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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