全氟己酸迁移反应机理的理论研究

Theoretical Study on the Migration Reaction Mechanism of Perfluorohexanoic Acid

全氟羧酸(PFCAs)化合物是自然界广泛存在的一类持久性有机污染物,理论研究发现PFCAs的支链结构较直链结构更稳定,更容易存在,而对自然界水体中直链和支链结构含量的实验测定发现,直链结构含量明显高于对应的支链结构含量,探究其不一致原因具有一定的现实意义。应用DFT方法,采用Gaussian 09计算程序,在B3LYP/6-31G(d,p)计算水平下,探究了直链全氟己酸(PFHxA)通过甲基迁移为支链PFCAs的6种反应路径及能量变化。研究发现,通过双三元环反应机理,PFHxA迁移为2-三氟甲基全氟戊酸的反应,具有相对较低的活化能,是主反应路径,其反应速率常数为1.28×10^(-58)/s。迁移反应速率与温度呈正相关,常温下,该迁移反应很难进行,高温有利于反应的进行。

Perfluorocarboxylic acids(PFCAs)compounds are a type of persistent organic pollutants which exist widely in nature.Theoretical studies had shown that the branched chain structure was more stable than the straight chain structure,but the experimental results showed that the content of the straight structure was significantly higher than the corresponding branched structure.Therefore,it was practical significance to explore the reasons.In this study,perfluorohexanoic acid(PFHxA)was taken as an example to explore the reactive mechanism of the migration of linear PFHxA to branched PFCAs.Using Gaussian 09 calculation program,simulated the migration path of linear PFCAs represented by PFHxA into branched PFCAs at the B3LYP/6-31G(d,p)level.The calculation results showed that PFHxA could migrate into the corresponding branched PFCAs through the double three-membered ring reaction mechanism.Among them,the reaction of migration toα-trifluoromethyl perfluorovaleric acid has relatively low energy.It was the main path,and its reaction rate constant was 1.28×10^(-58)/s.At room temperature,the migration reaction was difficult to proceed,but the rate of the migration reaction was positively correlated with the temperature,and thus high temperature was conducive to the progress of the reaction.