Banca de DEFESA: HEMILLY OLIVEIRA SOUZA

Investigation of Glycolaldehyde, Glyceraldehyde and Dihydroxyacetone Formation Models in the Interstellar Medium

Glycolaldehyde, ISM, Astrochemistry

From observational research, about 204 complex organic molecules have been detected in interstellar and circumstellar medium. Furthermore, a diversity of organic compounds, including biomolecules, has been identified in fragments of comets and meteorites. Among such biomolecules, sugars and their precursors are of special interest. The general objectives of this work are: to investigate reaction mechanisms, not yet elucidated and under astrochemical conditions for the abiotic synthesis of the sugar precursors: glycolaldehyde (GLA), glyceraldehyde (GLI) and dihydroxyacetone (DI) in gaseous phase and on surfaces . The mechanism of GLA synthesis in the gas phase was investigated at CCSD(T)/M06-2X/aug-cc-pVTZ level, considering the elementary reactions: R1: CO + CH3OH, R2: H2O + CH2CO, R3: HCO + CH2OH , R4: OH + CH2CHO and R5: H2CO + trans-HCOH, all leading to the formation of GLA, the R5 route being possible in two ways: the first through the abstraction of the hydrogen of O-H from trans-HCOH by the carbonyl oxygen (R5a ), with an energy barrier of 10.1 kcal/mol; and the second way by the abstraction of the hydrogen of C-H from trans-HCOH by the carbonyl carbon (R5b), in a reaction without a barrier. Rate coefficients, obtained in the range of 10 - 500 K, are expressed by: kR1(T) = 1,8010-15 T0,7exp ( -86,11/RT), kR2(T) = 5,1910-14 T-0,12exp (-78,76/RT), kR3(T) = 3,6610-17 T2,18exp (-0,22/RT), kR4(T) = 2,9010-16 T2,11exp (-0,12/RT), kR5a(T) = 1,3410-9 T-1,93exp (-9,57/RT)andkR5b(T) = 8,7110-17 T2,17exp (0,03/RT). Rate coefficient calculations suggest that the barrier-free pathways R3, R4, and R5b are the most likely pathways for glycolaldehyde formation. A better distinction of R3, R4 and R5b occurs at high temperatures (~500 K), where R4 is the fastest. Rate coefficients for R4 decrease significantly with the lowering of the temperature and, around 100 K, route R5b shows the highest rate coefficient, being indicated as the most probable route of GLA synthesis, in colder conditions. Route R5b was extended to the formation of glyceraldehyde (trans-HCOH + GLA → GLI) and R5a to the formation of dihydroxyacetone (trans-HCOH + GLA → DI), showing rate coefficients expressed by: kR5b-GLI(T) = 6,4310-18 T2,18exp (0,12/RT)and kR5a-DI(T) = 6,7710-18 T1,98exp (-0,08/RT), in the range of 10 to 500 K. It is known that heterogeneous reactions, on surfaces of astrochemical grains, play an important role in the synthesis of compounds of increasing molecular complexity. Thus, the R5 reaction was also studied on a forsterite surface and periodic calculations at PBE level, adopting Vanderbilt ultrasoft pseudopotentials (USPP), suggest that formaldehyde and trans-hydroxymethylene reagents are adsorbed on the surface, reacting by Langmuir–Hinshelwood mechanism , in a similar way to R5a, leading to the formation of glycolaldehyde, which is strongly adsorbed, with adsorption energy equal to -205.03 kcal/mol. The intense adsorption of this precursor should allow consecutive reactions, leading to an increase in the complexity of surface sugars. Finally, this study allowed conclusions about the possible routes of abiotic synthesis of glycolaldehyde, glyceraldehyde and dihydroxyacetone.