Article pubs.acs.org/JPCA
Systematic Search for Chemical Reactions in Gas Phase Contributing to Methanol Formation in Interstellar Space Victoria G. Gamez-Garcia and Annia Galano* Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina. Iztapalapa, C. P. 09340, México D. F. México S Supporting Information *
ABSTRACT: A massive search for chemical routes leading to methanol formation in gas phase has been conducted using computational chemistry, at the CBS-QB3 level of theory. The calculations were performed at five different temperatures (100, 80, 50, 20, and 10 K) and at three pressures (0.1, 0.01, and 0.001 atm) for each temperature. The search was focused on identifying reactions with the necessary features to be viable in the interstellar medium (ISM). A searching strategy was applied to that purpose, which allowed to reduce an initial set of 678 possible reactions to a subset of 11 chemical routes that are recommended, for the first time, as potential candidates for contributing to methanol formation in the gas phase of the ISM. They are all barrier-less, and thus they are expected to take place at collision rates. Hopefully, including these reactions in the currently available models, for the gas-phase methanol formation in the ISM, would help improving the predicted fractional abundance of this molecule in dark clouds. Further investigations, especially those dealing with grain chemistry and electronic excited states, would be crucial to get a complete picture of the methanol formation in the ISM.
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with very small or no activation energy.3,6−8 The reactions fulfilling such requirements frequently involve free radicals and/ or ions.5,9 More than 150 species have been identified, to date, in molecular clouds, being H2 the most abundant by far. On the other hand, some of the organic molecules present in the ISM may be considered as precursors of prebiotic molecules (PBM).5,9−15 Since it is assumed that PBM were created from simpler precursors,16 the chemical processes leading to the formation of such precursors are of particular interest. Different chemical routes toward molecular complexity have been proposed4,5,17,18 with the most widely accepted consisting of chain ion-neutral reactions leading to larger polyatomic species, which eventually react with electrons yielding neutral species by dissociative recombination.8 Methanol is among the molecules found in the ISM that are considered of biochemical relevance.10 It is considered as a potential precursor of, more complex, prebiotic species.19 Like other molecules containing a C−O bond, it might be considered as a potential building block for the synthesis of sugars and amino acids.9 Methanol has been found in diverse regions of the ISM, in significant amounts,5,17,20−22 and gasphase methanol is one of the most ubiquitous interstellar species.23 It has been observed in a wide range of interstellar
INTRODUCTION The interstellar medium (ISM) is a chemically fascinating region of our Universe, where reactions take place under conditions very different from those on the Earth. Such conditions make possible that the most likely chemical routes yielding a particular molecule in the ISM may be significantly different from those involved in the formation of the same molecule on the Earth. For example, ions, free radicals, energetic isomers and HCnN (n = 3, 5, 7, 9, etc.) molecules are common reactants in the ISM,1,2 while they are considered very reactive and short-lived species under earthly conditions. In addition, replicating the ISM conditions in experiments is a very difficult task. It is not surprising then that “One of the major challenges in the field of astrochemistry is to understand fully the synthetic mechanisms that are responsible for the gaseous molecules that have been observed in the ISM”.3 It is assumed that interstellar molecules are mainly formed by chemical reactions within dense clouds, also known as molecular clouds. They mainly consist of both gas and dust at very low temperature, pressure and density;4 albeit they are not uniform but highly structured with large variations in these properties.5 Chemical reactions in molecular clouds can take place both in the gas phase and on, or in, the dust grains.1 In addition, because of the low pressure and particle density, likely gas phase chemical reactions in these ISM regions should fulfill at least three important requirements. They are (i) the reactions must involve at most two reactants, (ii) they must be significantly exergonic, and (iii) they must be very fast, i.e., © 2017 American Chemical Society
Received: June 13, 2017 Revised: September 7, 2017 Published: September 8, 2017 7393
DOI: 10.1021/acs.jpca.7b05797 J. Phys. Chem. A 2017, 121, 7393−7400
Article
The Journal of Physical Chemistry A environments, with typical abundances relative to H2 of 10−6 to 10−7 in hot cores, 10−9 in dark clouds, and
