Effects of geometric isomerism in electron ionization mass spectra of dialkyl 1,4-cyclohexanedicarboxylates

Rationale

1,4-Cyclohexanedicarboxylic acid and its esters are widely used as building blocks in the production of polymers and copolymers. The properties of such compounds directly depend on the ratio of cis- and trans-isomers in the starting materials. The identification of such stereoisomers by mass spectrometry can be used for the analysis of complex reactions and pyrolysis mixtures.

Methods

1,4-Cyclohexanedicarboxylic acid propyl, isopropyl, butyl, 2-butyl, pentyl, 2-pentyl, 4-methylpentyl, heptyl, octyl, and nonyl esters were synthesized and analyzed by gas chromatography-electron ionization mass spectrometry. The general fragmentation patterns of the synthesized compounds in electron ionization conditions were determined using high-resolution mass spectrometry. The relative response factors for the isomers were established on the basis of NMR and GC–MS data.

Results

Regardless of the structure of the alkyl groups, the EI mass spectra of cis- and trans-dialkyl 1,4-cyclohexanedicarboxylates differ very sharply. The stereoselectivity of fragmentation processes is due to the steric proximity of the substituents in the case of cis-isomers. Elemental composition of all ions appearing in the mass spectra of the analytes under study was confirmed by high-resolution mass spectrometry. Despite the perceptible differences in the EI mass spectra of the isomers their ionization efficiencies are close.

Conclusions

The possibility of the unambiguous identification of stereoisomeric dialkyl cis and trans 1,4-cyclohexanedicarboxylates on the basis of their EI mass spectra was shown. The differences in mass spectra of the isomers are due to the conformational rigidity of the considered monocyclic molecules, which ensures the unique stereoselectivity of the elimination of neutral molecules owing to the 1,3-cis-diaxial proximity effect.