First-principles Computational Study On Gas Phase Synthesis Of Interstellar Pyrimidine Bases

In the extraterrestrial origin hypothesis of life,the chemical evolution of interstellar organic molecules is thought to be crucial for the formation of life-related compounds.In previous studies,pyrimidine bases,the basic units of RNA and DNA genetic code,were thought to evolve from large molecules like pyrimidines in interstellar ice.However,due to the instability of these molecules under ultraviolet radiation,the synthesis process of pyrimidine bases in the gaseous environment of molecular clouds is not clear at present.On the other hand,the abundance of interstellar molecules declines rapidly as their molecular weight increases,so it is important to understand the possibility that pyrimidine bases can be synthesized by small molecules in interstellar molecular clouds.In this paper,the possibility of forming cytosine（C₄H₅N₃O）,thymine（C₅H₅N₂O₂）and uracil（C₄H₄N₂O₂）by gas reaction of two kinds of small interstellar organic molecules formamide（H₂NCHO）and vinyl cyanide（H₂CCHCN）in the interstellar medium is investigated by first-principles calculations.In this paper,M06 functional is used to simulate the collision of gas phase molecules to find the optimal synthesis route for forming pyrimidine bases,and the calculation results are verified by ab initio M?ller–Plesset second-order perturbation theory.In this paper,we estimate the lowest reaction energy barriers based on the calculated reaction routes,and try to propose the specific interstellar environment favorable to the formation of pyrimidine bases according to the reaction rates calculated by these reaction energy barriers.In this paper,several synthetic routes of C₄H₅N₃O,C₅H₅N₂O₂ and C₄H₄N₂O₂were reported through the reaction of partial dehydrogenation H₂NCHO and H₂CCHCN.The optimal path was found in a molecular and radical reaction between HNCHO and H₂CCHCN.The reaction to generate 1H-Pyrimidin-2-one（C₄H₄N₂O）has a barrier of only about 19.3 kcal/mol.This means that the reaction rate coefficient can exceed 10^-11 cm³s^-1 above 180 K（generally considered the minimum reaction rate required for significant reactions in the interstellar medium）.The results suggest that pyrimidine bases can be synthesized from small molecules in hot molecular nuclei or protoplanetary disks in interstellar under gas phase conditions,without the（generally considered necessary）catalysis by ultraviolet light.In addition,by analyzing the calculated reaction paths,it is found that the hydrogen migration process in the reactants is the key process that determines the rate of base synthesis.In summary,gaseous synthesis of pyrimidines base in the interstellar medium was studied in this paper.Through quantum chemical calculation,we find the optimal reaction path of H₂NCHO and H₂CCHCN to synthesize C₄H₅N₃O,C₅H₅N₂O₂ and C₄H₄N₂O₂ under gas phase interstellar conditions,and demonstrate the possibility of these reactions being driven by heat energy in some interstellar regions.These results expand the scope of the certification of life related molecules in interstellar media.