Complex Organic Molecules in the Interstellar Medium

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18 Mayo 2023 - 10:30 Europe/London


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More than 200 species have been detected in the interstellar medium (ISM), among them many molecules, radicals and ions, containing the −C≡N functional group. Both linear and branched isomers of propyl cyanide (PrCN; C 3 H 7 CN) are ubiquitous in interstellar space. To date, PrCN is one of the most complex molecules found in the interstellar medium. Furthermore, it is the only one observed species to share the branched atomic backbone of amino acids, some of the building blocks of life. Radical-radical chemical reactions in gas phase and on an ice model are examined in detail using density functional theory M062X/6-311++g(d,p) and ab initio methods CCSD(T)-F12//MP2. The reaction mechanism involves the following radicals association: CH 3 CHCH 3 +CN, CH 3 +CH3CHCN for iso-PrCN and CH 3 CH 2 +CH 2 CN, CH 3 +CH 2 CH 2 CN, CN+CH 3 CH 2 CH 2 for n-PrCN formation. Rate constants (see Figure 1) are also reported for gas phase association reactions. All reaction paths are exoergic and barrier-less in the gas phase and on the ice-model, suggesting that the formation of iso-PrCN and n-PrCN is efficient on the water-ice model adopted.

 Another molecule : acetaldehyde (CH 3 CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids through reaction with nitrogen containing chemical species. Quantum chemical and reaction kinetics studies are reported for acetaldehyde formation from the chemical reaction of C(3 P) with a methanol molecule adsorbed at the eighth position of a cubic water cluster. We present extensive quantum chemical calculations by means of CCSD(T)//wB97XD/6-311++G(2d,p) for total spin S=1 and S=0. The rate limiting step for forming acetaldehyde is the CO bond breaking in CH 3 OCH to form adsorbed
CH 3 and HCO. We find two positions on the reaction path where spin crossing may be possible such that acetaldehyde can form in its singlet spin state.

1. I. BenChouikha, B. Kerkeni, et al. Quantum chemical study of the reaction paths and kinetics of acetaldehyde formation on a methanol-water ice model”, ACS Adv., 12,18994 (2022).
2. B. Kerkeni, V Gámez, G. Ouerfelli, M-L. Senent, and N. Feautrier “Understanding Propyl-cyanide and its isomers Formation: Ab initio Study of the Spectroscopy and Reaction Mechanisms.”, Mon. Not. Roy. Astron. Soc. (2023).