α-酮醛的Schiff碱类化合物的质谱断裂机理

A STUDY OF MS FRAGMENTATION MECHANISM ON α-KETO-ALDEHYDE SCHIFF BASES

本文通过低分辨、高分辨质谱及亚稳离子测定,对α-酮醛的 Schiff 碱类化合物的质谱断裂过程进行了研究.结果表明,这类化合物均不太稳定,分子离子峰的相对强度很小.分子的电子轰击质谱裂解有以下三种基本方式:(1)电离发生在碳氢不饱和双键的 N 原子上,得到 m/z186的离子及其子离子 m/z170,144,143;(2)电离发生在乙酰胺基的 N 原子上,经甲基上的 H 重排,丢失 COCH,得到 m/z[M-COCH]的离子及其骨架重排子离子;(3)电离发生在乙酰基的氧原子上,α断键产生 m/z43的离子.取代基 R 的性质决定了断裂所采取的主要方式.α-酮醛的 Schiff 碱类化合物与喹噁啉环类化合物是同一原料和合成路线在不同溶剂中反应的不同产物,而在质谱离子源中,Schiff碱也存在转变为喹噁啉环的趋势.

The mechanisms of fragmentation ofα-keto-aldehyde Sehiff bases were studied by means of accurate mass and motastable ion measurements.It was observed that these compounds are unstable under electron impact and only very low abundance of molecular ion could be obtained.The fragmentation mechanisms of the compounds were discussed in terms of charge radical localization theory as follows:(1)The ionization takes place at nitrogen atom connected to the double bound between carbon and nitrogen,subse-quently,m/z 186 ion and its daughter ions m/z 170,144,143 were formed.(2)If ionization takes place at nitrogen atom connected to acetamide group,the ion[M-COCH]and its skeletal rearranged daughter ions appeared through the rearrangement of the H atom of methyl group and loss of the fragment COCH.(3)When the ionization takes place at the oxygen atom of acetyl group,m/z 43 ion was formed withα-cleavage.The pathway of fragmentation depends on the property of the substituent R.Besides,these compounds undergo a special deoxidation process shown by the fragments.