Study On The Spectral Properties And Reaction Mechanism Of Several Interstellar Carbon-Containing Small Molecules

The discovery of interstellar organic molecules was one of the four great discoveries of astronomy in the 1960s.This discovery allows humans to gain insight into the composition of the Milky Way galaxy,infer the evolution of nebulae and stars,and increase the possibility of the existence of extraterrestrial life.It is the basis of interstellar chemistry,a branch of astronomy.Since the discovery of interstellar organic molecules,people have carried out extensive experimental and theoretical studies on their related properties.Especially in recent years,in the fields of organic chemistry,materials chemistry,inorganic chemistry and chemical kinetics,some small molecule compounds containing C atoms have attracted the attention of scholars.Since CH and CN free radicals were detected in the empty interstellar space,many physical and chemical methods have gradually appeared in the study of interstellar chemistry.With the continuous deepening and development of relevant researches,two hundred interstellar molecules have been discovered in the past few decades.Especially in the fields of interstellar chemistry and atmospheric chemistry,considerable research achievements have been made in the research of such small molecules,both in theory and in experimental detection.However,due to extreme conditions such as low pressure and low temperature in the interstellar,it is very difficult to simulate the interstellar environment in the laboratory.Based on this situation,the above-mentioned problems can be easily solved by using theoretical chemical calculation methods.In this paper,α-CHD,C₂N₂,C₂N₂⁺are taken as the specific research objects,and the thermodynamic and dynamic stability of these systems are analyzed and discussed by using the quantum chemical method.The purpose is to reveal the stability of isomers,the geometric structure of molecules and the corresponding spectral properties in these systems,so as to provide strong theoretical support for experimental research and interstellar exploration.The specific research contents are as follows:1.The potential energy surface（PES）of photodissociation reaction,from ground-stateα-cyclohexanedione（α-CHD）to cyclopentanone（c-C₅H₈O）and carbon monoxide（CO）,has been studied at CCSD（T）//B3LYP/6-311++G（d,p）level of theory,and the infrared spectra of cyclic isomers along the reaction path have also been computed.The optimal photodissociation path of theα-CHD→c-C₅H₈O+CO is determined in two aspects,the number of high-energy barriers and the relative energies of transition states,together with the infrared spectra by comparing with other studies.The infrared vibrational analysis of the cyclic isomers on the PES is carried out.The observed frequencies are assigned to various vibration modes on the basis of the infrared intensity,and the assignment of fundamental vibration frequencies is in good agreement with the experimental data.The calculation results propose a new optimal dissociation path for the ground stateα-CHD,and also enrich and perfect the spectral information of its cyclic isomers.2.The isomerization of C₂N₂molecular system and its structure and properties have been studied on the theoretical level of CCSD（T）//B3LYP.The isomers NCCN,CNCN,CNNC,CCNN with linear structure and an NNC three-membered ring structure with exocyclic carbon-carbon bonding are obtained.For the above-mentioned isomers,the vibration frequency and infrared absorption intensity have been calculated at the level of CCSD（T）/6-311G（d）.Comparing the calculated spectral data with the experimental data results,it shows a good agreement.At the same time,the structure and stability of C₂N₂⁺isomers have been studied on the theoretical level of B3LYP/6-311G（d）,five isomers were obtained,of which the isomer NCCN⁺is the most stable isomer.