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.
