The GRETOBAPE Gas-phase Reaction Network: The Importance of Being Exothermic

Abstract

The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and nonlinear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations, as well as reactions inferred by chemical intuition and the similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE, based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that ∼5% of the reactions in the original network are endothermic, although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red, to be used to simulate astrochemical situations where only C-, O-, N-, and S-bearing species with less than six atoms are needed. Finally, the new GRETOBAPE network, its reduced version, and the database with all the molecular properties are made publicly available. The species property database can be used in the future to test the feasibility of possibly new reactions.

Bridging the gap: viable reaction pathways from tetrahedrane to benzyne

“…However, since no leaving group is present that would result in isolation of c -C 6 H 4 , this formation pathway is less feasible. 96,97 While radiative stabilization of c -C 6 H 4 is possible in theory, such processes have yet to be observed experimentally for neutral–neutral reactions. 98 As procession through an available bimolecular exit channel, if it exists, is often much faster than the corresponding radiative stabilization process for small species, 99 c -C 6 H 4 is more likely to re-dissociate into reactants or generate nontargeted products on this pathway.…”

Spiers Memorial Lecture: Astrochemistry at high resolution

“…In this respect, Tinacci and collaborators have obtained the most extensive QM study and complete distribution of BE, obtained for ammonia and water over a large ice grain of 250 water molecules. 147,148 The example of the BE distribution of ammonia is reported in Fig. 10.…”

Formation of AlO, AlOH, and Al(OH)3 in the Interstellar Medium and Circumstellar Envelopes of AGB Stars

“…This results in the only available energy being from the initial reactants. 44 These conditions require the initial association of the reactants to be barrierless and the full reaction be exothermic. 43 The barrierless association of the reactants produces an excess of energy that must be dissipated in some manner.…”

“…The most common method for this is for a leaving group to carry the excess energy away as kinetic energy. 43 Radiative association, the addition of two molecules without a leaving group, is considered to be infeasible and typically results in the destruction of the product back into the starting materials 44 with the exception of very cold regions such as TMC-1 where neutral−neutral reactions are common. 45 Even though AlO is the most studied aluminum-containing molecule around AGB stars, 42 there is still much debate over how AlO is formed and what its subsequent chemistry will be.…”

“…There are two ways that reaction 2 above can occur: one with linear abstraction of the hydrogen directly onto the oxygen and the other where it goes through the cis/trans isomerization of HAlOH. Both of these pathways have a transition state that lies above the initial energy of the reactants 50 which is largely believed to prohibit such a reaction in most astronomical regions, 44 again, save for the innermost regions of the CSE of a star. The linear abstraction of the hydrogen results in a barrier of 45 kJ mol −1 (10.8 kcal mol −1 ), while the other path has a barrier of about 63 kJ mol −1 (15.1 kcal mol −1 ).…”