高等学校化学学报 ›› 2018, Vol. 39 ›› Issue (3): 435-446.doi: 10.7503/cjcu20170507

• 分析化学 • 上一篇    下一篇

质谱中反偶电子规则并含双电荷离子的两类联苯化合物的裂解机理

王毅1, 张苹1, 吴生秀1, 孙媛媛1, 赵彤1, 刘世熙2()   

  1. 1. 西安瑞联新材料股份有限公司, 西安 710077
    2. 云南大学化学科学与工程学院, 昆明 650091
  • 收稿日期:2017-07-26 出版日期:2018-03-10 发布日期:2018-01-17
  • 作者简介:联系人简介: 刘世熙, 男, 博士, 副教授, 主要从事计算化学方面的研究. E-mail:shxliu@ynu.edu.cn
  • 基金资助:
    国家自然科学基金(批准号: 21465025)资助

Fragmentation Mechanism of Two Kinds of Violated Even-electron Rule Compounds with Doubly Charged Ions in Mass Spectrometry

WANG Yi1, ZHANG Ping1, WU Shengxiu1, SUN Yuanyuan1, ZHAO Tong1, LIU Shixi2,*()   

  1. 1.Xi’an Manareco New Materials Co., Ltd., Xi’an 710077, China
    2.School of Chemical Science and Technology, Yunnan University, Kunming 650091, China
  • Received:2017-07-26 Online:2018-03-10 Published:2018-01-17
  • Contact: LIU Shixi E-mail:shxliu@ynu.edu.cn
  • Supported by:
    † Supported by the National Natural Science Foundation of China(No.21465025)

摘要:

利用气相色谱-质谱法分别对含C≡≡C桥键、 CF2O桥键及双CF2O桥键的联苯类化合物进行分析, 在各个化合物的谱图中均存在显著的奇电子自由基离子峰(反偶电子规则)和双电荷碎片离子峰. 根据所得实验结果, 归纳以上化合物中奇电子离子的生成途径分别为: (1) 游离基中心诱导苄基Csp3Csp3键均裂后再经另一苄基Csp3Csp3键均裂, 即α-σ裂解; (2) 电荷中心静电诱导Csp3Osp3键异裂后再经苄基Csp3Csp3键或Csp3Osp3键均裂, 即i-σ裂解. 推测引起σ键裂解的机制是由内转换引起的. 双电荷离子的生成受2个相互独立官能团上既含游离基又含电荷的活化反应中心共同诱导控制裂解得到, 生成途径分别为: (1) 2个相互独立的游离基中心分别诱导苄基Csp3Csp3键双均裂得到, 即αα双重控制裂解反应; (2) 独立的游离基中心诱导Csp3Csp3均裂及另一电荷中心静电诱导Csp3Osp3键异裂得到, 即双重控制裂解反应; (3) 2个相互独立的电荷中心分别静电诱导Csp3Osp3键双异裂, 即ii双重控制裂解反应. 采用密度泛函理论(DFT)方法对具有代表性的化合物的裂解途径进行了初步计算, 结果与前述裂解机制一致.

关键词: 气相色谱-质谱联用, 偶电子规则, 双电荷离子, 液晶化合物, 裂解机理

Abstract:

The formation of odd electron ion(OE) by cleavage of even electron ion(EE+) is an anomalous reaction violates the “even-electron rule”. Two kinds of liquid crystal compounds, which have the structures of biphenyl with C≡≡C or CF2O moiety as a linkage group, were analyzed by gas chromatography-mass spectrometer(GC-MS). The results show that the spectra of the compounds are obtained by remarkable odd electron radical ion peaks and doubly charged fragment ion peaks, so the cleavage pathways were similar for the two kinds of compounds. We noted a sequential loss of alkyl, alkyl, or phenoxyl radicals under electron impact, which seems to be frequent in the case of liquid crystal compounds. The achievement of a higher degree of aromaticity may be the driving force for these processes. For the alkyl biphenyl compounds with C≡≡C central-bridge-bond, the cleavage of the alkyl carbon-carbon bond is initiated by the loss of alkyl radical via the α-cleavage, followed by the the losing of the other alkyl radical by σ bond cleavage, i.e., α-σ cleavage. For the alkyl biphenyl compounds with CF2O central-bridge-bond, the cleavage of the carbon-oxygen bond is initiated by the loss of phenoxyl radical via the i-cleavage, followed by the losing of the other alkyl radical by σ bond cleavage, i.e., i-σ cleavage. For the biphenyl compounds with double CF2O central-bridge-bond, the cleavage of the carbon-oxygen bond is initiated by the loss of phenoxyl radical via the i-cleavage, followed by the losing of the other phenoxyl radical by σ-bond cleavage, i.e., i-σ cleavage. The obtained results suggest the reason of σ-bond breakage, which should be caused by an internal conversion(IC). Generally, doubly charged ions are found in the electron ionization(EI) mass spectra of hydrocarbons, but the intensities of them are weak and cannot be used in chemical analyses. In contrast, in this paper we showed two kinds of liquid crystal compounds, as interesting examples, which can eliminate two internal alkyl or phenoxyl groups as radicals and yield the most abundant doubly charged ions in their EI mass spectrum by αα/iα or ii cleavage reactions. As an example, the DFT method is adopted to simulating the cleavage pathway of compound 2d. A conclusion can be draw that the proposed fragmentation mechanism is agreed by both experimental data and theoretical research.

Key words: Gas chromatography-mass spectrometer, Even-electron rule, Doubly charged ions, Liquid crystal compound, Fragmentation mechanism

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