Large Millimeter Array) radiotelescope in the gas surrounding a young binary star such as IRAS 16293-2422 in 2012 [4] or NGC 1333 IRAS2A in 2015 [5]. This discovery, therefore, proves that these precursors of biomolecules, necessary for Life, already existed in the interstellar medium at the time of planet formation.
The knowledge of the chemistry in these environments is, therefore, essential to understand how new molecules are formed in Space. But how are these prebiotic molecules formed in the interstellar medium, where the gas temperature is ultra-low and ranges from 10 K (~ 263 ºC below zero) to 100 K (~173 ºC below zero)? There are two accepted mechanisms by the astrophysical community: chemical reactions on the surface of interstellar dust grains or ices or reactions between two molecules (ions, radicals or uncharged molecules) in the gas phase. For example, methanol (CH3OH) is formed by successive hydrogenation of carbon monoxide (CO) on the surface of the grains at temperatures below 30 K (~ 243 ºC below zero).
N. Balucani et al. proposed that some organic molecules, such as methyl formate (HC(O)OCH3) or dimethyl ether (DME, CH3OCH3), could be formed in the gas phase from the reaction of the hydroxyl radical (OH), present in the interstellar medium, with methanol [6]. In order to interpret the observed abundances of organic molecules in the interstellar medium, the understanding of the reaction rates at very low temperatures is essential. This kinetic information is incorporated into astrochemical gas-grain models that simulates how and how much these organic molecules are formed. Given that these models are made up of hundreds of reactions, there are many unknowns, especially at temperatures between 10 and 100 K. At these temperatures, most of the rate constants, k, (kinetic parameter to be measured experimentally) for A + B reactions (where normally A is an ion or a radical and B is an ion / radical or a molecule) are not known. The reason for that is the enormous experimental difficulty involved in obtaining a uniform gas flow in a chemical reactor at these ultra-cold temperatures, where by conventional cooling methods the organic molecules are in solid state. Hence, many of the rate constants used in this kind of simulations are those reported at room temperature (25º C) or extrapolated from expressions of k as a function of temperature (at T> 200 K). The experimental kinetic studies at T> 200 K have been carried out, mostly, in the 20th century due to their interest for the atmosphere of our planet or for Combustion Chemistry. Although the chemistry of our atmosphere is similar to that in interstellar clouds, there are important differences such as the physical conditions of the medium (temperature and pressure) or its composition. To simulate in the laboratory the temperature conditions of the interstellar medium, in the last decades, an unconventional cooling method has been used: the uniform supersonic expansion technique. This technique is based on the gas expansion at supersonic speed to obtain a uniform jet in temperature and pressure. With this technique, it has been possible to obtain gases at temperatures of down to 11 K [7-9], which, coupled with kinetic techniques, has allowed the database to be extended, particularly for radical-molecule reactions.
Recently the group of Prof. Dwayne E. Heard from the University of Leeds (United Kingdom) observed that the rate constant of the OH+CH3OH reaction was greatly accelerated at temperatures between 79 and 63 K [10], which implied a much faster formation of CH3O radicals than previously thought by the astrophysical community. In addition, CH3O formation could be explained exclusively by the gas-phase OH+CH3OH reaction. These observations were corroborated by our research group at the University of Castilla-La Mancha conducting kinetic studies at temperatures even lower down to 11.7 K. [9,11] The great challenge is to carry out studies under the vacuum conditions of the interstellar medium, where the pressure it is so extremely low that it would not be reached even with the best vacuum obtained on Earth. Hence, computational theoretical studies are required to simulate these conditions and the mechanism of these gas phase reactions is investigated.
What is true is that the experimental and theoretical chemical and physical community must join forces to contribute to the knowledge of the origin of the molecular Universe. This includes chemical kinetic investigations of the formation and disappearance processes of interstellar molecules and the mechanism by which these reactions take place. Quoting Carl Sagan “we are stardust who thinks about the stars. We are the way the Universe thinks of itself.

References:
[3] J. M. Hollis, F. J. Lovas, P. R. Jewell, Astrophysical Journal, 540:L107-L110 (2000).
[4] J. K. Jørgensen, C. Favre, S. E. Bisschop, T. L. Bourke, E. F. van Dishoeck, M. Schmalzl. The Astrophysical Journal Letters, 757: L4 (2012).
[5] A. Coutens, M. V. Persson, J. K. Jørgensen, S. F. Wampfler, J. M. Lykke. Astronomy and Astrophysics, 576: A5 (2015).
[6] N. Balucani, C. Ceccarelli, V. Taquet. Monthly Notices of the Royal Astronomical Society, 1-5 (2014).
[7] A. Potapov, A. Canosa, E. Jiménez, B. Rowe. Angewandte Chemie-International Edition, 56 (2017) 8618-8640.
[8] M. Tizniti, S. D. Le Picard, F. Lique, C. Berteloite, A. Canosa, M. H. Alexander, I. R. Sims, Nature Chemistry, 6 (2014) 141–145.
[9] A. J. Ocaña, S. Blázquez, A. Potapov, B. Ballesteros, A. Canosa, M. Antiñolo, L. Vereecken, J. Albaladejo, E. Jiménez. Physical Chemistry Chemical Physics, 21 (2019) 6942-6957.
[10] R.J. Shannon, M. A. Blitz, A. Goddard and D. E. Heard, Nature Chemistry, 5 (2013) 745–749.
[11] M. Antiñolo, M. Agúndez, E. Jiménez, B. Ballesteros, A. Canosa, J. Albaladejo, J. Cernicharo. Astrophysical Journal, 823:25 (2016) 1-8.


Elena Jiménez Martínez.
PhD in Chemistry.
Full professor, Department of Physical Chemistry.
University of Castilla-La Mancha. Faculty of Chemical Sciences and Technologies, Ciudad Real (Spain).


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